The OpenGL Shading Language 4.4 GLSLang Spec.4.40

User Manual: GLSLangSpec.4.40

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The OpenGL® Shading Language
Language Version: 4.40
Document Revision: 8
22-Jan-2014
Editor: John Kessenich, LunarG
Version 1.1 Authors: John Kessenich, Dave Baldwin, Randi Rost
Copyright (c) 2008-2014 The Khronos Group Inc. All Rights Reserved.
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ii
Table of Contents
1 Introduction.................................................................................................................................1
1.1 Acknowledgments................................................................................................................2
1.2 Changes................................................................................................................................3
1.2.1 Changes since revision 7 of GLSL version 4.40..........................................................3
1.2.2 Changes since revision 6 of GLSL version 4.40..........................................................4
1.2.3 Summary of Changes from Revision 9 of GLSL Version 4.30....................................5
1.3 Overview..............................................................................................................................7
1.4 Error Handling......................................................................................................................7
1.5 Typographical Conventions.................................................................................................7
1.6 Deprecation..........................................................................................................................7
2 Overview of OpenGL Shading....................................................................................................9
2.1 Vertex Processor..................................................................................................................9
2.2 Tessellation Control Processor.............................................................................................9
2.3 Tessellation Evaluation Processor......................................................................................10
2.4 Geometry Processor...........................................................................................................10
2.5 Fragment Processor............................................................................................................10
2.6 Compute Processor.............................................................................................................10
3 Basics........................................................................................................................................12
3.1 Character Set and Phases of Compilation..........................................................................12
3.2 Source Strings....................................................................................................................13
3.3 Preprocessor.......................................................................................................................14
3.4 Comments..........................................................................................................................19
3.5 Tokens................................................................................................................................20
3.6 Keywords............................................................................................................................20
3.7 Identifiers...........................................................................................................................22
3.8 Definitions..........................................................................................................................22
3.8.1 Static Use....................................................................................................................23
3.8.2 Uniform and Non-Uniform Control Flow..................................................................23
3.8.3 Dynamically Uniform Expressions.............................................................................23
4 Variables and Types..................................................................................................................24
4.1 Basic Types........................................................................................................................24
4.1.1 Void............................................................................................................................28
4.1.2 Booleans.....................................................................................................................28
4.1.3 Integers.......................................................................................................................28
4.1.4 Floating-Point Variables.............................................................................................30
4.1.5 Vectors........................................................................................................................31
4.1.6 Matrices......................................................................................................................31
4.1.7 Opaque Types.............................................................................................................32
iii
4.1.7.1 Samplers.............................................................................................................32
4.1.7.2 Images.................................................................................................................33
4.1.7.3 Atomic Counters.................................................................................................33
4.1.8 Structures....................................................................................................................33
4.1.9 Arrays.........................................................................................................................34
4.1.10 Implicit Conversions................................................................................................38
4.1.11 Initializers.................................................................................................................39
4.2 Scoping...............................................................................................................................41
4.3 Storage Qualifiers...............................................................................................................44
4.3.1 Default Storage Qualifier............................................................................................45
4.3.2 Constant Qualifier......................................................................................................45
4.3.3 Constant Expressions.................................................................................................45
4.3.4 Input Variables...........................................................................................................46
4.3.5 Uniform Variables......................................................................................................49
4.3.6 Output Variables.........................................................................................................49
4.3.7 Buffer Variables.........................................................................................................52
4.3.8 Shared Variables.........................................................................................................52
4.3.9 Interface Blocks..........................................................................................................53
4.4 Layout Qualifiers................................................................................................................57
4.4.1 Input Layout Qualifiers...............................................................................................59
4.4.1.1 Tessellation Evaluation Inputs............................................................................62
4.4.1.2 Geometry Shader Inputs......................................................................................64
4.4.1.3 Fragment Shader Inputs......................................................................................65
4.4.1.4 Compute Shader Inputs.......................................................................................66
4.4.2 Output Layout Qualifiers............................................................................................67
4.4.2.1 Transform Feedback Layout Qualifiers..............................................................69
4.4.2.2 Tessellation Control Outputs..............................................................................71
4.4.2.3 Geometry Outputs...............................................................................................72
4.4.2.4 Fragment Outputs...............................................................................................74
4.4.3 Uniform Variable Layout Qualifiers...........................................................................75
4.4.4 Subroutine Function Layout Qualifiers......................................................................76
4.4.5 Uniform and Shader Storage Block Layout Qualifiers...............................................76
4.4.6 Opaque-Uniform Layout Qualifiers............................................................................79
4.4.6.1 Atomic Counter Layout Qualifiers.....................................................................80
4.4.6.2 Format Layout Qualifiers....................................................................................81
4.5 Interpolation Qualifiers......................................................................................................83
4.5.1 Redeclaring Built-in Interpolation Variables in the Compatibility Profile.................84
4.6 Parameter Qualifiers...........................................................................................................85
4.7 Precision and Precision Qualifiers.....................................................................................85
iv
4.7.1 Range and Precision...................................................................................................85
4.7.2 Precision Qualifiers....................................................................................................86
4.7.3 Default Precision Qualifiers.......................................................................................87
4.7.4 Available Precision Qualifiers....................................................................................88
4.8 Variance and the Invariant Qualifier..................................................................................88
4.8.1 The Invariant Qualifier...............................................................................................88
4.8.2 Invariance of Constant Expressions...........................................................................89
4.9 The Precise Qualifier..........................................................................................................89
4.10 Memory Qualifiers...........................................................................................................92
4.11 Order and Repetition of Qualification..............................................................................95
5 Operators and Expressions........................................................................................................96
5.1 Operators............................................................................................................................96
5.2 Array Operations...............................................................................................................97
5.3 Function Calls....................................................................................................................97
5.4 Constructors.......................................................................................................................97
5.4.1 Conversion and Scalar Constructors..........................................................................97
5.4.2 Vector and Matrix Constructors.................................................................................98
5.4.3 Structure Constructors..............................................................................................100
5.4.4 Array Constructors...................................................................................................101
5.5 Vector and Scalar Components and Length.....................................................................101
5.6 Matrix Components..........................................................................................................103
5.7 Structure and Array Operations........................................................................................103
5.8 Assignments.....................................................................................................................104
5.9 Expressions......................................................................................................................105
5.10 Vector and Matrix Operations........................................................................................108
5.11 Out-of-Bounds Accesses................................................................................................109
6 Statements and Structure.........................................................................................................110
6.1 Function Definitions.........................................................................................................111
6.1.1 Function Calling Conventions..................................................................................114
6.1.2 Subroutines...............................................................................................................115
6.2 Selection...........................................................................................................................116
6.3 Iteration............................................................................................................................117
6.4 Jumps................................................................................................................................118
7 Built-in Variables....................................................................................................................120
7.1 Built-In Language Variables............................................................................................120
7.1.1 Compatibility Profile Built-In Language Variables..................................................128
7.2 Compatibility Profile Vertex Shader Built-In Inputs.......................................................131
7.3 Built-In Constants............................................................................................................132
7.3.1 Compatibility Profile Built-In Constants..................................................................134
v
7.4 Built-In Uniform State.....................................................................................................134
7.4.1 Compatibility Profile State.......................................................................................134
8 Built-in Functions...................................................................................................................138
8.1 Angle and Trigonometry Functions..................................................................................139
8.2 Exponential Functions......................................................................................................141
8.3 Common Functions..........................................................................................................142
8.4 Floating-Point Pack and Unpack Functions.....................................................................147
8.5 Geometric Functions........................................................................................................149
8.6 Matrix Functions..............................................................................................................151
8.7 Vector Relational Functions.............................................................................................153
8.8 Integer Functions..............................................................................................................155
8.9 Texture Functions.............................................................................................................157
8.9.1 Texture Query Functions..........................................................................................158
8.9.2 Texel Lookup Functions...........................................................................................161
8.9.3 Texture Gather Functions.........................................................................................167
8.9.4 Compatibility Profile Texture Functions..................................................................170
8.10 Atomic-Counter Functions.............................................................................................172
8.11 Atomic Memory Functions............................................................................................172
8.12 Image Functions.............................................................................................................173
8.13 Fragment Processing Functions......................................................................................177
8.13.1 Derivative Functions..............................................................................................177
8.13.2 Interpolation Functions...........................................................................................178
8.14 Noise Functions..............................................................................................................179
8.15 Geometry Shader Functions...........................................................................................180
8.16 Shader Invocation Control Functions.............................................................................182
8.17 Shader Memory Control Functions................................................................................183
9 Shading Language Grammar for Core Profile........................................................................185
10 Normative References...........................................................................................................201
vi
1 Introduction
This document specifies only version 4.40 of the OpenGL Shading Language. It requires __VERSION__
to substitute 440, and requires #version to accept only 440. If #version is declared with a smaller
number, the language accepted is a previous version of the shading language, which will be supported
depending on the version and type of context in the OpenGL API. See the OpenGL Graphics System
Specification, Version 4.4, for details on what language versions are supported.
Previous versions of the OpenGL Shading Language, as well as the OpenGL ES Shading Language, are
not strict subsets of the version specified here, particularly with respect to precision, name-hiding rules,
and treatment of interface variables. See the specification corresponding to a particular language version
for details specific to that version of the language.
All OpenGL Graphics System Specification references in this specification are to version 4.4.
1
1 Introduction
1.1 Acknowledgments
This specification is based on the work of those who contributed to past versions of the OpenGL
Language Specification, the OpenGL ES 2.0 Language Specification, and the following contributors to
this version:
Pat Brown, NVIDIA
Jeff Bolz, NVIDIA
Frank Chen
Pierre Boudier, AMD
Piers Daniell, NVIDIA
Chris Dodd, NVIDIA
Nick Haemel, NVIDIA
Jason Green, TransGaming
Brent Insko, Intel
Jon Leech
Bill Licea-Kane, AMD
Daniel Koch, TransGaming
Barthold Lichtenbelt, NVIDIA
Bruce Merry, ARM
Robert Ohannessian
Acorn Pooley, NVIDIA
Christophe Riccio, AMD
Kevin Rogovin
Ian Romanick, Intel
Greg Roth, Nvidia
Graham Sellers, AMD
Dave Shreiner, ARM
Jeremy Sandmel, Apple
Robert Simpson, Qualcomm
Eric Werness, NVIDIA
Mark Young, AMD
2
1 Introduction
1.2 Changes
1.2.1 Changes since revision 7 of GLSL version 4.40
•Bug 10440: Clarify that a name collision between members of two anonymous blocks, or
between a variable and a member of an anonymous block is an error.
•Bug 11009: Removed packed from the reserved word list.
•Bug 11299: Fixed textureOffset for sampler2DArrayShadow to take a ivec2 (not a vec2) for
the offset.
•Bug 11209: It is a compile-time error to use the same block name for more than one block
declaration in the same interface within one shader, even if the block contents are identical.
•Bug 11100: Simplify statement of what is written by EmitStreamVertex() to just say all built-in
and user-defined output variables.
•Bug 11096: gl_SampleMask can be sized to be no larger than the implementation-dependent
maximum sample-mask.
•Bug 10812: Missing text: Added the phrase “a pair of 16-bit signed integers” when describing
unpackSnorm2x16.
•Bug 10804: When a uniform layout location is used, it is not required that all declarations of that
name include the location; only that those that include a location use the same location.
•Bug 11001: Remove extraneous “g” from some gsampler..shadow types.
•Bug 10990: Remove old contradictory text requiring interpolation qualifiers to match cross
stage; they must only match within a stage.
•Bug 9999: Editorial: add explanatory text about optimizing in section 4.4.2.4 about fragment
output layout qualifiers: “This potentially includes skipping shader execution if the fragment is
discarded because it is occluded and the shader has no side effects.”
•Bug 10485: It is only geometry shaders whose input is sized by the input primitive layout
declaration.
•Bug 10903. Clarify that members of structures cannot be declared as atomic counter types.
•Put missing storage qualifiers in component examples.
•Bug 11457. Add missing “SHARED” to the layout_qualifier_id grammar in section 9. This was
already correctly reflected in the body of the specification.
•Bug 11392. Clarify that comments do eliminate new lines (but don't change the line count) and
that the preprocessing character set is bigger than the character set used in the resulting stream of
GLSL tokens.
•Bug 7343. Clarify interactions between comments, new lines, and preprocessing by explicitly
listing the logical phases of compilation.
3
1 Introduction
•Bug 11362: When counting locations consumed, clarify that the outer array level for geometry
shader inputs, tessellation control shader inputs and outputs, and tessellation evaluation inputs is
first removed before counting.
•Bug 10737: State more clearly which types are illegal for inputs and outputs.
•Bug 11178: Correct function overloading examples, which were from a different revision of the
spec. than the current rules.
•Bug 10593: Clarify that within a declaration, if inout is used, neither in nor out may be used,
and none of these can be repeated.
•Bug 11052: Make type matching across compilation units in the same program apply to all
declared variables (not just those statically used, etc.)
•Bug 10941: When accessing the same packed buffer across multiple stages in the same program,
it either works or you get a link error.
1.2.2 Changes since revision 6 of GLSL version 4.40
Deprecation
•Bug 384: Noise is now
◦defined to return 0, and
◦deprecated (not removed).
Changes
•Bug 10628: Subroutine arrays now require the index to be dynamically uniform.
•Bug 10440: Refine the link-time error: Within an interface, all declarations of the same global
name must be for the same object and must match in type and in whether they declare a variable
or member of a block with no instance name.
•Bug 10713: Update the offset/align example in section 4.4.5 to adhere to the std140 alignment
requirements.
•A few other examples corrected.
•Changed
◦gl_MaxComputeAtomicCounterBuffers to 8, and
◦gl_MaxCombinedTextureImageUnits to 96.
Clarifications
•Bug 10655: Clarification that opaque types (e.g., samplers) can be in a uniform (e.g., member in
a struct), not just a non-aggregate uniform variable.
•Bug 10659: Be even more clear that blocks generally cannot be redeclared as a way to size an
unsized array contained in the block.
•Bug 10735: Clarify that sampler type declarations can have precision qualifiers.
4
1 Introduction
•Bug 10682: Clarify that built-in functions with void return or out arguments are not included in
in the set of constant expressions.
1.2.3 Summary of Changes from Revision 9 of GLSL Version 4.30
Deprecations
•The built-in noise*() functions are deprecated. They are not removed, but are defined to return
0.
Changes
•Incorporate the ARB_enhanced_layouts extension, which adds
◦compile-time constant expressions for layout qualifier integers
◦new offset and align layout qualifiers for control over buffer block layouts
◦
add location layout qualifier for input and output blocks and block members
◦
new component layout qualifier for finer-grained layout control of input and output
variables and blocks
◦new xfb_buffer, xfb_stride, and xfb_offset layout qualifiers to allow the shader to control
transform feedback buffering.
•Bug 10530: To be consistent with ES, include sample types as valid in a precision statement.
Note the defaults are irrelevant, as precision qualifiers are not required or have any meaning.
•Bug 10628: Subroutine arrays now require the index to be dynamically uniform.
•Changed
◦gl_MaxComputeAtomicCounterBuffers to 8, and
◦gl_MaxCombinedTextureImageUnits to 96.
•Bug 11009: Removed packed from the reserved word list.
•Bug 11209: It is a compile-time error to use the same block name for more than one block
declaration in the same interface within one shader, even if the block contents are identical.
•Bug 11096: gl_SampleMask can be sized to be no larger than the implementation-dependent
maximum sample-mask.
•Bug 10804: When a uniform layout location is used, it is not required that all declarations of that
name include the location; only that those that include a location use the same location.
•Bug 11052: Make type matching across compilation units in the same program apply to all
declared variables (not just those statically used, etc.)
•Bug 10941: When accessing the same packed buffer across multiple stages in the same program,
it either works or you get a link error.
Clarifications and Typographical Errors
•Editorial: Added layout qualifier table for non-opaque type and interface layout qualifiers.
5
1 Introduction
•Editorial changes around compute shader group sizes for language consistency within the spec.
and extensions.
•Bug 10327: Editorial: Say character set is subset of Unicode, in UTF-8 encoding.
•Bug 11299: Fixed textureOffset for sampler2DArrayShadow to take a ivec2 (not a vec2) for
the offset.
•Bug 10440: Clarify that a name collision between members of two anonymous blocks or a
variable and a member of an anonymous block is an error.
•Bug 10655: Clarification that opaque types (e.g., samplers) can be in a uniform (e.g., member in
a struct), not just a non-aggregate uniform variable.
•Bug 10659: Be even more clear that blocks generally cannot be redeclared as a way to size an
unsized array contained in the block.
•Bug 10682: Clarify that built-in functions with void return or out arguments are not included in
in the set of constant expressions.
•Bug 11100: Editorial: Simplify statement of what is written by EmitStreamVertex() to just say
all built-in and user-defined output variables.
•Bug 10812: Missing text: Added the phrase “a pair of 16-bit signed integers” when describing
unpackSnorm2x16.
•Bug 11001: Remove extraneous “g” from some gsampler shadow types.
•Bug 10990: Remove old contradictory text requiring interpolation qualifiers to match cross
stage; they must only match within a stage.
•Bug 9999: Editorial: add explanatory text about optimizing in section 4.4.2.4 about fragment
output layout qualifiers: “This potentially includes skipping shader execution if the fragment is
discarded because it is occluded and the shader has no side effects.”
•Bug 10485: Clarify it is only geometry shaders whose input is sized by the input primitive layout
declaration.
•Bug 10903. Clarify that members of structures cannot be declared as atomic counter types.
•Put missing storage qualifiers in component examples.
•Bug 11457. Add missing “SHARED” to the layout_qualifier_id grammar in section 9. This was
already correctly reflected in the body of the specification.
•Bug 11392. Clarify that comments do eliminate new lines (but don't change the line count) and
that the preprocessing character set is bigger than the character set used in the resulting stream of
GLSL tokens.
•Bug 7343. Clarify interactions between comments, new lines, and preprocessing by explicitly
listing the logical phases of compilation.
•Bug 11362: When counting locations consumed, clarify that the outer array level for geometry
shader inputs, tessellation control shader inputs and outputs, and tessellation evaluation inputs is
first removed before counting.
6
1 Introduction
•Bug 10737: State more clearly which types are illegal for inputs and outputs.
•Bug 11178: Correct function overloading examples, which were from a different revision of the
spec. than the current rules.
•Bug 10593: Clarify that within a declaration, if inout is used, neither in nor out may be used,
and none of these can be repeated.
1.3 Overview
This document describes The OpenGL Shading Language, version 4.40.
Independent compilation units written in this language are called shaders. A program is a set of shaders
that are compiled and linked together, completely creating one or more of the programmable stages of the
OpenGL pipeline. All the shaders for a single programmable stage must be within the same program. A
complete set of programmable stages can be put into a single program or the stages can be partitioned
across multiple programs. The aim of this document is to thoroughly specify the programming language.
The OpenGL Graphics System Specification will specify the OpenGL entry points used to manipulate and
communicate with programs and shaders.
1.4 Error Handling
Compilers, in general, accept programs that are ill-formed, due to the impossibility of detecting all ill-
formed programs. Portability is only ensured for well-formed programs, which this specification
describes. Compilers are encouraged to detect ill-formed programs and issue diagnostic messages, but are
not required to do so for all cases. Compile-time errors must be returned for lexically or grammatically
incorrect shaders. Other errors are reported at compile time or link time as indicated. Code that is “dead”
must still be error checked. For example:
if (false) // changing false to true cannot uncover additional errors
statement; // statement must be error checked regardless
1.5 Typographical Conventions
Italic, bold, and font choices have been used in this specification primarily to improve readability. Code
fragments use a fixed width font. Identifiers embedded in text are italicized. Keywords embedded in text
are bold. Operators are called by their name, followed by their symbol in bold in parentheses. The
clarifying grammar fragments in the text use bold for literals and italics for non-terminals. The official
grammar in section 9 “Shading Language Grammar” uses all capitals for terminals and lower case for
non-terminals.
1.6 Deprecation
The OpenGL Shading Language has deprecated some features. These are clearly called out in this
specification as “deprecated”. They are still present in this version of the language, but are targeted for
potential removal in a future version of the shading language. The OpenGL API has a forward
compatibility mode that will disallow use of deprecated features. If compiling in a mode where use of
deprecated features is disallowed, their use causes compile-time or link-time errors. See the OpenGL
7
1 Introduction
Graphics System Specification for details on what causes deprecated language features to be accepted or
to return an error.
8
2 Overview of OpenGL Shading
The OpenGL Shading Language is actually several closely related languages. These languages are used
to create shaders for each of the programmable processors contained in the OpenGL processing pipeline.
Currently, these processors are the vertex, tessellation control, tessellation evaluation, geometry,
fragment, and compute processors.
Unless otherwise noted in this paper, a language feature applies to all languages, and common usage will
refer to these languages as a single language. The specific languages will be referred to by the name of
the processor they target: vertex, tessellation control, tessellation evaluation, geometry, fragment, or
compute.
Most OpenGL state is not tracked or made available to shaders. Typically, user-defined variables will be
used for communicating between different stages of the OpenGL pipeline. However, a small amount of
state is still tracked and automatically made available to shaders, and there are a few built-in variables for
interfaces between different stages of the OpenGL pipeline.
2.1 Vertex Processor
The vertex processor is a programmable unit that operates on incoming vertices and their associated data.
Compilation units written in the OpenGL Shading Language to run on this processor are called vertex
shaders. When a set of vertex shaders are successfully compiled and linked, they result in a vertex shader
executable that runs on the vertex processor.
The vertex processor operates on one vertex at a time. It does not replace graphics operations that require
knowledge of several vertices at a time.
2.2 Tessellation Control Processor
The tessellation control processor is a programmable unit that operates on a patch of incoming vertices
and their associated data, emitting a new output patch. Compilation units written in the OpenGL Shading
Language to run on this processor are called tessellation control shaders. When a set of tessellation
control shaders are successfully compiled and linked, they result in a tessellation control shader
executable that runs on the tessellation control processor.
The tessellation control shader is invoked for each vertex of the output patch. Each invocation can read
the attributes of any vertex in the input or output patches, but can only write per-vertex attributes for the
corresponding output patch vertex. The shader invocations collectively produce a set of per-patch
attributes for the output patch. After all tessellation control shader invocations have completed, the output
vertices and per-patch attributes are assembled to form a patch to be used by subsequent pipeline stages.
Tessellation control shader invocations run mostly independently, with undefined relative execution order.
However, the built-in function barrier() can be used to control execution order by synchronizing
invocations, effectively dividing tessellation control shader execution into a set of phases. Tessellation
control shaders will get undefined results if one invocation reads a per-vertex or per-patch attribute
9
2 Overview of OpenGL Shading
written by another invocation at any point during the same phase, or if two invocations attempt to write
different values to the same per-patch output in a single phase.
2.3 Tessellation Evaluation Processor
The tessellation evaluation processor is a programmable unit that evaluates the position and other
attributes of a vertex generated by the tessellation primitive generator, using a patch of incoming vertices
and their associated data. Compilation units written in the OpenGL Shading Language to run on this
processor are called tessellation evaluation shaders. When a set of tessellation evaluation shaders are
successfully compiled and linked, they result in a tessellation evaluation shader executable that runs on
the tessellation evaluation processor.
Each invocation of the tessellation evaluation executable computes the position and attributes of a single
vertex generated by the tessellation primitive generator. The executable can read the attributes of any
vertex in the input patch, plus the tessellation coordinate, which is the relative location of the vertex in the
primitive being tessellated. The executable writes the position and other attributes of the vertex.
2.4 Geometry Processor
The geometry processor is a programmable unit that operates on data for incoming vertices for a primitive
assembled after vertex processing and outputs a sequence of vertices forming output primitives.
Compilation units written in the OpenGL Shading Language to run on this processor are called geometry
shaders. When a set of geometry shaders are successfully compiled and linked, they result in a geometry
shader executable that runs on the geometry processor.
A single invocation of the geometry shader executable on the geometry processor will operate on a
declared input primitive with a fixed number of vertices. This single invocation can emit a variable
number of vertices that are assembled into primitives of a declared output primitive type and passed to
subsequent pipeline stages.
2.5 Fragment Processor
The fragment processor is a programmable unit that operates on fragment values and their associated
data. Compilation units written in the OpenGL Shading Language to run on this processor are called
fragment shaders. When a set of fragment shaders are successfully compiled and linked, they result in a
fragment shader executable that runs on the fragment processor.
A fragment shader cannot change a fragment's (x, y) position. Access to neighboring fragments is not
allowed. The values computed by the fragment shader are ultimately used to update framebuffer memory
or texture memory, depending on the current OpenGL state and the OpenGL command that caused the
fragments to be generated.
2.6 Compute Processor
The compute processor is a programmable unit that operates independently from the other shader
processors. Compilation units written in the OpenGL Shading Language to run on this processor are
called compute shaders. When a set of compute shaders are successfully compiled and linked, they result
in a compute shader executable that runs on the compute processor.
10
2 Overview of OpenGL Shading
A compute shader has access to many of the same resources as fragment and other shader processors,
including textures, buffers, image variables, and atomic counters. It does not have any predefined inputs
nor any fixed-function outputs. It is not part of the graphics pipeline and its visible side effects are
through changes to images, storage buffers, and atomic counters.
A compute shader operates on a group of work items called a work group. A work group is a collection
of shader invocations that execute the same code, potentially in parallel. An invocation within a work
group may share data with other members of the same work group through shared variables and issue
memory and control barriers to synchronize with other members of the same work group.
11
3 Basics
3.1 Character Set and Phases of Compilation
The source character set used for the OpenGL shading languages is Unicode in the UTF-8 encoding
scheme. After preprocessing, only the following characters are allowed in the resulting stream of GLSL
tokens:
The letters a-z, A-Z, and the underscore ( _ ).
The numbers 0-9.
The symbols period (.), plus (+), dash (-), slash (/), asterisk (*), percent (%), angled brackets (< and
>), square brackets ( [ and ] ), parentheses ( ( and ) ), braces ( { and } ), caret (^), vertical bar ( | ),
ampersand (&), tilde (~), equals (=), exclamation point (!), colon (:), semicolon (;), comma (,), and
question mark (?).
A compile-time error will be given if any other character is used in a GLSL token.
There are no digraphs or trigraphs. There are no escape sequences or uses of the backslash beyond use as
the line-continuation character.
Lines are relevant for compiler diagnostic messages and the preprocessor. They are terminated by
carriage-return or line-feed. If both are used together, it will count as only a single line termination. For
the remainder of this document, any of these combinations is simply referred to as a new line.
In general, the language’s use of this character set is case sensitive.
There are no character or string data types, so no quoting characters are included.
There is no end-of-file character.
More formally, compilation happens as if the following logical phases were executed in order:
1. Source strings are concatenated to form a single input. All provided new lines are retained.
2. Line numbering is noted, based on all present new lines, and does not change when new lines are
later eliminated.
3. Wherever a backslash ('\') occurs immediately before a new line, both are eliminated. (Note no
white space is substituted, allowing a single token to span a new line.) Any newly formed
backslash followed by a new line is not eliminated; only those pairs originally occurring after
phase 1 are eliminated.
4. All comments are replaced with a single space. (Note that '//' style comments end before their
terminating new lines and white space is generally relevant to preprocessing.)
5. Preprocessing is done, resulting in a sequence of GLSL tokens, formed from the character set
stated above.
6. GLSL processing is done on the sequence of GLSL tokens.
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Details that fully define source strings, comments, line numbering, new line elimination, and
preprocessing are all discussed in upcoming sections. Sections beyond those describe GLSL processing.
3.2 Source Strings
The source for a single shader is an array of strings of characters from the character set. A single shader
is made from the concatenation of these strings. Each string can contain multiple lines, separated by new
lines. No new lines need be present in a string; a single line can be formed from multiple strings. No new
lines or other characters are inserted by the implementation when it concatenates the strings to form a
single shader. Multiple shaders can be linked together to form a single program.
Diagnostic messages returned from compiling a shader must identify both the line number within a string
and which source string the message applies to. Source strings are counted sequentially with the first
string being string 0. Line numbers are one more than the number of new lines that have been processed,
including counting the new lines that will be removed by the line-continuation character ( \ ).
Lines separated by the line-continuation character preceding a new line are concatenated together before
either comment processing or preprocessing. No white space is substituted for the line-continuation
character. That is, a single token could be formed by the concatenation by taking the characters at the end
of one line concatenating them with the characters at the beginning of the next line.
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3 Basics
float f\
oo;
// forms a single line equivalent to “float foo;”
// (assuming '\' is the last character before the new line and “oo” are
// the first two characters of the next line)
3.3 Preprocessor
There is a preprocessor that processes the source strings as part of the compilation process. Except as
noted below, it behaves as the C++ standard preprocessor (see section 10 “Normative References”).
The complete list of preprocessor directives is as follows.
#
#define
#undef
#if
#ifdef
#ifndef
#else
#elif
#endif
#error
#pragma
#extension
#version
#line
The following operators are also available
defined
##
Each number sign (#) can be preceded in its line only by spaces or horizontal tabs. It may also be
followed by spaces and horizontal tabs, preceding the directive. Each directive is terminated by a new
line. Preprocessing does not change the number or relative location of new lines in a source string.
Preprocessing takes places after new lines have been removed by the line-continuation character.
The number sign (#) on a line by itself is ignored. Any directive not listed above will cause a diagnostic
message and make the implementation treat the shader as ill-formed.
#define and #undef functionality are defined as is standard for C++ preprocessors for macro definitions
both with and without macro parameters.
The following predefined macros are available
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3 Basics
__LINE__
__FILE__
__VERSION__
__LINE__ will substitute a decimal integer constant that is one more than the number of preceding new
lines in the current source string.
__FILE__ will substitute a decimal integer constant that says which source string number is currently
being processed.
__VERSION__ will substitute a decimal integer reflecting the version number of the OpenGL shading
language. The version of the shading language described in this document will have __VERSION__
substitute the decimal integer 440.
All macro names containing two consecutive underscores ( __ ) are reserved for future use as predefined
macro names. All macro names prefixed with “GL_” (“GL” followed by a single underscore) are also
reserved.
#if, #ifdef, #ifndef, #else, #elif, and #endif are defined to operate as is standard for C++ preprocessors.
Expressions following #if and #elif are further restricted to expressions operating on literal integer
constants, plus identifiers consumed by the defined operator. Character constants are not supported.
The operators available are as follows.
Precedence Operator class Operators Associativity
1 (highest) parenthetical grouping ( ) NA
2 unary defined
+ - ~ !
Right to Left
3 multiplicative * / % Left to Right
4 additive + - Left to Right
5 bit-wise shift << >> Left to Right
6 relational < > <= >= Left to Right
7 equality == != Left to Right
8 bit-wise and & Left to Right
9 bit-wise exclusive or ^ Left to Right
10 bit-wise inclusive or | Left to Right
11 logical and && Left to Right
12 (lowest) logical inclusive or | | Left to Right
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3 Basics
The defined operator can be used in either of the following ways:
defined identifier
defined ( identifier )
Two tokens in a macro can be concatenated into one token using the token pasting (##) operator, as is
standard for C++ preprocessors. The result must be a valid single token, which will then be subject to
macro expansion. That is, macro expansion happens only after token pasting. There are no other number
sign based operators (e.g., no # or #@), nor is there a sizeof operator.
The semantics of applying operators to integer literals in the preprocessor match those standard in the C+
+ preprocessor, not those in the OpenGL Shading Language.
Preprocessor expressions will be evaluated according to the behavior of the host processor, not the
processor targeted by the shader.
#error will cause the implementation to put a compile-time diagnostic message into the shader object’s
information log (see section 7.12 “Shader and Program Queries” in the OpenGL Graphics System
Specification for how to access a shader object’s information log). The message will be the tokens
following the #error directive, up to the first new line. The implementation must then consider the shader
to be ill-formed.
#pragma allows implementation dependent compiler control. Tokens following #pragma are not subject
to preprocessor macro expansion. If an implementation does not recognize the tokens following
#pragma, then it will ignore that pragma. The following pragmas are defined as part of the language.
#pragma STDGL
The STDGL pragma is used to reserve pragmas for use by future revisions of this language. No
implementation may use a pragma whose first token is STDGL.
#pragma optimize(on)
#pragma optimize(off)
can be used to turn off optimizations as an aid in developing and debugging shaders. It can only be used
outside function definitions. By default, optimization is turned on for all shaders. The debug pragma
#pragma debug(on)
#pragma debug(off)
can be used to enable compiling and annotating a shader with debug information, so that it can be used
with a debugger. It can only be used outside function definitions. By default, debug is turned off.
Shaders should declare the version of the language they are written to. The language version a shader is
written to is specified by
#version number profileopt
where number must be a version of the language, following the same convention as __VERSION__ above.
The directive “#version 440” is required in any shader that uses version 4.40 of the language. Any
number representing a version of the language a compiler does not support will cause a compile-time
error to be generated. Version 1.10 of the language does not require shaders to include this directive, and
shaders that do not include a #version directive will be treated as targeting version 1.10. Shaders that
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3 Basics
specify #version 100 will be treated as targeting version 1.00 of the OpenGL ES Shading Language.
Shaders that specify #version 300 will be treated as targeting version 3.00 of the OpenGL ES Shading
Language.
If the optional profile argument is provided, it must be the name of an OpenGL profile. Currently, there
are three choices:
core
compatibility
es
A profile argument can only be used with version 150 or greater. If no profile argument is provided and
the version is 150 or greater, the default is core. If version 300 is specified, the profile argument is not
optional and must be es, or a compile-time error results. The Language Specification for the es profile is
specified in The OpenGL ES Shading Language specification.
Shaders for the core or compatibility profiles that declare different versions can be linked together.
However, es profile shaders cannot be linked with non-es profile shaders or with es profile shaders of a
different version, or a link-time error will result. When linking shaders of versions allowed by these rules,
remaining link-time errors will be given as per the linking rules in the GLSL version corresponding to the
version of the context the shaders are linked under. Shader compile-time errors must still be given strictly
based on the version declared (or defaulted to) within each shader.
Unless otherwise specified, this specification is documenting the core profile, and everything specified for
the core profile is also available in the compatibility profile. Features specified as belonging specifically
to the compatibility profile are not available in the core profile.
There is a built-in macro definition for each profile the implementation supports. All implementations
provide the following macro:
#define GL_core_profile 1
Implementations providing the compatibility profile provide the following macro:
#define GL_compatibility_profile 1
Implementations providing the es profile provide the following macro:
#define GL_es_profile 1
The #version directive must occur in a shader before anything else, except for comments and white space.
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3 Basics
By default, compilers of this language must issue compile-time lexical and grammatical errors for shaders
that do not conform to this specification. Any extended behavior must first be enabled. Directives to
control the behavior of the compiler with respect to extensions are declared with the #extension directive
#extension extension_name : behavior
#extension all : behavior
where extension_name is the name of an extension. Extension names are not documented in this
specification. The token all means the behavior applies to all extensions supported by the compiler. The
behavior can be one of the following
behavior Effect
require Behave as specified by the extension extension_name.
Give a compile-time error on the #extension if the extension extension_name
is not supported, or if all is specified.
enable Behave as specified by the extension extension_name.
Warn on the #extension if the extension extension_name is not supported.
Give a compile-time error on the #extension if all is specified.
warn Behave as specified by the extension extension_name, except issue warnings
on any detectable use of that extension, unless such use is supported by other
enabled or required extensions.
If all is specified, then warn on all detectable uses of any extension used.
Warn on the #extension if the extension extension_name is not supported.
disable Behave (including issuing errors and warnings) as if the extension
extension_name is not part of the language definition.
If all is specified, then behavior must revert back to that of the non-extended
core version of the language being compiled to.
Warn on the #extension if the extension extension_name is not supported.
The extension directive is a simple, low-level mechanism to set the behavior for each extension. It does
not define policies such as which combinations are appropriate, those must be defined elsewhere. Order
of directives matters in setting the behavior for each extension: Directives that occur later override those
seen earlier. The all variant sets the behavior for all extensions, overriding all previously issued
extension directives, but only for the behaviors warn and disable.
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3 Basics
The initial state of the compiler is as if the directive
#extension all : disable
was issued, telling the compiler that all error and warning reporting must be done according to this
specification, ignoring any extensions.
Each extension can define its allowed granularity of scope. If nothing is said, the granularity is a shader
(that is, a single compilation unit), and the extension directives must occur before any non-preprocessor
tokens. If necessary, the linker can enforce granularities larger than a single compilation unit, in which
case each involved shader will have to contain the necessary extension directive.
Macro expansion is not done on lines containing #extension and #version directives.
#line must have, after macro substitution, one of the following forms:
#line line
#line line source-string-number
where line and source-string-number are constant integer expressions. After processing this directive
(including its new line), the implementation will behave as if it is compiling at line number line and source
string number source-string-number. Subsequent source strings will be numbered sequentially, until
another #line directive overrides that numbering.
3.4 Comments
Comments are delimited by /* and */, or by // and a new line. The begin comment delimiters (/* or //) are
not recognized as comment delimiters inside of a comment, hence comments cannot be nested. A /*
comment includes its terminating delimiter (*/). However, a // comment does not include (or eliminate)
its terminating new line.
Inside comments, any byte values may be used, except a byte whose value is 0. No errors will be given
for the content of comments and no validation on the content of comments need be done.
Removal of new lines by the line-continuation character ( \ ) logically occurs before comments are
processed. That is, a single-line comment ending in the line-continuation character ( \ ) includes the next
line in the comment.
// a single-line comment containing the next line \
a = b; // this is still in the first comment
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3 Basics
3.5 Tokens
The language, after preprocessing, is a sequence of GLSL tokens. A token can be
token:
keyword
identifier
integer-constant
floating-constant
operator
; { }
3.6 Keywords
The following are the language's keywords and (after preprocessing) can only be used as described in this
specification, or a compile-time error results:
attribute const uniform varying buffer shared
coherent volatile restrict readonly writeonly
atomic_uint
layout
centroid flat smooth noperspective
patch sample
break continue do for while switch case default
if else
subroutine
in out inout
float double int void bool true false
invariant precise
discard return
mat2 mat3 mat4 dmat2 dmat3 dmat4
mat2x2 mat2x3 mat2x4 dmat2x2 dmat2x3 dmat2x4
mat3x2 mat3x3 mat3x4 dmat3x2 dmat3x3 dmat3x4
mat4x2 mat4x3 mat4x4 dmat4x2 dmat4x3 dmat4x4
vec2 vec3 vec4 ivec2 ivec3 ivec4 bvec2 bvec3 bvec4 dvec2 dvec3 dvec4
uint uvec2 uvec3 uvec4
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3 Basics
lowp mediump highp precision
sampler1D sampler2D sampler3D samplerCube
sampler1DShadow sampler2DShadow samplerCubeShadow
sampler1DArray sampler2DArray
sampler1DArrayShadow sampler2DArrayShadow
isampler1D isampler2D isampler3D isamplerCube
isampler1DArray isampler2DArray
usampler1D usampler2D usampler3D usamplerCube
usampler1DArray usampler2DArray
sampler2DRect sampler2DRectShadow isampler2DRect usampler2DRect
samplerBuffer isamplerBuffer usamplerBuffer
sampler2DMS isampler2DMS usampler2DMS
sampler2DMSArray isampler2DMSArray usampler2DMSArray
samplerCubeArray samplerCubeArrayShadow isamplerCubeArray usamplerCubeArray
image1D iimage1D uimage1D
image2D iimage2D uimage2D
image3D iimage3D uimage3D
image2DRect iimage2DRect uimage2DRect
imageCube iimageCube uimageCube
imageBuffer iimageBuffer uimageBuffer
image1DArray iimage1DArray uimage1DArray
image2DArray iimage2DArray uimage2DArray
imageCubeArray iimageCubeArray uimageCubeArray
image2DMS iimage2DMS uimage2DMS
image2DMSArray iimage2DMSArray uimage2DMSArray
struct
The following are the keywords reserved for future use. Using them will result in a compile-time error:
common partition active
asm
class union enum typedef template this
resource
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3 Basics
goto
inline noinline public static extern external interface
long short half fixed unsigned superp
input output
hvec2 hvec3 hvec4 fvec2 fvec3 fvec4
sampler3DRect
filter
sizeof cast
namespace using
In addition, all identifiers containing two consecutive underscores (__) are reserved as possible future
keywords.
3.7 Identifiers
Identifiers are used for variable names, function names, structure names, and field selectors (field
selectors select components of vectors and matrices similar to structure members, as discussed in section
5.5 “Vector and Scalar Components” and section 5.6 “Matrix Components” ). Identifiers have the form
identifier
nondigit
identifier nondigit
identifier digit
nondigit: one of
_ a b c d e f g h i j k l m n o p q r s t u v w x y z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
digit: one of
0 1 2 3 4 5 6 7 8 9
Identifiers starting with “gl_” are reserved for use by OpenGL, and may not be declared in a shader as
either a variable or a function; this results in a compile-time error. However, as noted in the specification,
there are some cases where previously declared variables can be redeclared, and predeclared "gl_" names
are allowed to be redeclared in a shader only for these specific purposes. More generally, it is a compile-
time error to redeclare a variable, including those starting “gl_”.
3.8 Definitions
Some language rules described below depend on the following definitions.
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3 Basics
3.8.1 Static Use
A shader contains a static use of (or static assignment to) a variable x if, after preprocessing, the shader
contains a statement that would read (or write) x, whether or not run-time flow of control will cause that
statement to be executed.
3.8.2 Uniform and Non-Uniform Control Flow
When executing statements in a fragment shader, control flow starts as uniform control flow; all fragments
enter the same control path into main(). Control flow becomes non-uniform when different fragments
take different paths through control-flow statements (selection, iteration, and jumps). Control flow
subsequently returns to being uniform after such divergent sub-statements or skipped code completes,
until the next time different control paths are taken.
For example:
main()
{
float a = ...;// this is uniform flow control
if (a < b) { // this expression is true for some fragments, not all
....; // non-uniform flow control
} else {
....; // non-uniform flow control
}
....; // uniform flow control again
}
Other examples of non-uniform flow control can occur within switch statements and after conditional
breaks, continues, early returns, and after fragment discards, when the condition is true for some
fragments but not others. Loop iterations that only some fragments execute are also non-uniform flow
control.
This is similarly defined for other shader stages, based on the per-instance data items they process.
3.8.3 Dynamically Uniform Expressions
A fragment-shader expression is dynamically uniform if all fragments evaluating it get the same resulting
value. When loops are involved, this refers to the expression's value for the same loop iteration. When
functions are involved, this refers to calls from the same call point.
This is similarly defined for other shader stages, based on the per-instance data they process.
Note that constant expressions are trivially dynamically uniform. It follows that typical loop counters
based on these are also dynamically uniform.
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4 Variables and Types
All variables and functions must be declared before being used. Variable and function names are
identifiers.
There are no default types. All variable and function declarations must have a declared type, and
optionally qualifiers. A variable is declared by specifying its type followed by one or more names
separated by commas. In many cases, a variable can be initialized as part of its declaration by using the
assignment operator (=).
User-defined types may be defined using struct to aggregate a list of existing types into a single name.
The OpenGL Shading Language is type safe. There are some implicit conversions between types.
Exactly how and when this can occur is described in section 4.1.10 “Implicit Conversions” and as
referenced by other sections in this specification.
4.1 Basic Types
The OpenGL Shading Language supports the following basic data types, grouped as follows.
Transparent types
Type Meaning
void for functions that do not return a value
bool a conditional type, taking on values of true or false
int a signed integer
uint an unsigned integer
float a single-precision floating-point scalar
double a double-precision floating-point scalar
vec2 a two-component single-precision floating-point vector
vec3 a three-component single-precision floating-point vector
vec4 a four-component single-precision floating-point vector
dvec2 a two-component double-precision floating-point vector
dvec3 a three-component double-precision floating-point vector
dvec4 a four-component double-precision floating-point vector
bvec2 a two-component Boolean vector
bvec3 a three-component Boolean vector
bvec4 a four-component Boolean vector
ivec2 a two-component signed integer vector
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4 Variables and Types
Type Meaning
ivec3 a three-component signed integer vector
ivec4 a four-component signed integer vector
uvec2 a two-component unsigned integer vector
uvec3 a three-component unsigned integer vector
uvec4 a four-component unsigned integer vector
mat2 a 2×2 single-precision floating-point matrix
mat3 a 3×3 single-precision floating-point matrix
mat4 a 4×4 single-precision floating-point matrix
mat2x2 same as a mat2
mat2x3 a single-precision floating-point matrix with 2 columns and 3 rows
mat2x4 a single-precision floating-point matrix with 2 columns and 4 rows
mat3x2 a single-precision floating-point matrix with 3 columns and 2 rows
mat3x3 same as a mat3
mat3x4 a single-precision floating-point matrix with 3 columns and 4 rows
mat4x2 a single-precision floating-point matrix with 4 columns and 2 rows
mat4x3 a single-precision floating-point matrix with 4 columns and 3 rows
mat4x4 same as a mat4
dmat2 a 2×2 double-precision floating-point matrix
dmat3 a 3×3 double-precision floating-point matrix
dmat4 a 4×4 double-precision floating-point matrix
dmat2x2 same as a dmat2
dmat2x3 a double-precision floating-point matrix with 2 columns and 3 rows
dmat2x4 a double-precision floating-point matrix with 2 columns and 4 rows
dmat3x2 a double-precision floating-point matrix with 3 columns and 2 rows
dmat3x3 same as a dmat3
dmat3x4 a double-precision floating-point matrix with 3 columns and 4 rows
dmat4x2 a double-precision floating-point matrix with 4 columns and 2 rows
dmat4x3 a double-precision floating-point matrix with 4 columns and 3 rows
dmat4x4 same as a dmat4
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4 Variables and Types
Floating-Point Opaque Types
Type Meaning
sampler1D
image1D
a handle for accessing a 1D texture
sampler2D
image2D
a handle for accessing a 2D texture
sampler3D
image3D
a handle for accessing a 3D texture
samplerCube
imageCube
a handle for accessing a cube mapped texture
sampler2DRect
image2DRect
a handle for accessing a rectangle texture
sampler1DArray
image1DArray
a handle for accessing a 1D array texture
sampler2DArray
image2DArray
a handle for accessing a 2D array texture
samplerBuffer
imageBuffer
a handle for accessing a buffer texture
sampler2DMS
image2DMS
a handle for accessing a 2D multi-sample texture
sampler2DMSArray
image2DMSArray
a handle for accessing a 2D multi-sample array texture
samplerCubeArray
imageCubeArray
a handle for accessing a cube map array texture
sampler1DShadow a handle for accessing a 1D depth texture with comparison
sampler2DShadow a handle for accessing a 2D depth texture with comparison
sampler2DRectShadow a handle for accessing a rectangle texture with comparison
sampler1DArrayShadow a handle for accessing a 1D array depth texture with comparison
sampler2DArrayShadow a handle for accessing a 2D array depth texture with comparison
samplerCubeShadow a handle for accessing a cube map depth texture with comparison
samplerCubeArrayShadow a handle for accessing a cube map array depth texture with
comparison
Signed Integer Opaque Types
Type Meaning
isampler1D
iimage1D
a handle for accessing an integer 1D texture
26
4 Variables and Types
Type Meaning
isampler2D
iimage2D
a handle for accessing an integer 2D texture
isampler3D
iimage3D
a handle for accessing an integer 3D texture
isamplerCube
iimageCube
a handle for accessing an integer cube mapped texture
isampler2DRect
iimage2DRect
a handle for accessing an integer 2D rectangle texture
isampler1DArray
iimage1DArray
a handle for accessing an integer 1D array texture
isampler2DArray
iimage2DArray
a handle for accessing an integer 2D array texture
isamplerBuffer
iimageBuffer
a handle for accessing an integer buffer texture
isampler2DMS
iimage2DMS
a handle for accessing an integer 2D multi-sample texture
isampler2DMSArray
iimage2DMSArray
a handle for accessing an integer 2D multi-sample array texture
isamplerCubeArray
iimageCubeArray
a handle for accessing an integer cube map array texture
Unsigned Integer Opaque Types
Type Meaning
atomic_uint a handle for accessing an unsigned integer atomic counter
usampler1D
uimage1D
a handle for accessing an unsigned integer 1D texture
usampler2D
uimage2D
a handle for accessing an unsigned integer 2D texture
usampler3D
uimage3D
a handle for accessing an unsigned integer 3D texture
usamplerCube
uimageCube
a handle for accessing an unsigned integer cube mapped texture
usampler2DRect
uimage2DRect
a handle for accessing an unsigned integer rectangle texture
usampler1DArray
uimage1DArray
a handle for accessing an unsigned integer 1D array texture
usampler2DArray
uimage2DArray
a handle for accessing an unsigned integer 2D array texture
27
4 Variables and Types
Type Meaning
usamplerBuffer
uimageBuffer
a handle for accessing an unsigned integer buffer texture
usampler2DMS
uimage2DMS
a handle for accessing an unsigned integer 2D multi-sample texture
usampler2DMSArray
uimage2DMSArray
a handle for accessing an unsigned integer 2D multi-sample texture
array
usamplerCubeArray
uimageCubeArray
a handle for accessing an unsigned integer cube map array texture
In addition, a shader can aggregate these basic types using arrays and structures to build more complex
types.
There are no pointer types.
4.1.1 Void
Functions that do not return a value must be declared as void. There is no default function return type.
The keyword void cannot be used in any other declarations (except for empty formal or actual parameter
lists), or a compile-time error results.
4.1.2 Booleans
To make conditional execution of code easier to express, the type bool is supported. There is no
expectation that hardware directly supports variables of this type. It is a genuine Boolean type, holding
only one of two values meaning either true or false. Two keywords true and false can be used as literal
Boolean constants. Booleans are declared and optionally initialized as in the follow example:
bool success; // declare “success” to be a Boolean
bool done = false; // declare and initialize “done”
The right side of the assignment operator ( = ) must be an expression whose type is bool.
Expressions used for conditional jumps (if, for, ?:, while, do-while) must evaluate to the type bool.
4.1.3 Integers
Signed and unsigned integer variables are fully supported. In this document, the term integer is meant to
generally include both signed and unsigned integers. Unsigned integers have exactly 32 bits of precision.
Signed integers use 32 bits, including a sign bit, in two's complement form. Addition, subtraction, and
shift operations resulting in overflow or underflow will not cause any exception, nor will they saturate,
rather they will “wrap” to yield the low-order 32 bits of the result. Division and multiplication operations
resulting in overflow or underflow will not cause any exception but will result in an undefined value.
Integers are declared and optionally initialized with integer expressions, as in the following example:
int i, j = 42; // default integer literal type is int
uint k = 3u; // “u” establishes the type as uint
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4 Variables and Types
Literal integer constants can be expressed in decimal (base 10), octal (base 8), or hexadecimal (base 16)
as follows.
integer-constant :
decimal-constant integer-suffixopt
octal-constant integer-suffixopt
hexadecimal-constant integer-suffixopt
integer-suffix: one of
u U
decimal-constant :
nonzero-digit
decimal-constant digit
octal-constant :
0
octal-constant octal-digit
hexadecimal-constant :
0x hexadecimal-digit
0X hexadecimal-digit
hexadecimal-constant hexadecimal-digit
digit :
0
nonzero-digit
nonzero-digit : one of
1 2 3 4 5 6 7 8 9
octal-digit : one of
0 1 2 3 4 5 6 7
hexadecimal-digit : one of
0 1 2 3 4 5 6 7 8 9
a b c d e f
A B C D E F
No white space is allowed between the digits of an integer constant, including after the leading 0 or after
the leading 0x or 0X of a constant, or before the suffix u or U. When tokenizing, the maximal token
matching the above will be recognized before a new token is started. When the suffix u or U is present,
the literal has type uint, otherwise the type is int. A leading unary minus sign (-) is interpreted as an
arithmetic unary negation, not as part of the constant. Hence, literals themselves are always expressed
with non-negative syntax, though they could result in a negative value.
It is a compile-time error to provide a literal integer whose bit pattern cannot fit in 32 bits. The bit pattern
of the literal is always used unmodified. So a signed literal whose bit pattern includes a set sign bit
creates a negative value. For example,
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4 Variables and Types
int a = 0xffffffff; // 32 bits, a gets the value -1
int b = 0xffffffffU; // ERROR: can't convert uint to int
uint c = 0xffffffff; // 32 bits, c gets the value 0xFFFFFFFF
uint d = 0xffffffffU; // 32 bits, d gets the value 0xFFFFFFFF
int e = -1; // the literal is “1”, then negation is performed,
// and the resulting non-literal 32-bit signed
// bit pattern of 0xFFFFFFFF is assigned, giving e
// the value of -1.
uint f = -1u; // the literal is “1u”, then negation is performed,
// and the resulting non-literal 32-bit unsigned
// bit pattern of 0xFFFFFFFF is assigned, giving f
// the value of 0xFFFFFFFF.
int g = 3000000000; // a signed decimal literal taking 32 bits,
// setting the sign bit, g gets -1294967296
int h = 0xA0000000; // okay, 32-bit signed hexadecimal
int i = 5000000000; // ERROR: needs more than 32 bits
int j = 0xFFFFFFFFF; // ERROR: needs more than 32 bits
int k = 0x80000000; // k gets -2147483648 == 0x80000000
int l = 2147483648; // l gets -2147483648 (the literal set the sign bit)
Despite all these examples initializing variables, literals are recognized and given values and types
independently of their context.
4.1.4 Floating-Point Variables
Single-precision and double-precision floating point variables are available for use in a variety of scalar
calculations. Generally, the term floating-point will refer to both single- and double-precision floating
point. Floating-point variables are defined as in the following examples:
float a, b = 1.5; // single-precision floating-point
double c, d = 2.0LF; // double-precision floating-point
As an input value to one of the processing units, a single-precision or double-precision floating-point
variable is expected to match the corresponding IEEE 754 floating-point definition for precision and
dynamic range. Floating-point variables within a shader are also encoded according to the IEEE 754
specification for single-precision floating-point values (logically, not necessarily physically). While
encodings are logically IEEE 754, operations (addition, multiplication, etc.) are not necessarily performed
as required by IEEE 754. See section 4.7.1 “Range and Precision” for more details on precision and
usage of NaNs (Not a Number) and Infs (positive or negative infinities).
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4 Variables and Types
Floating-point constants are defined as follows.
floating-constant :
fractional-constant exponent-partopt floating-suffixopt
digit-sequence exponent-part floating-suffixopt
fractional-constant :
digit-sequence . digit-sequence
digit-sequence .
. digit-sequence
exponent-part :
e signopt digit-sequence
E signopt digit-sequence
sign : one of
+ –
digit-sequence :
digit
digit-sequence digit
floating-suffix: one of
f F lf LF
A decimal point ( . ) is not needed if the exponent part is present. No white space may appear anywhere
within a floating-point constant, including before a suffix. When tokenizing, the maximal token matching
the above will be recognized before a new token is started. When the suffix "lf" or "LF" is present, the
literal has type double. Otherwise, the literal has type float. A leading unary minus sign (-) is interpreted
as a unary operator and is not part of the floating-point constant.
4.1.5 Vectors
The OpenGL Shading Language includes data types for generic 2-, 3-, and 4-component vectors of
floating-point values, integers, or Booleans. Floating-point vector variables can be used to store colors,
normals, positions, texture coordinates, texture lookup results and the like. Boolean vectors can be used
for component-wise comparisons of numeric vectors. Some examples of vector declaration are:
vec2 texcoord1, texcoord2;
vec3 position;
vec4 myRGBA;
ivec2 textureLookup;
bvec3 less;
Initialization of vectors can be done with constructors, which are discussed shortly.
4.1.6 Matrices
The OpenGL Shading Language has built-in types for 2×2, 2×3, 2×4, 3×2, 3×3, 3×4, 4×2, 4×3, and 4×4
matrices of floating-point numbers. Matrix types beginning with "mat" have single-precision components
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4 Variables and Types
while matrix types beginning with "dmat" have double-precision components. The first number in the
type is the number of columns, the second is the number of rows. If there is only one number, the matrix
is square. Example matrix declarations:
mat2 mat2D;
mat3 optMatrix;
mat4 view, projection;
mat4x4 view; // an alternate way of declaring a mat4
mat3x2 m; // a matrix with 3 columns and 2 rows
dmat4 highPrecisionMVP;
dmat2x4 dm;
Initialization of matrix values is done with constructors (described in section 5.4 “Constructors” ) in
column-major order.
4.1.7 Opaque Types
The opaque types declare variables that are effectively opaque handles to other objects. These objects are
accessed through built-in functions, not through direct reading or writing of the declared variable. They
can only be declared as function parameters or in uniform-qualified variables. The only opaque types
that take memory qualifiers are the image types. Except for array indexing, structure member selection,
and parentheses, opaque variables are not allowed to be operands in expressions; such use results in a
compile-time error.
Opaque variables cannot be treated as l-values; hence cannot be used as out or inout function parameters,
nor can they be assigned into. Any such use results in a compile-time error. However, they can be passed
as in parameters with matching type and memory qualifiers. They are initialized only through the
OpenGL API; they cannot be declared with an initializer in a shader.
Because a single opaque type declaration effectively declares two objects, the opaque handle itself and the
object it is a handle to, there is room for both a storage qualifier and a memory qualifier. The storage
qualifier will qualify the opaque handle, while the memory qualifier will qualify the object it is a handle
to.
4.1.7.1 Samplers
Sampler types (e.g., sampler2D) are opaque types, declared and behaving as described above for opaque
types. When aggregated into arrays within a shader, samplers can only be indexed with a dynamically
uniform integral expression, otherwise results are undefined.
Sampler variables are handles to one-, two-, and three- dimensional textures, cube maps, depth textures
(shadowing), etc., as enumerated in the basic types tables. There are distinct sampler types for each
texture target, and for each of float, integer, and unsigned integer data types. Texture accesses are done
through built-in texture functions (described in section 8.9 “Texture Functions”) and samplers are used to
specify which texture to access and how it is to be filtered.
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4 Variables and Types
4.1.7.2 Images
Image types are opaque types, declared and behaving as described above for opaque types. They can be
further qualified with memory qualifiers. When aggregated into arrays within a shader, images can only
be indexed with a dynamically uniform integral expression, otherwise results are undefined.
Image variables are handles to one-, two-, or three-dimensional images corresponding to all or a portion
of a single level of a texture image bound to an image unit. There are distinct image types for each texture
target, and for each of float, integer, and unsigned integer data types. Image accesses should use an image
type that matches the target of the texture whose level is bound to the image unit, or for non-layered
bindings of 3D or array images should use the image type that matches the dimensionality of the layer of
the image (i.e., a layer of 3D, 2DArray, Cube, or CubeArray should use image2D, a layer of 1DArray
should use image1D, and a layer of 2DMSArray should use image2DMS). If the image target type does
not match the bound image in this manner, if the data type does not match the bound image, or if the
format layout qualifier does not match the image unit format as described in section 8.25 “Texture Image
Loads and Stores” of the OpenGL Specification, the results of image accesses are undefined but cannot
include program termination.
Image variables are used in the image load, store, and atomic functions described in section 8.12 "Image
Functions" to specify an image to access.
4.1.7.3 Atomic Counters
Atomic counter types (atomic_uint) are opaque handles to counters, declared and behaving as described
above for opaque types. The variables they declare specify which counter to access when using the built-
in atomic counter functions as described in section 8.10 “Atomic Counter Functions”. They are bound to
buffers as described in section 4.4.6.1 “Atomic Counter Layout Qualifiers”. When aggregated into arrays
within a shader, atomic counters can only be indexed with a dynamically uniform integral expression,
otherwise results are undefined. Members of structures cannot be declared as atomic counter types.
4.1.8 Structures
User-defined types can be created by aggregating other already defined types into a structure using the
struct keyword. For example,
struct light {
float intensity;
vec3 position;
} lightVar;
In this example, light becomes the name of the new type, and lightVar becomes a variable of type light.
To declare variables of the new type, use its name (without the keyword struct).
light lightVar2;
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4 Variables and Types
More formally, structures are declared as follows. However, the complete correct grammar is as given in
section 9 “Shading Language Grammar” .
struct-definition :
qualifieropt struct nameopt { member-list } declaratorsopt ;
member-list :
member-declaration;
member-declaration member-list;
member-declaration :
basic-type declarators;
where name becomes the user-defined type, and can be used to declare variables to be of this new type.
The name shares the same name space as other variables, types, and functions. All previously visible
variables, types, constructors, or functions with that name are hidden. The optional qualifier only applies
to any declarators, and is not part of the type being defined for name.
Structures must have at least one member declaration. Member declarators may contain precision
qualifiers, but use of any other qualifier results in a compile-time error. Bit fields are not supported.
Member types must be already defined (there are no forward references). A compile-time error results if a
member declaration contains an initializer. Member declarators can contain arrays. Such arrays must
have a size specified, and the size must be an integral constant expression that's greater than zero (see
section 4.3.3 “Constant Expressions”). Each level of structure has its own name space for names given in
member declarators; such names need only be unique within that name space.
Anonymous structures are not supported. Embedded structure definitions are not supported. These result
in compile-time errors.
struct S { float f; };
struct T {
S; // Error: anonymous structures disallowed
struct { ... }; // Error: embedded structures disallowed
S s; // Okay: nested structures with name are allowed
};
Structures can be initialized at declaration time using constructors, as discussed in section 5.4.3 “Structure
Constructors” .
Any restrictions on the usage of a type or qualifier also apply to any structure that contains a member of
that type or qualifier. This also applies to structure members that are structures, recursively.
4.1.9 Arrays
Variables of the same type can be aggregated into arrays by declaring a name followed by brackets ( [ ] )
enclosing an optional size. When an array size is specified in a declaration, it must be an integral constant
expression (see section 4.3.3 “Constant Expressions”) greater than zero. Except for the last declared
member of a shader storage block (section 4.3.9 “Interface Blocks”), the size of an array must be declared
before it is indexed with anything other than an integral constant expression. The size of any array must
be declared before passing it as an argument to a function. Violation of any of these rules result in
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4 Variables and Types
compile-time errors. It is legal to declare an array without a size and then later redeclare the same name
as an array of the same type and specify a size. However, unless noted otherwise, blocks cannot be
redeclared; an unsized array in a user-declared block cannot be sized by a block redeclaration. It is a
compile-time error to declare an array with a size, and then later (in the same shader) index the same array
with an integral constant expression greater than or equal to the declared size. It is also a compile-time
error to index an array with a negative constant expression. Arrays declared as formal parameters in a
function declaration must specify a size. Undefined behavior results from indexing an array with a non-
constant expression that’s greater than or equal to the array’s size or less than 0. Arrays only have a
single dimension (a single entry within "[ ]"), however, arrays of arrays can be declared. All types (basic
types, structures, arrays) can be formed into an array.
All arrays are inherently homogeneous; made of elements all having the same type and size, with one
exception. The exception is a shader storage block having an unsized array as its last member; an array
can be formed from such a shader storage block, even if the storage blocks have differing lengths for their
last member.
Some examples are:
float frequencies[3];
uniform vec4 lightPosition[4];
light lights[];
const int numLights = 2;
light lights[numLights];
// a shader storage block, introduced in section 4.3.7 “buffer variables”
buffer b {
float u[]; // an error, unless u gets statically sized by link time
vec4 v[]; // okay, v will be sized dynamically, if not statically
} name[3]; // when the block is arrayed, all u will be the same size,
// but not necessarily all v, if sized dynamically
An array type can be formed by specifying a type followed by square brackets ([ ]) and including a size:
float[5]
This type can be used anywhere any other type can be used, including as the return value from a function
float[5] foo() { }
as a constructor of an array
float[5](3.4, 4.2, 5.0, 5.2, 1.1)
as an unnamed parameter
void foo(float[5])
and as an alternate way of declaring a variable or function parameter.
float[5] a;
Arrays can have initializers formed from array constructors:
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4 Variables and Types
float a[5] = float[5](3.4, 4.2, 5.0, 5.2, 1.1);
float a[5] = float[](3.4, 4.2, 5.0, 5.2, 1.1); // same thing
An array of arrays can be declared as
vec4 a[3][2]; // size-3 array of size-2 array of vec4
which declares a one-dimensional array of size 3 of one-dimensional arrays of size 2 of vec4s. These
following declarations do the same thing:
vec4[2] a[3]; // size-3 array of size-2 array of vec4
vec4[3][2] a; // size-3 array of size-2 array of vec4
When in transparent memory (like in a uniform block), the layout is that the inner-most (right-most in
declaration) dimensions iterate faster than outer dimensions. That is, for the above, the order in memory
would be:
Low address : a[0][0] : a[0][1] : a[1][0] : a[1][1] : a[2][0] : a[2][1] : High address
The type of a needed for both constructors and nameless parameters is “vec4[3][2]”:
vec4 b[2] = vec4[2](vec4(0.0), vec4(0.1));
vec4[3][2] a = vec4[3][2](b, b, b); // constructor
void foo(vec4[3][2]); // prototype with unnamed parameter
Alternatively, the initializer-list syntax can be used to initialize an array of arrays:
vec4 a[3][2] = { vec4[2](vec4(0.0), vec4(1.0)),
vec4[2](vec4(0.0), vec4(1.0)),
vec4[2](vec4(0.0), vec4(1.0)) };
Unsized arrays can be explicitly sized by an initializer at declaration time:
float a[5];
...
float b[] = a; // b is explicitly size 5
float b[5] = a; // means the same thing
float b[] = float[](1,2,3,4,5); // also explicitly sizes to 5
However, it is a compile-time error to assign to an implicitly sized array. Note, this is a rare case that
initializers and assignments appear to have different semantics.
Arrays know the number of elements they contain. This can be obtained by using the length method:
float a[5];
a.length(); // returns 5
This returns a type int. If an array has been explicitly sized, the value returned by the length method is a
constant expression. If an array has not been explicitly sized and is not the last declared member of a
shader storage block, the value returned by the length method is not a constant expression and will be
determined when a program is linked. If an array has not been explicitly sized and is the last declared
member of a shader storage block, the value returned will not be a constant expression and will be
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4 Variables and Types
determined at run time based on the size of the buffer object providing storage for the block. For such
arrays, the value returned by the length method will be undefined if the array is contained in an array of
shader storage blocks that is indexed with a non-constant expression less than zero or greater than or
equal to the number of blocks in the array.
The length method cannot be called on an array that has not yet been explicitly sized; this results in a
compile-time error.
The length method works equally well for arrays of arrays:
vec4 a[3][2];
a.length() // this is 3
a[x].length() // this is 2
When the length method returns a constant, the expression in brackets (x above) will be evaluated and
subjected to the rules required for array indexes, but the array will not be dereferenced. Thus, behavior is
well defined even if the run-time value of the expression is out of bounds.
When the length method returns a run-time value, the array will be dereferenced with the value x. If x is
not a compile-time constant and is out of range, an undefined value results.
// for an array b containing a member array a:
b[++x].a.length(); // b is never dereferenced, but “++x” is evaluated
// for an array s of a shader storage object containing a member array a:
s[x].a.length(); // s is dereferenced; x needs to be a valid index
For unsized arrays, only the outermost dimension can be lacking a size. A type that includes an unknown
array size cannot be formed into an array until it gets an explicit size, except for shader storage blocks
where the only unsized array member is the last member of the block.
In a shader storage block, the last member may be declared without an explicit size. In this case, the
effective array size is inferred at run-time from the size of the data store backing the interface block. Such
unsized arrays may be indexed with general integer expressions. However, it is a compile-time error to
pass them as an argument to a function or index them with a negative constant expression.
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4 Variables and Types
4.1.10 Implicit Conversions
In some situations, an expression and its type will be implicitly converted to a different type. The
following table shows all allowed implicit conversions:
Type of expression Can be implicitly converted to
int uint
int
uint
float
int
uint
float
double
ivec2 uvec2
ivec3 uvec3
ivec4 uvec4
ivec2
uvec2
vec2
ivec3
uvec3
vec3
ivec4
uvec4
vec4
ivec2
uvec2
vec2
dvec2
ivec3
uvec3
vec3
dvec3
ivec4
uvec4
vec4
dvec4
mat2 dmat2
mat3 dmat3
mat4 dmat4
mat2x3 dmat2x3
mat2x4 dmat2x4
mat3x2 dmat3x2
mat3x4 dmat3x4
mat4x2 dmat4x2
mat4x3 dmat4x3
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4 Variables and Types
There are no implicit array or structure conversions. For example, an array of int cannot be implicitly
converted to an array of float.
When an implicit conversion is done, it is not a re-interpretation of the expression's bit pattern, but a
conversion of its value to an equivalent value in the new type. For example, the integer value -5 will be
converted to the floating-point value -5.0. Integer values having more bits of precision than a single-
precision floating-point mantissa will lose precision when converted to float.
When performing implicit conversion for binary operators, there may be multiple data types to which the
two operands can be converted. For example, when adding an int value to a uint value, both values can
be implicitly converted to uint, float, and double. In such cases, a floating-point type is chosen if either
operand has a floating-point type. Otherwise, an unsigned integer type is chosen if either operand has an
unsigned integer type. Otherwise, a signed integer type is chosen. If operands can be implicitly converted
to multiple data types deriving from the same base data type, the type with the smallest component size is
used.
The conversions in the table above are done only as indicated by other sections of this specification.
4.1.11 Initializers
At declaration, an initial value for an aggregate variable may be provided, specified as an equals (=)
followed by an initializer. The initializer is either an assignment-expression or a list of initializers
enclosed in curly braces. The grammar for the initializer is:
initializer :
assignment-expression
{ initializer-list }
{ initializer-list , }
initializer-list :
initializer
initializer-list , initializer
The assignment-expression is a normal expression except that a comma ( , ) outside parentheses is
interpreted as the end of the initializer, not as the sequence operator. As explained in more detail below,
this allows creation of nested initializers: The aggregate and its initializer must exactly match in terms of
nesting, number of components/elements/members present at each level, and types of
components/elements/members.
An assignment-expression in an initializer must be either the same type as the object it initializes or be a
type that can be converted to the object's type according to section 4.1.10 "Implicit Conversions". Since
these include constructors, an aggregate can be initialized by either a constructor or an initializer list; an
element in an initializer list can be a constructor.
If an initializer is a list of initializers enclosed in curly braces, the variable being declared must be a
vector, a matrix, an array, or a structure.
int i = { 1 }; // illegal, i is not an aggregate
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4 Variables and Types
A list of initializers enclosed in a matching set of curly braces is applied to one aggregate. This may be
the variable being declared or an aggregate contained in the variable being declared. Individual
initializers from the initializer list are applied to the elements/members of the aggregate, in order.
If the aggregate has a vector type, initializers from the list are applied to the components of the vector, in
order, starting with component 0. The number of initializers must match the number of components.
If the aggregate has a matrix type, initializers from the list must be vector initializers and are applied to
the columns of the matrix, in order, starting with column 0. The number of initializers must match the
number of columns.
If the aggregate has a structure type, initializers from the list are applied to the members of the structure,
in the order declared in the structure, starting with the first member. The number of initializers must
match the number of members.
Applying these rules, the following matrix declarations are equivalent:
mat2x2 a = mat2( vec2( 1.0, 0.0 ), vec2( 0.0, 1.0 ) );
mat2x2 b = { vec2( 1.0, 0.0 ), vec2( 0.0, 1.0 ) };
mat2x2 c = { { 1.0, 0.0 }, { 0.0, 1.0 } };
All of the following declarations result in a compile-time error.
float a[2] = { 3.4, 4.2, 5.0 }; // illegal
vec2 b = { 1.0, 2.0, 3.0 }; // illegal
mat3x3 c = { vec3(0.0), vec3(1.0), vec3(2.0), vec3(3.0) }; // illegal
mat2x2 d = { 1.0, 0.0, 0.0, 1.0 }; // illegal, can't flatten nesting
struct {
float a;
int b;
} e = { 1.2, 2, 3 }; // illegal
In all cases, the innermost initializer (i.e., not a list of initializers enclosed in curly braces) applied to an
object must have the same type as the object being initialized or be a type that can be converted to the
object's type according to section 4.1.10 "Implicit Conversions". In the latter case, an implicit conversion
will be done on the initializer before the assignment is done.
struct {
float a;
int b;
} e = { 1.2, 2 }; // legal, all types match
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4 Variables and Types
struct {
float a;
int b;
} e = { 1, 3 }; // legal, first initializer is converted
All of the following declarations result in a compile-time error.
int a = true; // illegal
vec4 b[2] = { vec4(0.0), 1.0 }; // illegal
mat4x2 c = { vec3(0.0), vec3(1.0) }; // illegal
struct S1 {
vec4 a;
vec4 b;
};
struct {
float s;
float t;
} d[] = { S1(vec4(0.0), vec4(1.1)) }; // illegal
If an initializer (of either form) is provided for an unsized array, the size of the array is determined by the
number of top-level (non-nested) initializers within the initializer. All of the following declarations create
arrays explicitly sized with five elements:
float a[] = float[](3.4, 4.2, 5.0, 5.2, 1.1);
float b[] = { 3.4, 4.2, 5.0, 5.2, 1.1 };
float c[] = a; // c is explicitly size 5
float d[5] = b; // means the same thing
It is a compile-time error to have too few or too many initializers in an initializer list for the aggregate
being initialized. That is, all elements of an array, all members of a structure, all columns of a matrix, and
all components of a vector must have exactly one initializer expression present, with no unconsumed
initializers.
4.2 Scoping
The scope of a variable is determined by where it is declared. If it is declared outside all function
definitions, it has global scope, which starts from where it is declared and persists to the end of the shader
it is declared in. If it is declared in a while test or a for statement, then it is scoped to the end of the
following sub-statement. If it is declared in an if or else statement, it is scoped to the end of that
statement. (See section 6.2 “Selection” and section 6.3 “Iteration” for the location of statements and sub-
statements.) Otherwise, if it is declared as a statement within a compound statement, it is scoped to the
end of that compound statement. If it is declared as a parameter in a function definition, it is scoped until
the end of that function definition. A function's parameter declarations and body together form a single
scope nested in the global scope. The if statement’s expression does not allow new variables to be
declared, hence does not form a new scope.
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4 Variables and Types
Within a declaration, the scope of a name starts immediately after the initializer if present or immediately
after the name being declared if not. Several examples:
int x = 1;
{
int x = 2, y = x; // y is initialized to 2
}
struct S
{
int x;
};
{
S S = S(0); // 'S' is only visible as a struct and constructor
S; // 'S' is now visible as a variable
}
int x = x; // Error if x has not been previously defined.
// If the previous definition of x was in this
// same scope, this causes a redeclaration error.
int f( /* nested scope begins here */ int k)
{
int k = k + 3; // redeclaration error of the name k
...
}
int f(int k)
{
{
int k = k + 3; // 2nd k is parameter, initializing nested first k
int m = k // use of new k, which is hiding the parameter
}
}
For both for and while loops, the sub-statement itself does not introduce a new scope for variable names,
so the following has a redeclaration compile-time error:
for ( /* nested scope begins here */ int i = 0; i < 10; i++) {
int i; // redeclaration error
}
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4 Variables and Types
The body of a do-while loop introduces a new scope lasting only between the do and while (not including
the while test expression), whether or not the body is simple or compound:
int i = 17;
do
int i = 4; // okay, in nested scope
while (i == 0); // i is 17, scoped outside the do-while body
All variable names, structure type names, and function names in a given scope share the same name space.
Function names can be redeclared in the same scope, with the same or different parameters, without error.
An implicitly sized array can be redeclared in the same scope as an array of the same base type.
Otherwise, within one compilation unit, a declared name cannot be redeclared in the same scope; doing so
results in a redeclaration compile-time error. If a nested scope redeclares a name used in an outer scope,
it hides all existing uses of that name. There is no way to access the hidden name or make it unhidden,
without exiting the scope that hid it.
The built-in functions are scoped in a scope outside the global scope users declare global variables in.
That is, a shader's global scope, available for user-defined functions and global variables, is nested inside
the scope containing the built-in functions. When a function name is redeclared in a nested scope, it hides
all functions declared with that name in the outer scope. Function declarations (prototypes) cannot occur
inside of functions; they must be at global scope, or for the built-in functions, outside the global scope,
otherwise a compile-time error results.
Shared globals are global variables declared with the same name in independently compiled units
(shaders) within the same language (i.e., same stage, e.g., vertex) that are linked together when making a
single program. (Globals forming the interface between two different shader languages are discussed in
other sections.) Shared globals share the same name space, and must be declared with the same type.
They will share the same storage.
Shared global arrays must have the same base type and the same explicit size. An array implicitly sized in
one shader can be explicitly sized by another shader in the same stage. If no shader in a stage has an
explicit size for the array, the largest implicit size (one more than the largest index used) in that stage is
used. There is no cross-stage array sizing. If there is no static access to an implicitly sized array within
the stage declaring it, then the array is given a size of 1, which is relevant when the array is declared
within an interface block that is shared with other stages or the application (other unused arrays might be
eliminated by the optimizer).
Shared global scalars must have exactly the same type name and type definition. Structures must have the
same name, sequence of type names, and type definitions, and member names to be considered the same
type. This rule applies recursively for nested or embedded types. If a shared global has multiple
initializers, the initializers must all be constant expressions, and they must all have the same value.
Otherwise, a link-time error will result. (A shared global having only one initializer does not require that
initializer to be a constant expression.)
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4 Variables and Types
4.3 Storage Qualifiers
Variable declarations may have at most one storage qualifier specified in front of the type. These are
summarized as
Storage Qualifier Meaning
< none: default > local read/write memory, or an input parameter to a function
const a variable whose value cannot be changed
in linkage into a shader from a previous stage, variable is copied in
out linkage out of a shader to a subsequent stage, variable is copied out
attribute compatibility profile only and vertex language only; same as in when in a
vertex shader
uniform value does not change across the primitive being processed, uniforms
form the linkage between a shader, OpenGL, and the application
varying compatibility profile only and vertex and fragment languages only; same
as out when in a vertex shader and same as in when in a fragment shader
buffer value is stored in a buffer object, and can be read or written both by
shader invocations and the OpenGL API
shared compute shader only; variable storage is shared across all work items in a
local work group
Some input and output qualified variables can be qualified with at most one additional auxiliary storage
qualifier:
Auxiliary Storage
Qualifier
Meaning
centroid centroid-based interpolation
sample per-sample interpolation
patch per-tessellation-patch attributes
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4 Variables and Types
Not all combinations of qualification are allowed. Which variable types can have which qualifiers are
specifically defined in upcoming sections.
Local variables can only use the const storage qualifier (or use no storage qualifier).
Function parameters can use const, in, and out qualifiers, but as parameter qualifiers. Parameter
qualifiers are discussed in section 6.1.1 “Function Calling Conventions”.
Function return types and structure members do not use storage qualifiers.
Initializers in global declarations may only be used in declarations of global variables with no storage
qualifier, with a const qualifier or with a uniform qualifier. Global variables without storage qualifiers
that are not initialized in their declaration or by the application will not be initialized by OpenGL, but
rather will enter main() with undefined values.
When comparing an output from one shader stage to an input of a subsequent shader stage, the input and
output don't match if their auxiliary qualifiers (or lack thereof) are not the same.
4.3.1 Default Storage Qualifier
If no qualifier is present on a global variable, then the variable has no linkage to the application or shaders
running on other pipeline stages. For either global or local unqualified variables, the declaration will
appear to allocate memory associated with the processor it targets. This variable will provide read/write
access to this allocated memory.
4.3.2 Constant Qualifier
Named compile-time constants or read-only variables can be declared using the const qualifier. The const
qualifier can be used with any of the non-void transparent basic data types, as well as with structures and
arrays of these. It is a compile-time error to write to a const variable outside of its declaration, so they
must be initialized when declared. For example,
const vec3 zAxis = vec3 (0.0, 0.0, 1.0);
const float ceiling = a + b; // a and b not necessarily constants
Structure members may not be qualified with const. Structure variables can be declared as const, and
initialized with a structure constructor or initializer.
Initializers for const declarations at global scope must be constant expressions, as defined in section 4.3.3
“Constant Expressions.”
4.3.3 Constant Expressions
A constant expression is one of
•a literal value (e.g., 5 or true)
•a variable declared with the const qualifier and an initializer, where the initializer is a constant
expression
•an expression formed by an operator on operands that are all constant expressions, including getting an
element of a constant array, or a member of a constant structure, or components of a constant vector.
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4 Variables and Types
However, the lowest precedence operators of the sequence operator ( , ) and the assignment operators
( =, +=, ...) are not included in the operators that can create a constant expression.
•valid use of the length() method on an explicitly sized object, whether or not the object itself is
constant (implicitly sized or unsized arrays do not return a constant expression)
•a constructor whose arguments are all constant expressions
•the value returned by a built-in function call whose arguments are all constant expressions, with the
exception of the texture lookup functions and the noise functions. This rule excludes functions with a
void return or functions that have an out parameter. The built-in functions dFdx, dFdy, and fwidth
must return 0 when evaluated with an argument that is a constant expression.
Function calls to user-defined functions (non-built-in functions) cannot be used to form constant
expressions.
An integral constant expression is a constant expression that evaluates to a scalar signed or unsigned
integer.
Constant expressions will be always be evaluated in an invariant way, independent of use of invariant
and precise qualification, so as to create the same value in multiple shaders when the same constant
expressions appear in those shaders. See section 4.8.1 “The Invariant Qualifier” and section 4.9 “The
Precise Qualifier” for more details on how to create invariant expressions. Constant expressions may be
evaluated by the compiler's host platform, and are therefore not required to compute the same value that
the same expression would evaluate to on the shader execution target. However, the host must use the
same or greater precision than the target would use.
4.3.4 Input Variables
Shader input variables are declared with the storage qualifier in. They form the input interface between
previous stages of the OpenGL pipeline and the declaring shader. Input variables must be declared at
global scope. Values from the previous pipeline stage are copied into input variables at the beginning of
shader execution. It is a compile-time error to write to a variable declared as an input.
Only the input variables that are statically read need to be written by the previous stage; it is allowed to
have superfluous declarations of input variables. This is shown in the following table.
Treatment of Mismatched Input
Variables
Consuming Shader (input variables)
No Declaration Declared but no
Static Use
Declared and
Static Use
Generating
Shader
(output
variables)
No Declaration Allowed Allowed Link-Time Error
Declared but no
Static Use Allowed Allowed Allowed
(values are undefined)
Declared and
Static Use Allowed Allowed
Allowed
(values are potentially
undefined)
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4 Variables and Types
Consumption errors are based on static use only. Compilation may generate a warning, but not an error,
for any dynamic use the compiler can deduce that might cause consumption of undefined values.
See section 7 “Built-in Variables” for a list of the built-in input names.
Vertex shader input variables (or attributes) receive per-vertex data. They are declared in a vertex shader
with the in qualifier. It is a compile-time error to use any auxiliary or interpolation qualifier on a vertex
shader input. The values copied in are established by the OpenGL API or through the use of the layout
identifier location. It is a compile-time error to declare a vertex shader input containing any of the
following:
•A Boolean type (bool, bvec2, bvec3, bvec4)
•An opaque type
•A structure
Example declarations in a vertex shader:
in vec4 position;
in vec3 normal;
in vec2 texCoord[4];
It is expected that graphics hardware will have a small number of fixed vector locations for passing vertex
inputs. Therefore, the OpenGL Shading language defines each non-matrix input variable as taking up one
such vector location. There is an implementation dependent limit on the number of locations that can be
used, and if this is exceeded it will cause a link-time error. (Declared input variables that are not statically
used do not count against this limit.) A scalar input counts the same amount against this limit as a vec4,
so applications may want to consider packing groups of four unrelated float inputs together into a vector
to better utilize the capabilities of the underlying hardware. A matrix input will use up multiple locations.
The number of locations used will equal the number of columns in the matrix.
Tessellation control, evaluation, and geometry shader input variables get the per-vertex values written out
by output variables of the same names in the previous active shader stage. For these inputs, centroid and
interpolation qualifiers are allowed, but have no effect. Since tessellation control, tessellation evaluation,
and geometry shaders operate on a set of vertices, each input variable (or input block, see interface blocks
below) needs to be declared as an array. For example,
in float foo[]; // geometry shader input for vertex “out float foo”
Each element of such an array corresponds to one vertex of the primitive being processed. Each array can
optionally have a size declared. For geometry shaders, the array size will be set by, (or if provided must
be consistent with) the input layout declaration(s) establishing the type of input primitive, as described
later in section 4.4.1 “Input Layout Qualifiers”.
Some inputs and outputs are arrayed, meaning that for an interface between two shader stages either the
input or output declaration requires an extra level of array indexing for the declarations to match. For
example, with the interface between a vertex shader and a geometry shader, vertex shader output variables
and geometry shader input variables of the same name must have matching types, except that the
geometry shader will have one more array dimension than the vertex shader, to allow for vertex indexing.
If such an arrayed interface variable is not declared with the necessary additional input or output array
dimension, a link-time error will result. Geometry shader inputs, tessellation control shader inputs and
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4 Variables and Types
outputs, and tessellation evaluation inputs all have an additional level of arrayness relative to other shader
inputs and outputs.
For non-arrayed interfaces (meaning array dimensionally stays the same between stages), it is a link-time
error if the input variable is not declared with the same type, including array dimensionality, as the
matching output variable.
The link-time type-matching rules apply to all declared input and output variables, whether or not they are
used.
Additionally, tessellation evaluation shaders support per-patch input variables declared with the patch and
in qualifiers. Per-patch input variables are filled with the values of per-patch output variables written by
the tessellation control shader. Per-patch inputs may be declared as one-dimensional arrays, but are not
indexed by vertex number. Applying the patch qualifier to inputs can only be done in tessellation
evaluation shaders. As with other input variables, per-patch inputs must be declared using the same type
and qualification as per-patch outputs from the previous (tessellation control) shader stage.
Fragment shader inputs get per-fragment values, typically interpolated from a previous stage's outputs.
They are declared in fragment shaders with the in storage qualifier. The auxiliary storage qualifiers
centroid and sample can also be applied, as well as the interpolation qualifiers flat, noperspective, and
smooth. It is a compile-time error to use patch in a fragment shader. It is a compile-time error to
declare a fragment shader input containing any of the following:
•A Boolean type (bool, bvec2, bvec3, bvec4)
•An opaque type
Fragment shader inputs that are signed or unsigned integers, integer vectors, or any double-precision
floating-point type must be qualified with the interpolation qualifier flat.
Fragment inputs are declared as in the following examples:
in vec3 normal;
centroid in vec2 TexCoord;
invariant centroid in vec4 Color;
noperspective in float temperature;
flat in vec3 myColor;
noperspective centroid in vec2 myTexCoord;
The fragment shader inputs form an interface with the last active shader in the vertex processing pipeline.
For this interface, the last active shader stage output variables and fragment shader input variables of the
same name must match in type and qualification, with a few exceptions: The storage qualifiers must, of
course, differ (one is in and one is out). Also, interpolation qualification (e.g., flat) and auxiliary
qualification (e.g. centroid) may differ. These mismatches are allowed between any pair of stages. When
interpolation or auxiliary qualifiers do not match, those provided in the fragment shader supersede those
provided in previous stages. If any such qualifiers are completely missing in the fragment shaders, then
the default is used, rather than any qualifiers that may have been declared in previous stages. That is,
what matters is what is declared in the fragment shaders, not what is declared in shaders in previous
stages.
When an interface between shader stages is formed using shaders from two separate program objects, it is
not possible to detect mismatches between inputs and outputs when the programs are linked. When there
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4 Variables and Types
are mismatches between inputs and outputs on such interfaces, the values passed across the interface will
be partially or completely undefined. Shaders can ensure matches across such interfaces either by using
input and output layout qualifiers (sections 4.4.1 “Input Layout Qualifiers” and 4.4.2 “Output Layout
Qualifiers”) or by using identical input and output declarations of blocks or variables. Complete rules for
interface matching between programs are found in section 7.4.1 “Shader Interface Matching” of the
OpenGL Graphics System Specification.
Compute shaders do not permit user-defined input variables and do not form a formal interface with any
other shader stage. See section 7.1 “Built-In Variables” for a description of built-in compute shader input
variables. All other input to a compute shader is retrieved explicitly through image loads, texture fetches,
loads from uniforms or uniform buffers, or other user supplied code. Redeclaration of built-in input
variables in compute shaders is not permitted.
4.3.5 Uniform Variables
The uniform qualifier is used to declare global variables whose values are the same across the entire
primitive being processed. All uniform variables are read-only and are initialized externally either at link
time or through the API. The link-time initial value is either the value of the variable's initializer, if
present, or 0 if no initializer is present. Opaque types cannot have initializers, or a compile-time error
results.
Example declarations are:
uniform vec4 lightPosition;
uniform vec3 color = vec3(0.7, 0.7, 0.2); // value assigned at link time
The uniform qualifier can be used with any of the basic data types, or when declaring a variable whose
type is a structure, or an array of any of these.
There is an implementation dependent limit on the amount of storage for uniforms that can be used for
each type of shader and if this is exceeded it will cause a compile-time or link-time error. Uniform
variables that are declared but not used do not count against this limit. The number of user-defined
uniform variables and the number of built-in uniform variables that are used within a shader are added
together to determine whether available uniform storage has been exceeded.
If multiple shaders are linked together, then they will share a single global uniform name space, including
within a language as well as across languages. Hence, the types and initializers of all declared uniform
variables with the same name must match across all shaders that are linked into a single program. While
this single uniform name space is cross stage, a uniform variable name's scope is per stage: If a uniform
variable name is declared in one stage (e.g., a vertex shader) but not in another (e.g., a fragment shader),
then that name is still available in the other stage for a different use.
It is legal for some shaders to provide an initializer for a particular uniform variable, while another shader
does not, but all provided initializers must be equal. Similarly, when a layout location is used, it is not
required that all declarations of that name include the location; only that those that include a location use
the same location.
4.3.6 Output Variables
Shader output variables are declared with a storage qualifier using the keyword out. They form the output
interface between the declaring shader and the subsequent stages of the OpenGL pipeline. Output
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4 Variables and Types
variables must be declared at global scope. During shader execution they will behave as normal
unqualified global variables. Their values are copied out to the subsequent pipeline stage on shader exit.
Only output variables that are read by the subsequent pipeline stage need to be written; it is allowed to
have superfluous declarations of output variables.
There is not an inout storage qualifier at global scope for declaring a single variable name as both input
and output to a shader. Also, a variable cannot be declared with both the in and the out qualifiers, this
will result in a compile-time or link-time error. Output variables must be declared with different names
than input variables. However, nesting an input or output inside an interface block with an instance name
allows the same names with one referenced through a block instance name.
Vertex, tessellation evaluation, and geometry output variables output per-vertex data and are declared
using the out storage qualifier. Applying patch to an output can only be done in a tessellation control
shader.
It is a compile-time error to declare a vertex, tessellation evaluation, tessellation control, or geometry
shader output that contains any of the following:
•A Boolean type (bool, bvec2, bvec3, bvec4)
•An opaque type
Individual vertex, tessellation evaluation, and geometry outputs are declared as in the following examples:
out vec3 normal;
centroid out vec2 TexCoord;
invariant centroid out vec4 Color;
noperspective out float temperature;
flat out vec3 myColor;
noperspective centroid out vec2 myTexCoord;
sample out vec4 perSampleColor;
These can also appear in interface blocks, as described in section 4.3.9 “Interface Blocks”. Interface
blocks allow simpler addition of arrays to the interface from vertex to geometry shader. They also allow a
fragment shader to have the same input interface as a geometry shader for a given vertex shader.
Tessellation control shader output variables are may be used to output per-vertex and per-patch data. Per-
vertex output variables are arrayed (see arrayed under 4.3.4 Inputs) and declared using the out qualifier
without the patch qualifier. Per-patch output variables are declared using the patch and out qualifiers.
Since tessellation control shaders produce an arrayed primitive comprising multiple vertices, each per-
vertex output variable (or output block, see interface blocks below) needs to be declared as an array. For
example,
out float foo[]; // feeds next stage input “in float foo[]”
Each element of such an array corresponds to one vertex of the primitive being produced. Each array can
optionally have a size declared. The array size will be set by (or if provided must be consistent with) the
output layout declaration(s) establishing the number of vertices in the output patch, as described later in
section 4.4.2.1 “Tessellation Control Outputs”.
Each tessellation control shader invocation has a corresponding output patch vertex, and may assign
values to per-vertex outputs only if they belong to that corresponding vertex. If a per-vertex output
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4 Variables and Types
variable is used as an l-value, it is a compile-time error if the expression indicating the vertex index is not
the identifier gl_InvocationID.
The order of execution of a tessellation control shader invocation relative to the other invocations for the
same input patch is undefined unless the built-in function barrier() is used. This provides some control
over relative execution order. When a shader invocation calls barrier(), its execution pauses until all
other invocations have reached the same point of execution. Output variable assignments performed by
any invocation executed prior to calling barrier() will be visible to any other invocation after the call to
barrier() returns.
Because tessellation control shader invocations execute in undefined order between barriers, the values of
per-vertex or per-patch output variables will sometimes be undefined. Consider the beginning and end of
shader execution and each call to barrier() as synchronization points. The value of an output variable
will be undefined in any of the three following cases:
1. At the beginning of execution.
2. At each synchronization point, unless
•the value was well-defined after the previous synchronization point and was not written by any
invocation since, or
•the value was written by exactly one shader invocation since the previous synchronization
point, or
•the value was written by multiple shader invocations since the previous synchronization point,
and the last write performed by all such invocations wrote the same value.
3. When read by a shader invocation, if
•the value was undefined at the previous synchronization point and has not been writen by the
same shader invocation since, or
•the output variable is written to by any other shader invocation between the previous and next
synchronization points, even if that assignment occurs in code following the read.
Fragment outputs output per-fragment data and are declared using the out storage qualifier. It is a
compile-time error to use auxiliary storage qualifiers or interpolation qualifiers on an output in a fragment
shader. It is a compile-time error to declare a fragment shader output that contains any of the following:
•A Boolean type (bool, bvec2, bvec3, bvec4)
•A double-precision scalar or vector (double, dvec2, dvec3, dvec4)
•An opaque type
•Any matrix type
•A structure
Fragment outputs are declared as in the following examples:
out vec4 FragmentColor;
out uint Luminosity;
Compute shaders have no built-in output variables, do not support user-defined output variables and do
not form a formal interface with any other shader stage. All outputs from a compute shader take the form
of the side effects such as image stores and operations on atomic counters.
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4 Variables and Types
4.3.7 Buffer Variables
The buffer qualifier is used to declare global variables whose values are stored in the data store of a
buffer object bound through the OpenGL API. Buffer variables can be read and written with the
underlying storage shared among all active shader invocations. Buffer variable memory reads and writes
within a single shader invocation are processed in order. However, the order of reads and writes
performed in one invocation relative to those performed by another invocation is largely undefined.
Buffer variables may be qualified with memory qualifiers affecting how the underlying memory is
accessed, as described in section 4.10 “Memory Qualifiers”.
The buffer qualifier can be used with any of the basic data types, or when declaring a variable whose type
is a structure, or an array of any of these.
Buffer variables may only be declared inside interface blocks (section 4.3.9 “Interface Blocks”), which
are then referred to as shader storage blocks. It is a compile-time error to declare buffer variables at
global scope (outside a block). Buffer variables cannot have initializers.
// use buffer to create a buffer block (shader storage block)
buffer BufferName { // externally visible name of buffer
int count; // typed, shared memory...
... // ...
vec4 v[]; // last element may be an array that is not sized
// until after link time (dynamically sized)
} Name; // name of block within the shader
There are implementation-dependent limits on the number of shader storage blocks used for each type of
shader, the combined number of shader storage blocks used for a program, and the amount of storage
required by each individual shader storage block. If any of these limits are exceeded, it will cause a
compile-time or link-time error.
If multiple shaders are linked together, then they will share a single global buffer variable name space,
including within a language as well as across languages. Hence, the types of all declared buffer variables
with the same name must match across all shaders that are linked into a single program.
4.3.8 Shared Variables
The shared qualifier is used to declare variables that have storage shared between all work items
compute shader local work group. Variables declared as shared may only be used in compute shaders
(see section 2.6 “Compute Processor”). Shared variables are implicitly coherent. That is, writes to shared
variables from one shader invocation will eventually be seen by other invocations within the same local
work group.
Variables declared as shared may not have initializers and their contents are undefined at the beginning
of shader execution. Any data written to shared variables will be visible to other shaders executing the
same shader within the same local work group. Order of execution with respect to reads and writes to the
same shared variable by different invocations of a shader is not defined. In order to achieve ordering with
respect to reads and writes to shared variables, memory barriers must be employed using the barrier()
function (see section 8.16 “Shader Invocation Control Functions”).
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4 Variables and Types
There is a limit to the total size of all variables declared as shared in a single program. This limit,
expressed in units of basic machine units may be determined by using the OpenGL API to query the value
of MAX_COMPUTE_SHARED_MEMORY_SIZE.
4.3.9 Interface Blocks
Input, output, uniform, and buffer variable declarations can be grouped into named interface blocks to
provide coarser granularity backing than is achievable with individual declarations. They can have an
optional instance name, used in the shader to reference their members. An output block of one
programmable stage is backed by a corresponding input block in the subsequent programmable stage. A
uniform block is backed by the application with a buffer object. A block of buffer variables, called a
shader storage block, is also backed by the application with a buffer object. It is a compile-time error to
have an input block in a vertex shader or an output block in a fragment shader; these uses are reserved for
future use.
An interface block is started by an in, out, uniform, or buffer keyword, followed by a block name,
followed by an open curly brace ( { ) as follows:
interface-block :
layout-qualifieropt interface-qualifier block-name { member-list } instance-nameopt ;
interface-qualifier :
in
out
uniform
buffer
member-list :
member-declaration
member-declaration member-list
member-declaration :
layout-qualifieropt qualifiersopt type declarators ;
instance-name :
identifier
identifier [ ]
identifier [ integral-constant-expression ]
Each of the above elements is discussed below, with the exception of layout qualifiers (layout-qualifier),
which are defined in the next section.
First, an example,
uniform Transform {
mat4 ModelViewMatrix;
mat4 ModelViewProjectionMatrix;
uniform mat3 NormalMatrix; // allowed restatement of qualifier
float Deformation;
};
The above establishes a uniform block named “Transform” with four uniforms grouped inside it.
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4 Variables and Types
Types and declarators are the same as for other input, output, uniform, and buffer variable declarations
outside blocks, with these exceptions:
•initializers are not allowed
•opaque types are not allowed
•structure definitions cannot be nested inside a block
Any of these would result in a compile-time error. Otherwise, built-in types, previously declared
structures, and arrays of these are allowed as the type of a declarator in the same manner they are allowed
outside a block.
If no optional qualifier is used in a member-declaration, the qualification of the variable is just in, out,
uniform, or buffer as determined by interface-qualifier. If optional qualifiers are used, they can include
interpolation qualifiers, auxiliary storage qualifiers, and storage qualifiers and they must declare an input,
output, or uniform variable consistent with the interface qualifier of the block: Input variables, output
variables, uniform variables, and buffer variables can only be in in blocks, out blocks, uniform blocks,
and shader storage blocks, respectively. Repeating the in, out, uniform, or buffer interface qualifier for
a member's storage qualifier is optional. For example,
in Material {
smooth in vec4 Color1; // legal, input inside in block
smooth vec4 Color2; // legal, 'in' inherited from 'in Material'
vec2 TexCoord; // legal, TexCoord is an input
uniform float Atten; // illegal, mismatched storage qualifier
};
For this section, define an interface to be one of these
•All the uniform variables and uniform blocks declared in a program. This spans all compilation units
linked together within one program.
•All the buffer blocks declared in a program.
•The boundary between adjacent programmable pipeline stages: This spans all the outputs declared in
all compilation units of the first stage and all the inputs declared in all compilation units of the second
stage.
The block name (block-name) is used to match interfaces: an output block of one pipeline stage will be
matched to an input block with the same name in the subsequent pipeline stage. For uniform blocks, the
application uses the block name to identify the block. Block names have no other use within a shader
beyond interface matching; it is a compile-time error to use a block name at global scope for anything
other than as a block name (e.g., use of a block name for a global variable name or function name is
currently reserved). It is a compile-time error to use the same block name for more than one block
declaration in the same interface (as defined above) within one shader, even if the block contents are
identical.
Matched block names within an interface (as defined above) must match in terms of having the same
number of declarations with the same sequence of types and the same sequence of member names, as well
as having the same member-wise layout qualification (see next section). Matched uniform block names
(but not input or output block names) must also either all be lacking an instance name or all having an
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4 Variables and Types
instance name, putting their members at the same scoping level. When instance names are present on
matched block names, it is allowed for the instance names to differ; they need not match for the blocks to
match. Furthermore, if a matching block is declared as an array, then the array sizes must also match (or
follow array matching rules for the interface between a vertex and a geometry shader). Any mismatch
will generate a link-time error. A block name is allowed to have different definitions in different
interfaces within the same shader, allowing, for example, an input block and output block to have the
same name.
If an instance name (instance-name) is not used, the names declared inside the block are scoped at the
global level and accessed as if they were declared outside the block. If an instance name (instance-name)
is used, then it puts all the members inside a scope within its own name space, accessed with the field
selector ( . ) operator (analogously to structures). For example,
in Light {
vec4 LightPos;
vec3 LightColor;
};
in ColoredTexture {
vec4 Color;
vec2 TexCoord;
} Material; // instance name
vec3 Color; // different Color than Material.Color
vec4 LightPos; // illegal, already defined
...
... = LightPos; // accessing LightPos
... = Material.Color; // accessing Color in ColoredTexture block
Outside the shading language (i.e., in the API), members are similarly identified except the block name is
always used in place of the instance name (API accesses are to interfaces, not to shaders). If there is no
instance name, then the API does not use the block name to access a member, just the member name.
Within an interface, all declarations of the same global name must be for the same object and must match
in type and in whether they declare a variable or member of a block with no instance name. The API also
needs this name to uniquely identify an object in the interface. It is a link-time error if any particular
interface contains
•two different blocks, each having no instance name, and each having a member of the same
name, or
•a variable outside a block, and a block with no instance name, where the variable has the same
name as a member in the block.
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4 Variables and Types
out Vertex {
vec4 Position; // API transform/feedback will use “Vertex.Position”
vec2 Texture;
} Coords; // shader will use “Coords.Position”
out Vertex2 {
vec4 Color; // API will use “Color”
float Color2;
};
// in same program as Vertex2 above:
out Vertex3 {
float Intensity;
vec4 Color; // ERROR, name collision with Color in Vertex2
};
float Color2; // ERROR, collides with Color2 in Vertex2
For blocks declared as arrays, the array index must also be included when accessing members, as in this
example
uniform Transform { // API uses “Transform[2]” to refer to instance 2
mat4 ModelViewMatrix;
mat4 ModelViewProjectionMatrix;
vec4 a[]; // array will get implicitly sized
float Deformation;
} transforms[4];
...
... = transforms[2].ModelViewMatrix; // shader access of instance 2
// API uses “Transform.ModelViewMatrix” to query an offset or other query
transforms[x].a.length(); // same length for 'a' for all x
Transform[x]; // illegal, must use 'transforms'
Transform.a.length(); // illegal, must use 'transforms'
...transforms[2].a[3]... // if these are the only two dereferences of 'a',
...transforms[3].a[7]... // then 'a' must be size 8, for all transforms[x]
For uniform or shader storage blocks declared as an array, each individual array element corresponds to a
separate buffer-object bind range, backing one instance of the block. As the array size indicates the
number of buffer objects needed, uniform and shader storage block array declarations must specify an
array size. A uniform or shader storage block array can only be indexed with a dynamically uniform
integral expression, otherwise results are undefined.
When using OpenGL API entry points to identify the name of an individual block in an array of blocks,
the name string must include an array index (e.g., Transform[2]). When using OpenGL API entry points
to refer to offsets or other characteristics of a block member, an array index must not be specified (e.g.,
Transform.ModelViewMatrix).
Geometry shader input blocks must be declared as arrays and follow the array declaration and linking
rules for all geometry shader inputs. All other input and output block arrays must specify an array size.
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There are implementation dependent limits on the number of uniform blocks and the number of shader
storage blocks that can be used per stage. If either limit is exceeded, it will cause a link-time error.
4.4 Layout Qualifiers
Layout qualifiers can appear in several forms of declaration. They can appear as part of an interface
block definition or block member, as shown in the grammar in the previous section. They can also appear
with just an interface qualifier (a storage qualifier that is in, out, or uniform) to establish layouts of other
declarations made with that interface qualifier:
layout-qualifier interface-qualifier ;
Or, they can appear with an individual variable declared with an interface qualifier:
layout-qualifier interface-qualifier declaration ;
Declarations of layouts can only be made at global scope, and only where indicated in the following
subsections; their details are specific to what the interface qualifier is, and are discussed individually.
The layout-qualifier expands to
layout-qualifier :
layout ( layout-qualifier-id-list )
layout-qualifier-id-list :
layout-qualifier-id
layout-qualifier-id , layout-qualifier-id-list
layout-qualifier-id
layout-qualifier-name
layout-qualifier-name = layout-qualifier-value
shared
The tokens used for layout-qualifier-name are identifiers, not keywords, however, the shared keyword is
allowed as a layout-qualifier-id. Generally, they can be listed in any order. Order-dependent meanings
exist only if explicitly called out below. Similarly, these identifiers are not case sensitive, unless
explicitly noted otherwise.
More than one layout qualifier may appear in a single declaration. Additionally, the same layout-
qualifier-name can occur multiple times within a layout qualifier or across multiple layout qualifiers in the
same declaration. When the same layout-qualifier-name occurs multiple times, in a single declaration, the
last occurrence overrides the former occurrence(s). Further, if such a layout-qualifier-name will effect
subsequent declarations or other observable behavior, it is only the last occurrence that will have any
effect, behaving as if the earlier occurrence(s) within the declaration are not present. This is also true for
overriding layout-qualifier-names, where one overrides the other (e.g., row_major vs. column_major);
only the last occurrence has any effect.
The following table summarizes the use of layout qualifiers applied to non-opaque types. It shows for
each one what kinds of declarations it may be applied to. These are all discussed in detail in the following
sections. Layout qualifiers applied to opaque types are not show in this table, but are discussed
subsequently in section 4.4.6 “Opaque-Uniform Layout Qualifiers”.
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4 Variables and Types
Layout Qualifier Qualifier
Only
Individual
Variable Block
Block
Member Allowed Interfaces
shared
packed
std140
std430
X X
uniform/buffer
row_major
column_major X X X
binding = opaque types
only X
offset = X
align = X X
location = Xuniform/buffer and
subroutine variables
location = X X X all in/out, except for
compute
component = X X
index = Xfragment out
and subroutine functions
triangles
quads
isolines
X tessellation evaluation in
equal_spacing
fractional_even_spacing
fractional_odd_spacing
X tessellation evaluation in
cw
ccw X tessellation evaluation in
point_mode X tessellation evaluation in
points X geometry in/out
[ points ]
lines
lines_adjacency
triangles
triangles_adjacency
X geometry in
invocations = X geometry in
origin_upper_left
pixel_center_integer
gl_FragCoord
only fragment in
early_fragment_tests X
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4 Variables and Types
Layout Qualifier Qualifier
Only
Individual
Variable Block
Block
Member Allowed Interfaces
local_size_x =
local_size_y =
local_size_z = Xcompute in
xfb_buffer =
xfb_stride = X X X X vertex, tessellation, and
geometry out
xfb_offset = X X X
vertices = X tessellation control out
[ points ]
line_strip
triangle_strip X
geometry out
max_vertices = X
stream = X X X X
depth_any
depth_greater
depth_less
depth_unchanged
gl_FragDepth
only fragment out
4.4.1 Input Layout Qualifiers
Some input layout qualifiers apply to all shader languages and some apply only to specific languages.
The latter are discussed in separate sections below.
All shaders, except compute shaders, allow location layout qualifiers on input variable declarations, input
block declarations, and input block member declarations. Of these, variables and block members (but not
blocks) additionally allow the component layout qualifier.
The layout qualifier identifiers for inputs are:
layout-qualifier-id
location = integer-constant-expression
component = integer-constant-expression
Where integral-constant-expression is defined in section 4.3.3 “Constant Expressions” as “integral
constant expression”.
For example,
layout(location = 3) in vec4 normal;
const int start = 6;
layout(location = start + 2) int vec4 v;
will establish that the shader input normal is assigned to vector location number 3 and v is assigned
location number 8. For vertex shader inputs, the location specifies the number of the generic vertex
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4 Variables and Types
attribute from which input values are taken. For inputs of all other shader types, the location specifies a
vector number that can be used to match against outputs from a previous shader stage, even if that shader
is in a different program object.
The following language describes how many locations are consumed by a given type. However, geometry
shader inputs, tessellation control shader inputs and outputs, and tessellation evaluation inputs all have an
additional level of arrayness relative to other shader inputs and outputs. This outer array level is removed
from the type before considering how many locations the type consumes.
If a vertex shader input is any scalar or vector type, it will consume a single location. If a non-vertex
shader input is a scalar or vector type other than dvec3 or dvec4, it will consume a single location, while
types dvec3 or dvec4 will consume two consecutive locations. Inputs of type double and dvec2 will
consume only a single location, in all stages.
If the declared input (after potentially removing an outer array level as just described above) is an array of
size n and each element takes m locations, it will be assigned m * n consecutive locations starting with the
location specified. For example,
layout(location = 6) in vec4 colors[3];
will establish that the shader input colors is assigned to vector location numbers 6, 7, and 8.
If the declared input is an n x m single- or double-precision matrix, it will be assigned multiple locations
starting with the location specified. The number of locations assigned for each matrix will be the same as
for an n-element array of m-component vectors. For example,
layout(location = 9) in mat4 transforms[2];
will establish that shader input transforms is assigned to vector locations 9-16, with transforms[0] being
assigned to locations 9-12 and transforms[1] being assigned to locations 13-16.
If the declared input is a structure or block, its members will be assigned consecutive locations in their
order of declaration, with the first member assigned the location provided in the layout qualifier. For a
structure, this process applies to the entire structure. It is a compile-time error to use a location qualifier
on a member of a structure. For a block, this process applies to the entire block, or until the first member
is reached that has a location layout qualifier. When a block member is declared with a location
qualifier, its location comes from that qualifier: The member's location qualifier overrides the block-level
declaration. Subsequent members are again assigned consecutive locations, based on the newest location,
until the next member declared with a location qualifier. The values used for locations do not have to be
declared in increasing order.
If a block has no block-level location layout qualifier, it is required that either all or none of its members
have a location layout qualifier, or a compile-time error results.
The locations consumed by block and structure members are determined by applying the rules above
recursively as though the structure member were declared as an input variable of the same type. For
example:
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4 Variables and Types
layout(location = 3) in struct S {
vec3 a; // gets location 3
mat2 b; // gets locations 4 and 5
vec4 c[2]; // gets locations 6 and 7
layout (location = 8) vec2 A; // ERROR, can't use on struct member
} s;
layout(location = 4) in block {
vec4 d; // gets location 4
vec4 e; // gets location 5
layout(location = 7) vec4 f; // gets location 7
vec4 g; // gets location 8
layout (location = 1) vec4 h; // gets location 1
vec4 i; // gets location 2
vec4 j; // gets location 3
vec4 k; // ERROR, location 4 already used
};
The number of input locations available to a shader is limited. For vertex shaders, the limit is the
advertised number of vertex attributes. For all other shaders, the limit is implementation-dependent and
must be no less than one fourth of the advertised maximum input component count. A program will fail to
link if any attached shader uses a location greater than or equal to the number of supported locations,
unless device-dependent optimizations are able to make the program fit within available hardware
resources.
A program will fail to link if explicit location assignments leave the linker unable to find space for other
variables without explicit assignments.
For the purposes of determining if a non-vertex input matches an output from a previous shader stage, the
location layout qualifier (if any) must match.
If a vertex shader input variable with no location assigned in the shader text has a location specified
through the OpenGL API, the API-assigned location will be used. Otherwise, such variables will be
assigned a location by the linker. See section 11.1.1 “Vertex Attributes” of the OpenGL Graphics System
Specification for more details. A link-time error will occur if an input variable is declared in multiple
shaders of the same language with conflicting locations.
The component qualifier allows the location to be more finely specified for scalars and vectors, down to
the individual components within a location that are consumed. It is a compile-time error to use
component without also specifying the location qualifier (order does not matter). The components
within a location are 0, 1, 2, and 3. A variable or block member starting at component N will consume
components N, N+1, N+2, ... up through its size. It is a compile-time error if this sequence of components
gets larger than 3. For example:
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4 Variables and Types
// a consumes components 2 and 3 of location 4
layout(location = 4, component = 2) in vec2 a;
// b consumes component 1 of location 4
layout(location = 4, component = 1) in float b;
// ERROR: c overflows components 2 and 3
layout(location = 3, component = 2) in vec3 c;
If the variable is an array, each element of the array, in order, is assigned to consecutive locations, but all
at the same specified component within each location. For example:
// component 3 in 6 locations are consumed
layout(location = 2, component = 3) in float d[6];
That is, location 2 component 3 will hold d[0], location 3 component 3 will hold d[1], …, up through
location 7 component 3 holding d[5].
This allows packing of two arrays into the same set of locations:
// e consumes beginning (components 0, 1 and 2) of each of 6 slots
layout(location = 0, component = 0) in vec3 e[6];
// f consumes last component of the same 6 slots
layout(location = 0, component = 3) in float f[6];
If applying this to an array of arrays, all levels of arrayness are removed to get to the elements that are
assigned per location to the specified component. These non-arrayed elements will fill the locations in the
order specified for arrays of arrays in section 4.1.9 "Arrays".
It is a compile-time error to apply the component qualifier to a matrix, a structure, a block, or an array
containing any of these. It is a link-time error to specify different components for the same variable
within a program.
Location aliasing is causing two variables or block members to have the same location number.
Component aliasing is assigning the same (or overlapping) component numbers for two location aliases.
(Recall if component is not used, component's are assigned starting with 0.) With one exception, location
aliasing is allowed only if it does not cause component aliasing; it is a compile-time or link-time error to
cause component aliasing. Further, when location aliasing, the aliases sharing the location must have the
same underlying numerical type (floating-point or integer) and the same auxiliary storage and
interpolation qualification. The one exception where component aliasing is permitted is for two input
variables (not block members) to a vertex shader, which are allowed to have component aliasing. This
vertex-variable component aliasing is intended only to support vertex shaders where each execution path
accesses at most one input per each aliased component. Implementations are permitted, but not required,
to generate link-time errors if they detect that every path through the vertex shader executable accesses
multiple inputs aliased to any single component.
4.4.1.1 Tessellation Evaluation Inputs
Additional input layout qualifier identifiers allowed for tessellation evaluation shaders are:
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4 Variables and Types
layout-qualifier-id
triangles
quads
isolines
equal_spacing
fractional_even_spacing
fractional_odd_spacing
cw
ccw
point_mode
One subset of these identifiers, primitive mode, is used to specify a tessellation primitive mode to be used
by the tessellation primitive generator. To specify a primitive mode, the identifier must be one of
triangles, quads, or isolines, which specify that the tessellation primitive generator should subdivide a
triangle into smaller triangles, a quad into triangles, or a quad into a collection of lines, respectively.
A second subset of these identifiers, vertex spacing, is used to specify the spacing used by the tessellation
primitive generator when subdividing an edge. To specify vertex spacing, the identifier must be one of
the following.
equal_spacing signifying that edges should be divided into a collection of equal-sized segments.
fractional_even_spacing signifying that edges should be divided into an even number of equal-
length segments plus two additional shorter "fractional" segments.
fractional_odd_spacing signifying that edges should be divided into an odd number of equal-
length segments plus two additional shorter "fractional" segments.
A third subset of these identifiers, ordering, specifies whether the tessellation primitive generator
produces triangles in clockwise or counter-clockwise order, according to the coordinate system depicted
in the OpenGL specification. The ordering identifiers cw and ccw indicate clockwise and counter-
clockwise triangles, respectively. If the tessellation primitive generator does not produce triangles,
ordering is ignored.
Finally, point mode, is specified with the identifier point_mode indicating the tessellation primitive
generator should produce a point for each unique vertex in the subdivided primitive, rather than
generating lines or triangles.
Any or all of these identifiers may be specified one or more times in a single input layout declaration. If
primitive mode, vertex spacing, or ordering is declared more than once in the tessellation evaluation
shaders of a program, all such declarations must use the same identifier.
At least one tessellation evaluation shader (compilation unit) in a program must declare a primitive mode
in its input layout. Declaring vertex spacing, ordering, or point mode identifiers is optional. It is not
required that all tessellation evaluation shaders in a program declare a primitive mode. If spacing or
vertex ordering declarations are omitted, the tessellation primitive generator will use equal spacing or
counter-clockwise vertex ordering, respectively. If a point mode declaration is omitted, the tessellation
primitive generator will produce lines or triangles according to the primitive mode.
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4 Variables and Types
4.4.1.2 Geometry Shader Inputs
Additional layout qualifier identifiers for geometry shader inputs include primitive identifiers and an
invocation count identifier:
layout-qualifier-id
points
lines
lines_adjacency
triangles
triangles_adjacency
invocations = integer-constant-expression
The identifiers points, lines, lines_adjacency, triangles, and triangles_adjacency are used to specify the
type of input primitive accepted by the geometry shader, and only one of these is accepted. At least one
geometry shader (compilation unit) in a program must declare this input primitive layout, and all geometry
shader input layout declarations in a program must declare the same layout. It is not required that all
geometry shaders in a program declare an input primitive layout.
The identifier invocations is used to specify the number of times the geometry shader executable is
invoked for each input primitive received. Invocation count declarations are optional. If no invocation
count is declared in any geometry shader in a program, the geometry shader will be run once for each
input primitive. If an invocation count is declared, all such declarations must specify the same count. If a
shader specifies an invocation count greater than the implementation-dependent maximum, it will fail to
compile.
For example,
layout(triangles, invocations = 6) in;
will establish that all inputs to the geometry shader are triangles and that the geometry shader executable
is run six times for each triangle processed.
All geometry shader input unsized array declarations will be sized by an earlier input primitive layout
qualifier, when present, as per the following table.
Layout Size of Input Arrays
points 1
lines 2
lines_adjacency 4
triangles 3
triangles_adjacency 6
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4 Variables and Types
The intrinsically declared input array gl_in[] will also be sized by any input primitive-layout declaration.
Hence, the expression
gl_in.length()
will return the value from the table above.
For inputs declared without an array size, including intrinsically declared inputs (i.e., gl_in), a layout must
be declared before any use of the method length or other any array use that requires the array size to be
known.
It is a compile-time error if a layout declaration's array size (from table above) does not match all the
explicit array sizes specified in declarations of an input variables in the same shader. The following
includes examples of compile-time errors:
// code sequence within one shader...
in vec4 Color1[]; // legal, size still unknown
in vec4 Color2[2]; // legal, size is 2
in vec4 Color3[3]; // illegal, input sizes are inconsistent
layout(lines) in; // legal for Color2, input size is 2, matching Color2
in vec4 Color4[3]; // illegal, contradicts layout of lines
layout(lines) in; // legal, matches other layout() declaration
layout(triangles) in;// illegal, does not match earlier layout() declaration
It is a link-time error if not all provided sizes (sized input arrays and layout size) match across all
geometry shaders in a program.
4.4.1.3 Fragment Shader Inputs
Additional fragment layout qualifier identifiers include the following for gl_FragCoord
layout-qualifier-id
origin_upper_left
pixel_center_integer
By default, gl_FragCoord assumes a lower-left origin for window coordinates and assumes pixel centers
are located at half-pixel coordinates. For example, the (x, y) location (0.5, 0.5) is returned for the lower-
left-most pixel in a window. The origin can be changed by redeclaring gl_FragCoord with the
origin_upper_left identifier, moving the origin of gl_FragCoord to the upper left of the window, with y
increasing in value toward the bottom of the window. The values returned can also be shifted by half a
pixel in both x and y by pixel_center_integer so it appears the pixels are centered at whole number pixel
offsets. This moves the (x, y) value returned by gl_FragCoord of (0.5, 0.5) by default, to (0.0, 0.0) with
pixel_center_integer.
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4 Variables and Types
Redeclarations are done as follows
in vec4 gl_FragCoord; // redeclaration that changes nothing is allowed
// All the following are allowed redeclaration that change behavior
layout(origin_upper_left) in vec4 gl_FragCoord;
layout(pixel_center_integer) in vec4 gl_FragCoord;
layout(origin_upper_left, pixel_center_integer) in vec4 gl_FragCoord;
If gl_FragCoord is redeclared in any fragment shader in a program, it must be redeclared in all the
fragment shaders in that program that have a static use gl_FragCoord. All redeclarations of
gl_FragCoord in all fragment shaders in a single program must have the same set of qualifiers. Within
any shader, the first redeclarations of gl_FragCoord must appear before any use of gl_FragCoord. The
built-in gl_FragCoord is only predeclared in fragment shaders, so redeclaring it in any other shader
language results in a compile-time error.
Redeclaring gl_FragCoord with origin_upper_left and/or pixel_center_integer qualifiers only affects
gl_FragCoord.x and gl_FragCoord.y. It has no affect on rasterization, transformation, or any other part
of the OpenGL pipeline or language features.
Fragment shaders also allow the following layout qualifier on in only (not with variable declarations)
layout-qualifier-id
early_fragment_tests
to request that fragment tests be performed before fragment shader execution, as described in section
15.2.4 “Early Fragment Tests” of the OpenGL Specification.
For example,
layout(early_fragment_tests) in;
Specifying this will make per-fragment tests be performed before fragment shader execution. If this is not
declared, per-fragment tests will be performed after fragment shader execution. Only one fragment shader
(compilation unit) need declare this, though more than one can. If at least one declares this, then it is
enabled.
4.4.1.4 Compute Shader Inputs
There are no layout location qualifiers for compute shader inputs.
Layout qualifier identifiers for compute shader inputs are the work-group size qualifiers:
layout-qualifier-id
local_size_x = integer-constant-expression
local_size_y = integer-constant-expression
local_size_z = integer-constant-expression
The local_size_x, local_size_y, and local_size_z qualifiers are used to declare a fixed local group size by
the compute shader in the first, second, and third dimension, respectively. The default size in each
dimension is 1. If a shader does not specify a size for one of the dimensions, that dimension will have a
size of 1.
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4 Variables and Types
For example, the following declaration in a compute shader
layout (local_size_x = 32, local_size_y = 32) in;
is used to declare a two-dimensional compute shader with a local size of 32 X 32 elements, which is
equivalent to a three-dimensional compute shader where the third dimension has size one.
As another example, the declaration
layout (local_size_x = 8) in;
effectively specifies that a one-dimensional compute shader is being compiled, and its size is 8 elements.
If the fixed local group size of the shader in any dimension is greater than the maximum size supported by
the implementation for that dimension, a compile-time error results. Also, if such a layout qualifier is
declared more than once in the same shader, all those declarations must set the same set of local work-
group sizes and set them to the same values; otherwise a compile-time error results. If multiple compute
shaders attached to a single program object declare a fixed local group size, the declarations must be
identical; otherwise a link-time error results.
Furthermore, if a program object contains any compute shaders, at least one must contain an input layout
qualifier specifying a fixed local group size for the program, or a link-time error will occur.
4.4.2 Output Layout Qualifiers
Some output layout qualifiers apply to all shader languages and some apply only to specific languages.
The latter are discussed in separate sections below.
As with input layout qualifiers, all shaders except compute shaders allow location layout qualifiers
on output variable declarations, output block declarations, and output block member declarations. Of
these, variables and block members (but not blocks) additionally allow the component layout qualifier.
The layout qualifier identifiers for outputs are:
layout-qualifier-id
location = integer-constant-expression
component = integer-constant-expression
The usage and rules for using the component qualifier, and applying location qualifier to blocks and
structures, are exactly as described in section 4.4.1 "Input Layout Qualifiers". Additionally, for fragment
shader outputs, if two variables are placed within the same location, they must have the same underlying
type (floating-point or integer). No component aliasing of output variables or members is allowed.
Fragment shaders allow an additional index output layout qualifiers:
layout-qualifier-id
index = integer-constant-expression
Each of these qualifiers may appear at most once. If index is specified, location must also be specified.
If index is not specified, the value 0 is used. For example, in a fragment shader,
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4 Variables and Types
layout(location = 3) out vec4 color;
will establish that the fragment shader output color is assigned to fragment color 3 as the first (index zero)
input to the blend equation. And,
layout(location = 3, index = 1) out vec4 factor;
will establish that the fragment shader output factor is assigned to fragment color 3 as the second (index
one) input to the blend equation.
For fragment-shader outputs, the location and index specify the color output number and index receiving
the values of the output. For outputs of all other shader stages, the location specifies a vector number that
can be used to match against inputs in a subsequent shader stage, even if that shader is in a different
program object.
If a declared output is a scalar or vector type other than dvec3 or dvec4, it will consume a single location.
Outputs of type dvec3 or dvec4 will consume two consecutive locations. Outputs of type double and
dvec2 will consume only a single location, in all stages.
If the declared output is an array, it will be assigned consecutive locations starting with the location
specified. For example,
layout(location = 2) out vec4 colors[3];
will establish that colors is assigned to vector location numbers 2, 3, and 4.
If the declared output is an n x m single- or double-precision matrix, it will be assigned multiple locations
starting with the location specified. The number of locations assigned will be the same as for an n-
element array of m-component vectors.
If the declared output is a structure, its members will be assigned consecutive locations in the order of
declaration, with the first member assigned the location specified for the structure. The number of
locations consumed by a structure member is determined by applying the rules above recursively as
though the structure member were declared as an output variable of the same type.
Location layout qualifiers may be used on output variables declared as structures, but not on individual
members. Location layout qualifiers may not be used on output blocks or output block members.
Compile-time errors result if these rules are not followed.
The number of output locations available to a shader is limited. For fragment shaders, the limit is the
advertised number of draw buffers. For all other shaders, the limit is implementation-dependent and must
be no less than one fourth of the advertised maximum output component count. (Compute shaders have
no outputs.) A program will fail to link if any attached shader uses a location greater than or equal to the
number of supported locations, unless device-dependent optimizations are able to make the program fit
within available hardware resources. Compile-time errors may also be given if at compile time it is
known the link will fail. A negative output location will result in a compile-time error. It is also a
compile-time error if a fragment shader sets a layout index to less than 0 or greater than 1.
A program will fail to link if any of the following occur:
•any two fragment shader output variables are assigned to the same location and index, or
•any two geometry shader output variables are assigned the same location and stream, or
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4 Variables and Types
•if any two output variables from the same vertex or tessellation shader stage are assigned to the
same location.
For fragment shader outputs, locations can be assigned using either a layout qualifier or via the OpenGL
API. For all shader types, a program will fail to link if explicit location assignments leave the linker
unable to find space for other variables without explicit assignments.
If an output variable with no location or index assigned in the shader text has a location specified through
the OpenGL API, the API-assigned location will be used. Otherwise, such variables will be assigned a
location by the linker. All such assignments will have a color index of zero. See section 15.2 “Shader
Execution” of the OpenGL Graphics System Specification for more details. A link-time error will occur if
an output variable is declared in multiple shaders of the same language with conflicting location or index
values.
For the purposes of determining if a non-fragment output matches an input from a subsequent shader
stage, the location layout qualifier (if any) must match.
4.4.2.1 Transform Feedback Layout Qualifiers
The vertex, tessellation, and geometry stages allow shaders to control transform feedback. When doing
this, shaders will dictate which transform feedback buffers are in use, which output variables will be
written to which buffers, and how each buffer is laid out. To accomplish this, shaders allow the following
layout qualifier identifiers on output declarations:
layout-qualifier-id
xfb_buffer = integer-constant-expression
xfb_offset = integer-constant-expression
xfb_stride = integer-constant-expression
Any shader making any static use (after preprocessing) of any of these xfb_ qualifiers will cause the
shader to be in a transform feedback capturing mode and hence responsible for describing the transform
feedback setup. This mode will capture any output selected by xfb_offset, directly or indirectly, to a
transform feedback buffer.
The xfb_buffer qualifier specifies which transform feedback buffer will capture outputs selected with
xfb_offset. The xfb_buffer qualifier can be applied to the qualifier out, to output variables, to output
blocks, and to output block members. Shaders in the transform feedback capturing mode have an initial
global default of
layout(xfb_buffer = 0) out;
This default can be changed by declaring a different buffer with xfb_buffer on the interface qualifier out.
This is the only way the global default can be changed. When a variable or output block is declared
without an xfb_buffer qualifier, it inherits the global default buffer. When a variable or output block is
declared with an xfb_buffer qualifier, it has that declared buffer. All members of a block inherit the
block's buffer. A member is allowed to declare an xfb_buffer, but it must match the buffer inherited from
its block, or a compile-time error results.
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4 Variables and Types
layout(xfb_buffer=2, xfb_offset=0) out block { // block's buffer is 2
layout(xfb_buffer = 2) vec4 v; // okay, matches the inherited 2
layout(xfb_buffer = 3) vec4 u; // ERROR, mismatched buffer
vec4 w; // inherited
};
layout (xfb_offset=16) out vec4 t; // initial default is buffer 0
layout (xfb_buffer=1) out; // new global default of 1
out block { // block has buffer 1
vec4 x; // x has buffer 1 (not captured)
layout(xfb_buffer = 1) vec4 y; // okay (not captured)
layout(xfb_buffer = 0) vec4 z; // ERROR, mismatched buffer
};
layout(xfb_offset=0) out vec4 g; // g has buffer 1
layout(xfb_buffer=2) out vec4 h; // does not change global default
layout(xfb_offset=16) out vec4 j; // j has buffer 1
Note this means all members of a block that go to a transform feedback buffer will go to the same buffer.
It is a compile-time error to specify an xfb_buffer that is greater than the implementation-dependent
constant gl_MaxTransformFeedbackBuffers.
The xfb_offset qualifier assigns a byte offset within a transform feedback buffer. Only variables, block
members, or blocks can be qualified with xfb_offset. If a block is qualified with xfb_offset, all its
members are assigned transform feedback buffer offsets. If a block is not qualified with xfb_offset, any
members of that block not qualified with an xfb_offset will not be assigned transform feedback buffer
offsets. Only variables and block members that are assigned offsets will be captured (thus, a proper
subset of a block can be captured). Each time such a variable or block member is written in a shader, the
written value is captured at the assigned offset. If such a block member or variable is not written during a
shader invocation, the buffer contents at the assigned offset will be undefined. Even if there are no static
writes to a variable or member that is assigned a transform feedback offset, the space is still allocated in
the buffer and still affects the stride.
Variables and block members qualified with xfb_offset can be scalars, vectors, matrices, structures, and
(sized) arrays of these. The offset must be a multiple of the size of the first component of the first
qualified variable or block member, or a compile-time error results. Further, if applied to an aggregate
containing a double, the offset must also be a multiple of 8, and the space taken in the buffer will be a
multiple of 8. The given offset applies to the first component of the first member of the qualified entity.
Then, within the qualified entity, subsequent components are each assigned, in order, to the next available
offset aligned to a multiple of that component's size. Aggregate types are flattened down to the
component level to get this sequence of components. It is a compile-time error to apply xfb_offset to the
declaration of an unsized array.
No aliasing in output buffers is allowed: It is a compile-time or link-time error to specify variables with
overlapping transform feedback offsets.
The xfb_stride qualifier specifies how many bytes are consumed by each captured vertex. It applies to
the transform feedback buffer for that declaration, whether it is inherited or explicitly declared. It can be
applied to variables, blocks, block members, or just the qualifier out. If the buffer is capturing any
outputs with double-precision components, the stride must be a multiple of 8, otherwise it must be a
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4 Variables and Types
multiple of 4, or a compile-time or link-time error results. It is a compile-time or link-time error to have
any xfb_offset that overflows xfb_stride, whether stated on declarations before or after the xfb_stride, or
in different compilation units. While xfb_stride can be declared multiple times for the same buffer, it is a
compile-time or link-time error to have different values specified for the stride for the same buffer.
For example:
// buffer 1 has 32-byte stride
layout (xfb_buffer = 1, xfb_stride = 32) out;
// same as previous example; order within layout does not matter
layout (xfb_stride = 32, xfb_buffer = 1) out;
// everything in this block goes to buffer 0
layout (xfb_buffer = 0, xfb_stride = 32) out block1 {
layout (xfb_offset = 0) vec4 a; // a goes to byte offset 0 of buffer 0
layout (xfb_offset = 16) vec4 b; // b goes to offset 16 of buffer 0
};
layout (xfb_buffer = 3, xfb_offset = 12) out block2 {
vec4 v; // v will be written to byte offsets 12 through 27 of buffer
float u; // u will be written to offset 28
layout(xfb_offset = 40) vec4 w;
vec4 x; // x will be written to offset 56, the next available offset
};
layout (xfb_buffer = 2, xfb_stride = 32) out block3 {
layout (xfb_offset = 12) vec3 c;
layout (xfb_offset = 24) vec3 d; // ERROR, requires stride of 36
layout (xfb_offset = 0) vec3 g; // okay, increasing order not required
};
When no xfb_stride is specified for a buffer, the stride of the buffer will be the smallest needed to hold
the variable placed at the highest offset, including any required padding. For example:
// if there no other declarations for buffer 3, it has stride 32
layout (xfb_buffer = 3) out block4 {
layout (xfb_offset = 0) vec4 e;
layout (xfb_offset = 16) vec4 f;
};
The resulting stride (implicit or explicit), when divided by 4, must be less than or equal to the
implementation-dependent constant gl_MaxTransformFeedbackInterleavedComponents.
4.4.2.2 Tessellation Control Outputs
Other than for the transform feedback layout qualifiers, tessellation control shaders allow output layout
qualifiers only on the interface qualifier out, not on an output block, block member, or variable
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declaration. The output layout qualifier identifiers allowed for tessellation control shaders include the
vertex-count layout qualifier:
layout-qualifier-id
vertices = integer-constant-expression
The identifier vertices specifies the number of vertices in the output patch produced by the tessellation
control shader, which also specifies the number of times the tessellation control shader is invoked. It is a
compile- or link-time error for the output vertex count to be less than or equal to zero, or greater than the
implementation-dependent maximum patch size.
The intrinsically declared tessellation control output array gl_out[] will also be sized by any output layout
declaration. Hence, the expression
gl_out.length()
will return the output patch vertex count specified in a previous output layout qualifier. For outputs
declared without an array size, including intrinsically declared outputs (i.e., gl_out), a layout must be must
be declared before any use of the method length() or other array use requires its size be known.
It is a compile-time error if the output patch vertex count specified in an output layout qualifier does not
match the array size specified in any output variable declaration in the same shader.
All tessellation control shader layout declarations in a program must specify the same output patch vertex
count. There must be at least one layout qualifier specifying an output patch vertex count in any program
containing tessellation control shaders; however, such a declaration is not required in all tessellation
control shaders.
4.4.2.3 Geometry Outputs
Geometry shaders can have three additional types of output layout identifiers: an output primitive type, a
maximum output vertex count, and per-output stream numbers. The primitive type and vertex count
identifiers are allowed only on the interface qualifier out, not on an output block, block member, or
variable declaration. The stream identifier is allowed on the interface qualifier out, on output blocks, and
on variable declarations.
The layout qualifier identifiers for geometry shader outputs are
layout-qualifier-id
points
line_strip
triangle_strip
max_vertices = integer-constant-expression
stream = integer-constant-expression
The primitive type identifiers points, line_strip, and triangle_strip are used to specify the type of output
primitive produced by the geometry shader, and only one of these is accepted. At least one geometry
shader (compilation unit) in a program must declare an output primitive type, and all geometry shader
output primitive type declarations in a program must declare the same primitive type. It is not required
that all geometry shaders in a program declare an output primitive type.
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The vertex count identifier max_vertices is used to specify the maximum number of vertices the shader
will ever emit in a single invocation. At least one geometry shader (compilation unit) in a program must
declare a maximum output vertex count, and all geometry shader output vertex count declarations in a
program must declare the same count. It is not required that all geometry shaders in a program declare a
count.
In this example,
layout(triangle_strip, max_vertices = 60) out; // order does not matter
layout(max_vertices = 60) out; // redeclaration okay
layout(triangle_strip) out; // redeclaration okay
layout(points) out; // error, contradicts triangle_strip
layout(max_vertices = 30) out; // error, contradicts 60
all outputs from the geometry shader are triangles and at most 60 vertices will be emitted by the shader. It
is an error for the maximum number of vertices to be greater than gl_MaxGeometryOutputVertices.
The identifier stream is used to specify that a geometry shader output variable or block is associated with
a particular vertex stream (numbered beginning with zero). A default stream number may be declared at
global scope by qualifying interface qualifier out as in this example:
layout(stream = 1) out;
The stream number specified in such a declaration replaces any previous default and applies to all
subsequent block and variable declarations until a new default is established. The initial default stream
number is zero.
Each output block or non-block output variable is associated with a vertex stream. If the block or variable
is declared with the stream identifier, it is associated with the specified stream; otherwise, it is associated
with the current default stream. A block member may be declared with a stream identifier, but the
specified stream must match the stream associated with the containing block. One example:
layout(stream=1) out; // default is now stream 1
out vec4 var1; // var1 gets default stream (1)
layout(stream=2) out Block1 { // "Block1" belongs to stream 2
layout(stream=2) vec4 var2; // redundant block member stream decl
layout(stream=3) vec2 var3; // ILLEGAL (must match block stream)
vec3 var4; // belongs to stream 2
};
layout(stream=0) out; // default is now stream 0
out vec4 var5; // var5 gets default stream (0)
out Block2 { // "Block2" gets default stream (0)
vec4 var6;
};
layout(stream=3) out vec4 var7; // var7 belongs to stream 3
Each vertex emitted by the geometry shader is assigned to a specific stream, and the attributes of the
emitted vertex are taken from the set of output blocks and variables assigned to the targeted stream. After
each vertex is emitted, the values of all output variables become undefined. Additionally, the output
variables associated with each vertex stream may share storage. Writing to an output variable associated
with one stream may overwrite output variables associated with any other stream. When emitting each
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vertex, a geometry shader should write to all outputs associated with the stream to which the vertex will
be emitted and to no outputs associated with any other stream.
If a geometry shader output block or variable is declared more than once, all such declarations must
associate the variable with the same vertex stream. If any stream declaration specifies a non-existent
stream number, the shader will fail to compile.
Built-in geometry shader outputs are always associated with vertex stream zero.
All geometry shader output layout declarations in a program must declare the same layout and same value
for max_vertices. If geometry shaders are in a program, there must be at least one geometry output
layout declaration somewhere in that program, but not all geometry shaders (compilation units) are
required to declare it.
4.4.2.4 Fragment Outputs
The built-in fragment shader variable gl_FragDepth may be redeclared using one of the following layout
qualifiers.
layout-qualifier-id
depth_any
depth_greater
depth_less
depth_unchanged
For example:
layout (depth_greater) out float gl_FragDepth;
The layout qualifier for gl_FragDepth constrains intentions of the final value of gl_FragDepth written
by any shader invocation. GL implementations are allowed to perform optimizations assuming that the
depth test fails (or passes) for a given fragment if all values of gl_FragDepth consistent with the layout
qualifier would fail (or pass). This potentially includes skipping shader execution if the fragment is
discarded because it is occluded and the shader has no side effects. If the final value of gl_FragDepth is
inconsistent with its layout qualifier, the result of the depth test for the corresponding fragment is
undefined. However, no error will be generated in this case. If the depth test passes and depth writes are
enabled, the value written to the depth buffer is always the value of gl_FragDepth, whether or not it is
consistent with the layout qualifier.
By default, gl_FragDepth is qualified as depth_any. When the layout qualifier for gl_FragDepth is
depth_any, the shader compiler will note any assignment to gl_FragDepth modifying it in an unknown
way, and depth testing will always be performed after the shader has executed. When the layout qualifier
is depth_greater, the GL can assume that the final value of gl_FragDepth is greater than or equal to the
fragment's interpolated depth value, as given by the z component of gl_FragCoord. When the layout
qualifier is depth_less, the GL can assume that any modification of gl_FragDepth will only decrease its
value. When the layout qualifier is depth_unchanged, the shader compiler will honor any modification to
gl_FragDepth, but the rest of the GL can assume that gl_FragDepth is not assigned a new value.
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4 Variables and Types
Redeclarations of gl_FragDepth are performed as follows:
// redeclaration that changes nothing is allowed
out float gl_FragDepth;
// assume it may be modified in any way
layout (depth_any) out float gl_FragDepth;
// assume it may be modified such that its value will only increase
layout (depth_greater) out float gl_FragDepth;
// assume it may be modified such that its value will only decrease
layout (depth_less) out float gl_FragDepth;
// assume it will not be modified
layout (depth_unchanged) out float gl_FragDepth;
If gl_FragDepth is redeclared in any fragment shader in a program, it must be redeclared in all fragment
shaders in that program that have static assignments to gl_FragDepth. All redeclarations of
gl_FragDepth in all fragment shaders in a single program must have the same set of qualifiers. Within
any shader, the first redeclarations of gl_FragDepth must appear before any use of gl_FragDepth. The
built-in gl_FragDepth is only predeclared in fragment shaders, so redeclaring it in any other shader
language results in a compile-time error.
4.4.3 Uniform Variable Layout Qualifiers
Layout qualifiers can be used for uniform variables and subroutine uniforms. The layout qualifier
identifiers for uniform variables and subroutine uniforms are:
layout-qualifier-id
location = integer-constant-expression
The location identifier can be used with default-block uniform variables and subroutine uniforms. The
location specifies the location by which the OpenGL API can reference the uniform and update its value.
Individual elements of a uniform array are assigned consecutive locations with the first element taking
location location. No two default-block uniform variables in the program can have the same location,
even if they are unused, otherwise a compile-time or link-time error will be generated. No two subroutine
uniform variables can have the same location in the same shader stage, otherwise a compile-time or link-
time error will be generated. Valid locations for default-block uniform variable locations are in the range
of 0 to the implementation-defined maximum number of uniform locations minus one. Valid locations for
subroutine uniforms are in the range of 0 to the implementation-defined per-stage maximum number of
subroutine uniform locations minus one.
Locations can be assigned to default-block uniform arrays and structures. The first inner-most scalar,
vector or matrix member or element takes the specified location and the compiler assigns the next inner-
most member or element the next incremental location value. Each subsequent inner-most member or
element gets incremental locations for the entire structure or array. This rule applies to nested structures
and arrays and gives each inner-most scalar, vector, or matrix type a unique location. For arrays without
an explicit size, the size is calculated based on its static usage. When the linker generates locations for
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4 Variables and Types
uniforms without an explicit location, it assumes for all uniforms with an explicit location all their array
elements and structure members are used and the linker will not generate a conflicting location, even if
that element of member is deemed unused.
4.4.4 Subroutine Function Layout Qualifiers
Layout qualifiers can be used for subroutine functions. The layout qualifier identifiers for subroutine
functions are:
layout-qualifier-id
index = integer-constant-expression
Each subroutine with an index qualifier in the shader must be given a unique index, otherwise a compile-
or link-time error will be generated. The indices must be in the range of 0 to the implementation defined
maximum number of subroutines minus one. It is recommended, but not required, that the shader assigns
a range of tightly packed index values starting from zero so that the OpenGL subroutine function
enumeration API returns a non-empty name for all active indices.
4.4.5 Uniform and Shader Storage Block Layout Qualifiers
Layout qualifiers can be used for uniform and shader storage blocks, but not for non-block uniform
declarations. The layout qualifier identifiers (and shared keyword) for uniform and shader storage blocks
are
layout-qualifier-id
shared
packed
std140
std430
row_major
column_major
binding = integer-constant-expression
offset = integer-constant-expression
align = integer-constant-expression
None of these have any semantic affect at all on the usage of the variables being declared; they only
describe how data is laid out in memory. For example, matrix semantics are always column-based, as
described in the rest of this specification, no matter what layout qualifiers are being used.
Uniform and shader storage block layout qualifiers can be declared for global scope, on a single uniform
or shader storage block, or on a single block member declaration.
Default layouts for shared, packed, std140, std430, row_major, and column_major are established at
global scope for uniform blocks as
layout(layout-qualifier-id-list) uniform;
and for shader storage blocks as
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4 Variables and Types
layout(layout-qualifier-id-list) buffer;
When this is done, the previous default qualification is first inherited and then overridden as per the
override rules listed below for each qualifier listed in the declaration. The result becomes the new default
qualification scoped to subsequent uniform or shader storage block definitions.
The initial state of compilation is as if the following were declared:
layout(shared, column_major) uniform;
layout(shared, column_major) buffer;
Explicitly declaring this in a shader will return defaults back to their initial state.
Uniform and shader storage blocks can be declared with optional layout qualifiers, and so can their
individual member declarations. Such block layout qualification is scoped only to the content of the
block. As with global layout declarations, block layout qualification first inherits from the current default
qualification and then overrides it. Similarly, individual member layout qualification is scoped just to the
member declaration, and inherits from and overrides the block's qualification.
The shared qualifier overrides only the std140, std430, and packed qualifiers; other qualifiers are
inherited. The compiler/linker will ensure that multiple programs and programmable stages containing
this definition will share the same memory layout for this block, as long as all arrays are declared with
explicit sizes and all matrices have matching row_major and/or column_major qualifications (which may
come from a declaration outside the block definition). This allows use of the same buffer to back the same
block definition across different programs.
The packed qualifier overrides only std140, std430, and shared; other qualifiers are inherited. When
packed is used, no shareable layout is guaranteed. The compiler and linker can optimize memory use
based on what variables actively get used and on other criteria. Offsets must be queried, as there is no
other way of guaranteeing where (and which) variables reside within the block. Accessing the same
packed uniform or shader storage block in multiple stages within a program may result in a link-time
error. If no link-time error is given, then members will have the same offsets across stages and reads will
be well defined. Accessing the same packed uniform or shader storage block across programs can result
in conflicting member offsets and in undefined values being read. However, implementations may aid
application management of packed blocks by using canonical layouts for packed blocks.
The std140 and std430 qualifiers override only the packed, shared, std140, and std430 qualifiers; other
qualifiers are inherited. The std430 qualifier is supported only for shader storage blocks; using std430 on
a uniform block will result in a compile-time error. The layout is explicitly determined by this, as
described in section 7.6.2 “Uniform Blocks" under Standard Uniform Block Layout of the OpenGL
Graphics System Specification. Hence, as in shared above, the resulting layout is shareable across
programs.
Layout qualifiers on member declarations cannot use the shared, packed, std140, or std430 qualifiers.
These can only be used at global scope or on a block declaration, or a compile-time error results.
The row_major and column_major qualifiers affect the layout of matrices, including all matrices
contained in structures and arrays they are applied to, to all depths of nesting. These qualifiers can be
applied to other types, but will have no effect.
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4 Variables and Types
The row_major qualifier overrides only the column_major qualifier; other qualifiers are inherited.
Elements within a matrix row will be contiguous in memory.
The column_major qualifier overrides only the row_major qualifier; other qualifiers are inherited.
Elements within a matrix column will be contiguous in memory.
The binding identifier specifies the uniform buffer binding point corresponding to the uniform or shader
storage block, which will be used to obtain the values of the member variables of the block. It is a
compile-time error to specify the binding identifier for the global scope or for block member declarations.
Any uniform or shader storage block declared without a binding identifier is initially assigned to block
binding point zero. After a program is linked, the binding points used for uniform and shader storage
blocks declared with or without a binding identifier can be updated by the OpenGL API.
If the binding identifier is used with a uniform or shader storage block instanced as an array, the first
element of the array takes the specified block binding and each subsequent element takes the next
consecutive uniform block binding point.
If the binding point for any uniform or shader storage block instance is less than zero, or greater than or
equal to the implementation-dependent maximum number of uniform buffer bindings, a compile-time
error will occur. When the binding identifier is used with a uniform or shader storage block instanced as
an array of size N, all elements of the array from binding through binding + N – 1 must be within this
range.
When multiple arguments are listed in a layout declaration, the effect will be the same as if they were
declared one at a time, in order from left to right, each in turn inheriting from and overriding the result
from the previous qualification.
For example
layout(row_major, column_major)
results in the qualification being column_major. Other examples:
layout(shared, row_major) uniform; // default is now shared and row_major
layout(std140) uniform Transform { // layout of this block is std140
mat4 M1; // row_major
layout(column_major) mat4 M2; // column major
mat3 N1; // row_major
};
uniform T2 { // layout of this block is shared
...
};
layout(column_major) uniform T3 { // shared and column_major
mat4 M3; // column_major
layout(row_major) mat4 m4; // row major
mat3 N2; // column_major
};
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The offset qualifier can only be used on block members of blocks declared with std140 or std430 layouts.
The offset qualifier forces the qualified member to start at or after the specified integral-constant-
expression, which will be its byte offset from the beginning of the buffer. It is a compile-time error to
specify an offset that is smaller than the offset of the previous member in the block or that lies within the
previous member of the block. Two blocks linked together in the same program with the same block
name must have the exact same set of members qualified with offset and their integral-constant-
expression values must be the same, or a link-time error results. The specified offset must be a multiple
of the base alignment of the type of the block member it qualifies, or a compile-time error results.
The align qualifier can only be used on blocks or block members, and only for blocks declared with
std140 or std430 layouts. The align qualifier makes the start of each block member have a minimum
byte alignment. It does not affect the internal layout within each member, which will still follow the
std140 or std430 rules. The specified alignment must be a power of 2, or a compile-time error results.
The actual alignment of a member will be the greater of the specified align alignment and the standard
(e.g., std140) base alignment for the member's type. The actual offset of a member is computed as
follows: If offset was declared, start with that offset, otherwise start with the next available offset. If the
resulting offset is not a multiple of the actual alignment, increase it to the first offset that is a multiple of
the actual alignment. This results in the actual offset the member will have.
When align is applied to an array, it effects only the start of the array, not the array's internal stride. Both
an offset and an align qualifier can be specified on a declaration.
The align qualifier, when used on a block, has the same effect as qualifying each member with the same
align value as declared on the block, and gets the same compile-time results and errors as if this had been
done. As described in general earlier, an individual member can specify its own align, which overrides
the block-level align, but just for that member.
Examples:
layout(std140) uniform block {
vec4 a; // a takes offsets 0-15
layout(offset = 20) vec3 b; // b takes offsets 32-43
layout(offset = 40) vec2 c; // ERROR, lies within previous member
layout(offset = 48) vec2 d; // d takes offsets 48-55
layout(align = 16) float e; // e takes offsets 64-67
layout(align = 2) double f; // f takes offsets 72-79
layout(align = 6) double g; // ERROR, 6 is not a power of 2
layout(offset = 80) float h; // h takes offsets 80-83
layout(align = 64) dvec3 i; // i takes offsets 128-151
layout(offset = 153, align = 8)
float j; // j takes offsets 160-163
};
4.4.6 Opaque-Uniform Layout Qualifiers
Uniform layout qualifiers can be used to bind opaque uniform variables to specific buffers or units.
Texture image units can be bound to samplers, image units can be bound to images, and atomic counters
can be bound to buffers.
Details for specific to image formats and atomic counter bindings are given in the subsections below.
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4 Variables and Types
Image and sampler types both take the uniform layout qualifier identifier for binding:
layout-qualifier-id
binding = integer-constant-expression
The identifier binding specifies which unit will be bound. Any uniform sampler or image variable
declared without a binding qualifier is initially bound to unit zero. After a program is linked, the unit
referenced by a sampler or image uniform variable declared with or without a binding identifier can be
updated by the OpenGL API.
If the binding identifier is used with an array, the first element of the array takes the specified unit and
each subsequent element takes the next consecutive unit.
If the binding is less than zero, or greater than or equal to the implementation-dependent maximum
supported number of units, a compile-time error will occur. When the binding identifier is used with an
array of size N, all elements of the array from binding through binding + N - 1 must be within this range.
A link-time error will result if two compilation units in a program specify different integer-constant-
expression bindings for the same opaque-uniform name. However, it is not an error to specify a binding
on some but not all declarations for the same name, as shown in the examples below.
// in one compilation unit...
layout(binding=3) uniform sampler2D s; // s bound to unit 3
// in another compilation unit...
uniform sampler2D s; // okay, s still bound at 3
// in another compilation unit...
layout(binding=4) uniform sampler2D s; // ERROR: contradictory bindings
4.4.6.1 Atomic Counter Layout Qualifiers
The atomic counter qualifiers are
layout-qualifier-id
binding = integer-constant-expression
offset = integer-constant-expression
For example,
layout (binding = 2, offset = 4) uniform atomic_uint a;
will establish that the opaque handle to the atomic counter a will be bound to atomic counter buffer
binding point 2 at an offset of 4 basic machine units into that buffer. The default offset for binding point 2
will be post incremented by 4 (the size of an atomic counter).
A subsequent atomic counter declaration will inherit the previous (post incremented) offset. For example,
a subsequent declaration of
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4 Variables and Types
layout (binding = 2) uniform atomic_uint bar;
will establish that the atomic counter bar has a binding to buffer binding point 2 at an offset of 8 basic
machine units into that buffer. The offset for binding point 2 will again be post-incremented by 4 (the size
of an atomic counter).
When multiple variables are listed in a layout declaration, the effect will be the same as if they were
declared one at a time, in order from left to right.
Binding points are not inherited, only offsets. Each binding point tracks its own current default offset for
inheritance of subsequent variables using the same binding. The initial state of compilation is that all
binding points have an offset of 0. The offset can be set per binding point at global scope (without
declaring a variable). For example,
layout (binding = 2, offset = 4) uniform atomic_uint;
Establishes that the next atomic_uint declaration for binding point 2 will inherit offset 4 (but does not
establish a default binding):
layout (binding = 2) uniform atomic_uint bar; // offset is 4
layout (offset = 8) uniform atomic_uint bar; // error, no default binding
Atomic counters may share the same binding point, but if a binding is shared, their offsets must be either
explicitly or implicitly (from inheritance) unique and non overlapping.
Example valid uniform declarations, assuming top of shader:
layout (binding=3, offset=4) uniform atomic_uint a; // offset = 4
layout (binding=2) uniform atomic_uint b; // offset = 0
layout (binding=3) uniform atomic_uint c; // offset = 8
layout (binding=2) uniform atomic_uint d; // offset = 4
Example of an invalid uniform declaration:
layout (offset=4) … // error, must include binding
layout (binding=1, offset=0) … a; // okay
layout (binding=2, offset=0) … b; // okay
layout (binding=1, offset=0) … c; // error, offsets must not be shared
// between a and c
layout (binding=1, offset=2) … d; // error, overlaps offset 0 of a
It is a compile-time error to bind an atomic counter with a binding value greater than or equal to
gl_MaxAtomicCounterBindings.
4.4.6.2 Format Layout Qualifiers
Format layout qualifiers can be used on image variable declarations (those declared with a basic type
having “image” in its keyword). The format layout qualifier identifiers for image variable declarations
are
layout-qualifier-id
float-image-format-qualifier
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4 Variables and Types
int-image-format-qualifier
uint-image-format-qualifier
binding = integer-constant-expression
float-image-format-qualifier
rgba32f
rgba16f
rg32f
rg16f
r11f_g11f_b10f
r32f
r16f
rgba16
rgb10_a2
rgba8
rg16
rg8
r16
r8
rgba16_snorm
rgba8_snorm
rg16_snorm
rg8_snorm
r16_snorm
r8_snorm
int-image-format-qualifier
rgba32i
rgba16i
rgba8i
rg32i
rg16i
rg8i
r32i
r16i
r8i
uint-image-format-qualifier
rgba32ui
rgba16ui
rgb10_a2ui
rgba8ui
rg32ui
rg16ui
rg8ui
r32ui
r16ui
r8ui
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4 Variables and Types
A format layout qualifier specifies the image format associated with a declared image variable. Only one
format qualifier may be specified for any image variable declaration. For image variables with floating-
point component types (keywords starting with “image”), signed integer component types (keywords
starting with “iimage”), or unsigned integer component types (keywords starting with “uimage”), the
format qualifier used must match the float-image-format-qualifier, int-image-format-qualifier, or uint-
image-format-qualifier grammar rules, respectively. It is a compile-time error to declare an image
variable where the format qualifier does not match the image variable type.
Any image variable used for image loads or atomic operations must specify a format layout qualifier; it is
a compile-time error to pass an image uniform variable or function parameter declared without a format
layout qualifier to an image load or atomic function.
The binding identifier was described in section 4.4.5 “Uniform and Shader Storage Block Layout
Qualifiers”.
Uniforms not qualified with writeonly must have a format layout qualifier. Note that an image variable
passed to a function for read access cannot be declared as writeonly and hence must have been declared
with a format layout qualifier.
4.5 Interpolation Qualifiers
Inputs and outputs that could be interpolated can be further qualified by at most one of the following
interpolation qualifiers:
Qualifier Meaning
smooth perspective correct interpolation
flat no interpolation
noperspective linear interpolation
The presence of and type of interpolation is controlled by the above interpolation qualifiers as well as the
auxiliary storage qualifiers centroid and sample. The auxiliary storage qualifier patch is not used for
interpolation; it is a compile-time error to use interpolation qualifiers with patch.
A variable qualified as flat will not be interpolated. Instead, it will have the same value for every
fragment within a triangle. This value will come from a single provoking vertex, as described by the
OpenGL Graphics System Specification. A variable may be qualified as flat can also be qualified as
centroid or sample, which will mean the same thing as qualifying it only as flat.
A variable qualified as smooth will be interpolated in a perspective-correct manner over the primitive
being rendered. Interpolation in a perspective correct manner is specified in equation 14.7 in the OpenGL
Graphics System Specification, section 14.5 “Line Segments”.
A variable qualified as noperspective must be interpolated linearly in screen space, as described in
equation 3.7 in the OpenGL Graphics System Specification, section 3.5 “Line Segments”.
When multi-sample rasterization is disabled, or for fragment shader input variables qualified with neither
centroid nor sample, the value of the assigned variable may be interpolated anywhere within the pixel
and a single value may be assigned to each sample within the pixel, to the extent permitted by the
OpenGL Graphics System Specification.
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When multisample rasterization is enabled, centroid and sample may be used to control the location and
frequency of the sampling of the qualified fragment shader input. If a fragment shader input is qualified
with centroid, a single value may be assigned to that variable for all samples in the pixel, but that value
must be interpolated to a location that lies in both the pixel and in the primitive being rendered, including
any of the pixel's samples covered by the primitive. Because the location at which the variable is
interpolated may be different in neighboring pixels, and derivatives may be computed by computing
differences between neighboring pixels, derivatives of centroid-sampled inputs may be less accurate than
those for non-centroid interpolated variables. If a fragment shader input is qualified with sample, a
separate value must be assigned to that variable for each covered sample in the pixel, and that value must
be sampled at the location of the individual sample.
It is a link-time error if, within the same stage, the interpolation qualifiers of variables of the same name
do not match.
4.5.1 Redeclaring Built-in Interpolation Variables in the Compatibility Profile
The following predeclared variables can be redeclared with an interpolation qualifier when using the
compatibility profile:
Vertex, tessellation control, tessellation evaluation, and geometry languages:
gl_FrontColor
gl_BackColor
gl_FrontSecondaryColor
gl_BackSecondaryColor
Fragment language:
gl_Color
gl_SecondaryColor
For example,
in vec4 gl_Color; // predeclared by the fragment language
flat in vec4 gl_Color; // redeclared by user to be flat
flat in vec4 gl_FrontColor; // input to geometry shader, no “gl_in[]”
flat out vec4 gl_FrontColor; // output from geometry shader
Ideally, these are redeclared as part of the redeclaration of an interface block, as described in section 7.1.1
“Compatibility Profile Built-In Language Variables”. However, for the above purpose, they can be
redeclared as individual variables at global scope, outside an interface block. Such redeclarations also
allow adding the transform-feedback qualifiers xfb_buffer, xfb_stride, and xfb_offset to output
variables. (Using xfb_buffer on a variable does not change the global default buffer.) A compile-time
error will result if a shader has both an interface block redeclaration and a separate redeclaration of a
member of that interface block outside the interface block redeclaration.
If gl_Color is redeclared with an interpolation qualifier, then gl_FrontColor and gl_BackColor (if they
are written to) must also be redeclared with the same interpolation qualifier, and vice versa. If
gl_SecondaryColor is redeclared with an interpolation qualifier, then gl_FrontSecondaryColor and
gl_BackSecondaryColor (if they are written to) must also be redeclared with the same interpolation
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4 Variables and Types
qualifier, and vice versa. This qualifier matching on predeclared variables is only required for variables
that are statically used within the shaders in a program.
4.6 Parameter Qualifiers
In addition to precision qualifiers and memory qualifiers, parameters can have these parameter qualifiers.
Qualifier Meaning
< none: default > same is in
const for function parameters that cannot be written to
in for function parameters passed into a function
out for function parameters passed back out of a function, but not initialized
for use when passed in
inout for function parameters passed both into and out of a function
Parameter qualifiers are discussed in more detail in section 6.1.1 “Function Calling Conventions”.
4.7 Precision and Precision Qualifiers
Precision qualifiers are added for code portability with OpenGL ES, not for functionality. They have the
same syntax as in OpenGL ES, as described below, but they have no semantic meaning, which includes no
effect on the precision used to store or operate on variables.
If an extension adds in the same semantics and functionality in the OpenGL ES 2.0 specification for
precision qualifiers, then the extension is allowed to reuse the keywords below for that purpose.
For the purposes of determining if an output from one shader stage matches an input of the next stage, the
precision qualifier need not match.
4.7.1 Range and Precision
The precision of stored single- and double-precision floating-point variables is defined by the IEEE 754
standard for 32-bit and 64-bit floating-point numbers. This includes support for NaNs (Not a Number)
and Infs (positive or negative infinities).
The following rules apply to both single and double-precision operations: Infinities and zeros are
generated as dictated by IEEE, but subject to the precisions allowed in the following table and subject to
allowing positive and negative zeros to be interchanged. However, dividing a non-zero by 0 results in the
appropriately signed IEEE Inf: If both positive and negative zeros are implemented, the correctly signed
Inf will be generated, otherwise positive Inf is generated. Any denormalized value input into a shader or
potentially generated by any operation in a shader can be flushed to 0. The rounding mode cannot be set
and is undefined. NaNs are not required to be generated. Support for signaling NaNs is not required and
exceptions are never raised. Operations and built-in functions that operate on a NaN are not required to
return a NaN as the result.
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4 Variables and Types
Precisions are expressed in terms of maximum relative error in units of ULP (units in the last place),
unless otherwise noted.
For single precision operations, precisions are required as follows:
Operation Precision
a + b, a – b, a * bCorrectly rounded.
<, <=, ==, >, >= Correct result.
a / b, 1.0 / b2.5 ULP for b in the range [2-126, 2126].
a * b + cCorrectly rounded single operation or sequence of
two correctly rounded operations.
fma() Inherited from a * b + c.
pow(x, y) Inherited from exp2 (x * log2 (y)).
exp (x), exp2 (x) (3 + 2 * |x|) ULP.
log (), log2 () 3 ULP outside the range [0.5, 2.0].
Absolute error < 2-21 inside the range [0.5, 2.0].
sqrt () Inherited from 1.0 / inversesqrt().
inversesqrt () 2 ULP.
implicit and explicit
conversions between types
Correctly rounded.
Built-in functions defined in the specification with an equation built from the above operations inherit the
above errors. These include, for example, the geometric functions, the common functions, and many of
the matrix functions. Built-in functions not listed above and not defined as equations of the above have
undefined precision. These include, for example, the trigonometric functions and determinant.
The precision of double-precision operations is at least that of single precision.
4.7.2 Precision Qualifiers
Any single-precision floating-point declaration, integer declaration, or sampler declaration can have the
type preceded by one of these precision qualifiers:
Qualifier Meaning
highp None.
mediump None.
lowp None.
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4 Variables and Types
For example:
lowp float color;
out mediump vec2 P;
lowp ivec2 foo(lowp mat3);
highp mat4 m;
Literal constants do not have precision qualifiers. Neither do Boolean variables. Neither do floating-point
constructors nor integer constructors when none of the constructor arguments have precision qualifiers.
Precision qualifiers, as with other qualifiers, do not effect the basic type of the variable. In particular,
there are no constructors for precision conversions; constructors only convert types. Similarly, precision
qualifiers, as with other qualifiers, do not contribute to function overloading based on parameter types. As
discussed in the next chapter, function input and output is done through copies, and therefore qualifiers do
not have to match.
4.7.3 Default Precision Qualifiers
The precision statement
precision precision-qualifier type;
can be used to establish a default precision qualifier. The type field can be either int, or float, or any of
the sampler types, and the precision-qualifier can be lowp, mediump, or highp. Any other types or
qualifiers will result in a compile-time error. If type is float, the directive applies to non-precision-
qualified single-precision floating-point type (scalar, vector, and matrix) declarations. If type is int, the
directive applies to all non-precision-qualified integer type (scalar, vector, signed, and unsigned)
declarations. This includes global variable declarations, function return declarations, function parameter
declarations, and local variable declarations.
Non-precision qualified declarations will use the precision qualifier specified in the most recent precision
statement that is still in scope. The precision statement has the same scoping rules as variable
declarations. If it is declared inside a compound statement, its effect stops at the end of the innermost
statement it was declared in. Precision statements in nested scopes override precision statements in outer
scopes. Multiple precision statements for the same basic type can appear inside the same scope, with later
statements overriding earlier statements within that scope.
The vertex, tessellation, and geometry languages have the following predeclared globally scoped default
precision statements:
precision highp float;
precision highp int;
The fragment language has the following predeclared globally scoped default precision statements:
precision mediump int;
precision highp float;
There are no errors for omission of a precision qualifier; so the above is just for reference of what may
happen in OpenGL ES versions of the shading languages.
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4 Variables and Types
4.7.4 Available Precision Qualifiers
The built-in macro GL_FRAGMENT_PRECISION_HIGH is defined to 1:
#define GL_FRAGMENT_PRECISION_HIGH 1
This macro is available in the vertex, tessellation, geometry, and fragment languages.
4.8 Variance and the Invariant Qualifier
In this section, variance refers to the possibility of getting different values from the same expression in
different programs. For example, say two vertex shaders, in different programs, each set gl_Position with
the same expression in both shaders, and the input values into that expression are the same when both
shaders run. It is possible, due to independent compilation of the two shaders, that the values assigned to
gl_Position are not exactly the same when the two shaders run. In this example, this can cause problems
with alignment of geometry in a multi-pass algorithm.
In general, such variance between shaders is allowed. When such variance does not exist for a particular
output variable, that variable is said to be invariant.
4.8.1 The Invariant Qualifier
To ensure that a particular output variable is invariant, it is necessary to use the invariant qualifier. It can
either be used to qualify a previously declared variable as being invariant
invariant gl_Position; // make existing gl_Position be invariant
out vec3 Color;
invariant Color; // make existing Color be invariant
or as part of a declaration when a variable is declared
invariant centroid out vec3 Color;
Only variables output from a shader (including those that are then input to a subsequent shader) can be
candidates for invariance. This includes user-defined output variables and the built-in output variables.
As only outputs need be declared with invariant, an output from one shader stage will still match an input
of a subsequent stage without the input being declared as invariant.
Input or output instance names on blocks are not used when redeclaring built-in variables.
The invariant keyword can be followed by a comma separated list of previously declared identifiers. All
uses of invariant must be at the global scope, and before any use of the variables being declared as
invariant.
To guarantee invariance of a particular output variable across two programs, the following must also be
true:
•The output variable is declared as invariant in both programs.
•The same values must be input to all shader input variables consumed by expressions and flow control
contributing to the value assigned to the output variable.
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•The texture formats, texel values, and texture filtering are set the same way for any texture function
calls contributing to the value of the output variable.
•All input values are all operated on in the same way. All operations in the consuming expressions and
any intermediate expressions must be the same, with the same order of operands and same
associativity, to give the same order of evaluation. Intermediate variables and functions must be
declared as the same type with the same explicit or implicit precision qualifiers. Any control flow
affecting the output value must be the same, and any expressions consumed to determine this control
flow must also follow these invariance rules.
•All the data flow and control flow leading to setting the invariant output variable reside in a single
compilation unit.
Essentially, all the data flow and control flow leading to an invariant output must match.
Initially, by default, all output variables are allowed to be variant. To force all output variables to be
invariant, use the pragma
#pragma STDGL invariant(all)
before all declarations in a shader. If this pragma is used after the declaration of any variables or
functions, then the set of outputs that behave as invariant is undefined. It is a compile-time error to use
this pragma in a fragment shader.
Generally, invariance is ensured at the cost of flexibility in optimization, so performance can be degraded
by use of invariance. Hence, use of this pragma is intended as a debug aid, to avoid individually declaring
all output variables as invariant.
4.8.2 Invariance of Constant Expressions
Invariance must be guaranteed for constant expressions. A particular constant expression must evaluate to
the same result if it appears again in the same shader or a different shader. This includes the same
expression appearing two shaders of the same language or shaders of two different languages.
Constant expressions must evaluate to the same result when operated on as already described above for
invariant variables, whether or not the invariant qualifier is used.
4.9 The Precise Qualifier
Some algorithms require floating-point computations to exactly follow the order of operations specified in
the source code and to treat all operations consistently, even if the implementation supports optimizations
that could produce nearly equivalent results with higher performance. For example, many GL
implementations support a "multiply-add" instruction that can compute a floating-point expression such as
result = (a * b) + (c * d);
in two operations instead of three operations; one multiply and one multiply-add instead of two multiplies
and one add. The result of a floating-point multiply-add might not always be identical to first doing a
multiply yielding a floating-point result and then doing a floating-point add. Hence, in this example, the
two multiply operations would not be treated consistently; the two multiplies could effectively appear to
have differing precisions.
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4 Variables and Types
The key computation that needs to be made consistent appears when tessellating, where intermediate
points for subdivision are synthesized in different directions, yet need to yield the same result, as shown in
the diagram below.
Without any qualifiers, implementations are permitted to perform such optimizations that effectively
modify the order or number of operations used to evaluate an expression, even if those optimizations may
produce slightly different results relative to unoptimized code.
The qualifier precise will ensure that operations contributing to a variable's value are done in their stated
order and are done with operator consistency. Order is determined by operator precedence and
parenthesis, as described in section 5.1 “Operators”. Operator consistency means for each particular
operator, for example the multiply operator ( * ), its operation is always computed with the same
precision. Specifically, values computed by compiler-generated code must adhere to the following
identities:
1. a + b = b + a
2. a * b = b * a
3. a * b + c * d = b * a + c* d = d * c + b * a = <any other mathematically valid combination>
While the following are prevented:
4. a + (b + c) is not allowed to become (a + b) + c
5. a * (b * c) is not allowed to become (a * b) * c
6. a * b + c is not allowed to become a single operation fma(a, b, c)
Where a, b, c, and d, are scalars or vectors, not matrices. (Matrix multiplication generally does not
commute.) It is the shader writer's responsibility to express the computation in terms of these rules and
the compiler's responsibility to follow these rules. See the description of gl_TessCoord for the rules the
tessellation stages are responsible for following, which in conjunction with the above allow avoiding
cracking when subdividing.
90
Opposing directions
of edge walking
for subdivision
Subdivision points
need to land on the
same location to
prevent cracking
Corner points
start with same values
Corner points
start with same values
4 Variables and Types
For example,
precise out vec4 position;
declares that operations used to produce the value of position must be performed in exactly the order
specified in the source code and with all operators being treated consistently. As with the invariant
qualifier (section 4.8.1 “The Invariant Qualifier”), the precise qualifier may be used to qualify a built-in or
previously declared user-defined variable as being precise:
out vec3 Color;
precise Color; // make existing Color be precise
This qualifier will affect the evaluation of an r-value in a particular function if and only if the result is
eventually consumed in the same function by an l-value qualified as precise. Any other expressions
within a function are not affected, including return values and output parameters not declared as precise
but that are eventually consumed outside the function by an variable qualified as precise.
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Some examples of the use of precise:
in vec4 a, b, c, d;
precise out vec4 v;
float func(float e, float f, float g, float h)
{
return (e*f) + (g*h); // no constraint on order or
// operator consistency
}
float func2(float e, float f, float g, float h)
{
precise float result = (e*f) + (g*h); // ensures same precision for
// the two multiplies
return result;
}
float func3(float i, float j, precise out float k)
{
k = i * i + j; // precise, due to <k> declaration
}
void main()
{
vec3 r = vec3(a * b); // precise, used to compute v.xyz
vec3 s = vec3(c * d); // precise, used to compute v.xyz
v.xyz = r + s; // precise
v.w = (a.w * b.w) + (c.w * d.w); // precise
v.x = func(a.x, b.x, c.x, d.x); // values computed in func()
// are NOT precise
v.x = func2(a.x, b.x, c.x, d.x); // precise!
func3(a.x * b.x, c.x * d.x, v.x); // precise!
}
For the purposes of determining if an output from one shader stage matches an input of the next stage, the
precise qualifier need not match between the input and the output.
All constant expressions are evaluated as if precise was present, whether or not it is present. However, as
described in section 4.3.3 “Constant Expressions”, there is no requirement that a compile-time constant
expression evaluates to the same value as a corresponding non-constant expression.
4.10 Memory Qualifiers
Variables declared as image types (the basic opaque types with “image” in their keyword) can be further
qualified with one or more of the following memory qualifiers:
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4 Variables and Types
Qualifier Meaning
coherent memory variable where reads and writes are coherent with reads and
writes from other shader invocations
volatile memory variable whose underlying value may be changed at any point
during shader execution by some source other than the current shader
invocation
restrict memory variable where use of that variable is the only way to read
and write the underlying memory in the relevant shader stage
readonly memory variable that can be used to read the underlying memory, but
cannot be used to write the underlying memory
writeonly memory variable that can be used to write the underlying memory, but
cannot be used to read the underlying memory
Memory accesses to image variables declared using the coherent qualifier are performed coherently with
similar accesses from other shader invocations. In particular, when reading a variable declared as
coherent, the values returned will reflect the results of previously completed writes performed by other
shader invocations. When writing a variable declared as coherent, the values written will be reflected in
subsequent coherent reads performed by other shader invocations. As described in section 7.11 “Shader
Memory Access” of the OpenGL Specification, shader memory reads and writes complete in a largely
undefined order. The built-in function memoryBarrier() can be used if needed to guarantee the
completion and relative ordering of memory accesses performed by a single shader invocation.
When accessing memory using variables not declared as coherent, the memory accessed by a shader may
be cached by the implementation to service future accesses to the same address. Memory stores may be
cached in such a way that the values written might not be visible to other shader invocations accessing the
same memory. The implementation may cache the values fetched by memory reads and return the same
values to any shader invocation accessing the same memory, even if the underlying memory has been
modified since the first memory read. While variables not declared as coherent might not be useful for
communicating between shader invocations, using non-coherent accesses may result in higher
performance.
Memory accesses to image variables declared using the volatile qualifier must treat the underlying
memory as though it could be read or written at any point during shader execution by some source other
than the executing shader invocation. When a volatile variable is read, its value must be re-fetched from
the underlying memory, even if the shader invocation performing the read had previously fetched its value
from the same memory. When a volatile variable is written, its value must be written to the underlying
memory, even if the compiler can conclusively determine that its value will be overwritten by a
subsequent write. Since the external source reading or writing a volatile variable may be another shader
invocation, variables declared as volatile are automatically treated as coherent.
Memory accesses to image variables declared using the restrict qualifier may be compiled assuming that
the variable used to perform the memory access is the only way to access the underlying memory using
the shader stage in question. This allows the compiler to coalesce or reorder loads and stores using
restrict-qualified image variables in ways that wouldn't be permitted for image variables not so qualified,
because the compiler can assume that the underlying image won't be read or written by other code.
Applications are responsible for ensuring that image memory referenced by variables qualified with
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4 Variables and Types
restrict will not be referenced using other variables in the same scope; otherwise, accesses to restrict-
qualified variables will have undefined results.
Memory accesses to image variables declared using the readonly qualifier may only read the underlying
memory, which is treated as read-only memory and cannot be written to. It is a compile-time error to pass
an image variable qualified with readonly to imageStore() or other built-in functions that modify image
memory.
Memory accesses to image variables declared using the writeonly qualifier may only write the underlying
memory; the underlying memory cannot be read. It is a compile-time error to pass an image variable
qualified with writeonly to imageLoad() or other built-in functions that read image memory. A variable
could be qualified as both readonly and writeonly, disallowing both read and write, but still be passed to
imageSize() to have the size queried.
The memory qualifiers coherent, volatile, restrict, readonly, and writeonly may be used in the
declaration of buffer variables (i.e., members of shader storage blocks). When a buffer variable is
declared with a memory qualifier, the behavior specified for memory accesses involving image variables
described above applies identically to memory accesses involving that buffer variable. It is a compile-
time error to assign to a buffer variable qualified with readonly or to read from a buffer variable qualified
with writeonly.
Additionally, memory qualifiers may also be used in the declaration of shader storage blocks. When a
block declaration is qualified with a memory qualifier, it is as if all of its members were declared with the
same memory qualifier. For example, the block declaration
coherent buffer Block {
readonly vec4 member1;
vec4 member2;
};
is equivalent to
buffer Block {
coherent readonly vec4 member1;
coherent vec4 member2;
};
Memory qualifiers are only supported in the declarations of image variables, buffer variables, and shader
storage blocks; it is an error to use such qualifiers in any other declarations.
The values of image variables qualified with coherent, volatile, restrict, readonly, or writeonly may not
be passed to functions whose formal parameters lack such qualifiers. (See section 6.1 “Function
Definitions” for more detail on function calling.) It is legal to have additional qualifiers on a formal
parameter, but not to have fewer.
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4 Variables and Types
vec4 funcA(restrict image2D a) { ... }
vec4 funcB(image2D a) { ... }
layout(rgba32f) uniform image2D img1;
layout(rgba32f) coherent uniform image2D img2;
funcA(img1); // OK, adding "restrict" is allowed
funcB(img2); // illegal, stripping "coherent" is not
Layout qualifiers cannot be used on formal function parameters, but they are not included in parameter
matching.
Note that the use of const in an image variable declaration is qualifying the const-ness of variable being
declared, not the image it refers to: The qualifier readonly qualifies the image memory (as accessed
through that variable) while const qualifiers the variable itself.
4.11 Order and Repetition of Qualification
When multiple qualifiers are present in a variable or parameter declaration, they may appear in any order,
but they must all appear before the type. The layout qualifier is the only qualifier that can appear more
than once. Further, a declaration can have at most one storage qualifier, at most one auxiliary storage
qualifier, and at most one interpolation qualifier. If inout is used, neither in nor out may be used.
Multiple memory qualifiers can be used. Any violation of these rules will cause a compile-time error.
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5 Operators and Expressions
5.1 Operators
The OpenGL Shading Language has the following operators.
Precedence Operator Class Operators Associativity
1 (highest) parenthetical grouping ( ) NA
2
array subscript
function call and constructor structure
field or method selector, swizzle
post fix increment and decrement
[ ]
( )
.
++ --
Left to Right
3
prefix increment and decrement
unary
++ --
+ - ~ !
Right to Left
4 multiplicative * / % Left to Right
5 additive + - Left to Right
6 bit-wise shift << >> Left to Right
7 relational < > <= >= Left to Right
8 equality == != Left to Right
9 bit-wise and &Left to Right
10 bit-wise exclusive or ^Left to Right
11 bit-wise inclusive or |Left to Right
12 logical and && Left to Right
13 logical exclusive or ^^ Left to Right
14 logical inclusive or | | Left to Right
15 selection ? : Right to Left
16
Assignment
arithmetic assignments
=
+= -=
*= /=
%= <<= >>=
&= ^= |=
Right to Left
17 (lowest) sequence ,Left to Right
There is no address-of operator nor a dereference operator. There is no typecast operator; constructors
are used instead.
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5 Operators and Expressions
5.2 Array Operations
These are now described in section 5.7 “Structure and Array Operations”.
5.3 Function Calls
If a function returns a value, then a call to that function may be used as an expression, whose type will be
the type that was used to declare or define the function.
Function definitions and calling conventions are discussed in section 6.1 “Function Definitions” .
5.4 Constructors
Constructors use the function call syntax, where the function name is a type, and the call makes an object
of that type. Constructors are used the same way in both initializers and expressions. (See section 9
“Shading Language Grammar” for details.) The parameters are used to initialize the constructed value.
Constructors can be used to request a data type conversion to change from one scalar type to another
scalar type, or to build larger types out of smaller types, or to reduce a larger type to a smaller type.
In general, constructors are not built-in functions with predetermined prototypes. For arrays and
structures, there must be exactly one argument in the constructor for each element or member. For the
other types, the arguments must provide a sufficient number of components to perform the initialization,
and it is a compile-time error to include so many arguments that they cannot all be used. Detailed rules
follow. The prototypes actually listed below are merely a subset of examples.
5.4.1 Conversion and Scalar Constructors
Converting between scalar types is done as the following prototypes indicate:
int(uint) // converts an unsigned integer to a signed integer
int(bool) // converts a Boolean value to an int
int(float) // converts a float value to an int
int(double) // converts a double value to a signed integer
uint(int) // converts a signed integer value to an unsigned integer
uint(bool) // converts a Boolean value to an unsigned integer
uint(float) // converts a float value to an unsigned integer
uint(double) // converts a double value to an unsigned integer
bool(int) // converts a signed integer value to a Boolean
bool(uint) // converts an unsigned integer value to a Boolean value
bool(float) // converts a float value to a Boolean
bool(double) // converts a double value to a Boolean
float(int) // converts a signed integer value to a float
float(uint) // converts an unsigned integer value to a float value
float(bool) // converts a Boolean value to a float
float(double)// converts a double value to a float
double(int) // converts a signed integer value to a double
double(uint) // converts an unsigned integer value to a double
double(bool) // converts a Boolean value to a double
double(float)// converts a float value to a double
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When constructors are used to convert any floating-point type to an integer type, the fractional part of the
floating-point value is dropped. It is undefined to convert a negative floating-point value to an uint.
When a constructor is used to convert any integer or floating-point type to a bool, 0 and 0.0 are converted
to false, and non-zero values are converted to true. When a constructor is used to convert a bool to any
integer or floating-point type, false is converted to 0 or 0.0, and true is converted to 1 or 1.0.
The constructor int(uint) preserves the bit pattern in the argument, which will change the argument's
value if its sign bit is set. The constructor uint(int) preserves the bit pattern in the argument, which will
change its value if it is negative.
Identity constructors, like float(float) are also legal, but of little use.
Scalar constructors with non-scalar parameters can be used to take the first element from a non-scalar.
For example, the constructor float(vec3) will select the first component of the vec3 parameter.
5.4.2 Vector and Matrix Constructors
Constructors can be used to create vectors or matrices from a set of scalars, vectors, or matrices. This
includes the ability to shorten vectors.
If there is a single scalar parameter to a vector constructor, it is used to initialize all components of the
constructed vector to that scalar’s value. If there is a single scalar parameter to a matrix constructor, it is
used to initialize all the components on the matrix’s diagonal, with the remaining components initialized
to 0.0.
If a vector is constructed from multiple scalars, one or more vectors, or one or more matrices, or a mixture
of these, the vector's components will be constructed in order from the components of the arguments. The
arguments will be consumed left to right, and each argument will have all its components consumed, in
order, before any components from the next argument are consumed. Similarly for constructing a matrix
from multiple scalars or vectors, or a mixture of these. Matrix components will be constructed and
consumed in column major order. In these cases, there must be enough components provided in the
arguments to provide an initializer for every component in the constructed value. It is a compile-time
error to provide extra arguments beyond this last used argument.
If a matrix is constructed from a matrix, then each component (column i, row j) in the result that has a
corresponding component (column i, row j) in the argument will be initialized from there. All other
components will be initialized to the identity matrix. If a matrix argument is given to a matrix constructor,
it is a compile-time error to have any other arguments.
If the basic type (bool, int, float, or double) of a parameter to a constructor does not match the basic type
of the object being constructed, the scalar construction rules (above) are used to convert the parameters.
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Some useful vector constructors are as follows:
vec3(float) // initializes each component of the vec3 with the float
vec4(ivec4) // makes a vec4 with component-wise conversion
vec4(mat2) // the vec4 is column 0 followed by column 1
vec2(float, float) // initializes a vec2 with 2 floats
ivec3(int, int, int) // initializes an ivec3 with 3 ints
bvec4(int, int, float, float) // uses 4 Boolean conversions
vec2(vec3) // drops the third component of a vec3
vec3(vec4) // drops the fourth component of a vec4
vec3(vec2, float) // vec3.x = vec2.x, vec3.y = vec2.y, vec3.z = float
vec3(float, vec2) // vec3.x = float, vec3.y = vec2.x, vec3.z = vec2.y
vec4(vec3, float)
vec4(float, vec3)
vec4(vec2, vec2)
Some examples of these are:
vec4 color = vec4(0.0, 1.0, 0.0, 1.0);
vec4 rgba = vec4(1.0); // sets each component to 1.0
vec3 rgb = vec3(color); // drop the 4th component
To initialize the diagonal of a matrix with all other elements set to zero:
mat2(float)
mat3(float)
mat4(float)
That is, result[i][j] is set to the float argument for all i = j and set to 0 for all
i≠j.
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To initialize a matrix by specifying vectors or scalars, the components are assigned to the matrix elements
in column-major order.
mat2(vec2, vec2); // one column per argument
mat3(vec3, vec3, vec3); // one column per argument
mat4(vec4, vec4, vec4, vec4); // one column per argument
mat3x2(vec2, vec2, vec2); // one column per argument
dmat2(dvec2, dvec2);
dmat3(dvec3, dvec3, dvec3);
dmat4(dvec4, dvec4, dvec4, dvec4);
mat2(float, float, // first column
float, float); // second column
mat3(float, float, float, // first column
float, float, float, // second column
float, float, float); // third column
mat4(float, float, float, float, // first column
float, float, float, float, // second column
float, float, float, float, // third column
float, float, float, float); // fourth column
mat2x3(vec2, float, // first column
vec2, float); // second column
dmat2x4(dvec3, double, // first column
double, dvec3) // second column
A wide range of other possibilities exist, to construct a matrix from vectors and scalars, as long as enough
components are present to initialize the matrix. To construct a matrix from a matrix:
mat3x3(mat4x4); // takes the upper-left 3x3 of the mat4x4
mat2x3(mat4x2); // takes the upper-left 2x2 of the mat4x4, last row is 0,0
mat4x4(mat3x3); // puts the mat3x3 in the upper-left, sets the lower right
// component to 1, and the rest to 0
5.4.3 Structure Constructors
Once a structure is defined, and its type is given a name, a constructor is available with the same name to
construct instances of that structure. For example:
struct light {
float intensity;
vec3 position;
};
light lightVar = light(3.0, vec3(1.0, 2.0, 3.0));
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The arguments to the constructor will be used to set the structure's members, in order, using one argument
per member. Each argument must be the same type as the member it sets, or be a type that can be
converted to the member's type according to section 4.1.10 “Implicit Conversions.”
Structure constructors can be used as initializers or in expressions.
5.4.4 Array Constructors
Array types can also be used as constructor names, which can then be used in expressions or initializers.
For example,
const float c[3] = float[3](5.0, 7.2, 1.1);
const float d[3] = float[](5.0, 7.2, 1.1);
float g;
...
float a[5] = float[5](g, 1, g, 2.3, g);
float b[3];
b = float[3](g, g + 1.0, g + 2.0);
There must be exactly the same number of arguments as the size of the array being constructed. If no size
is present in the constructor, then the array is explicitly sized to the number of arguments provided. The
arguments are assigned in order, starting at element 0, to the elements of the constructed array. Each
argument must be the same type as the element type of the array, or be a type that can be converted to the
element type of the array according to section 4.1.10 “Implicit Conversions.”
Arrays of arrays are similarly constructed, but only the outer-most dimension is optional:
vec4 b[2] = ...;
vec4[3][2](b, b, b); // constructor
vec4[][2](b, b, b); // constructor, valid, size deduced
vec4[3][](b, b, b); // compile-time error, invalid type constructed
5.5 Vector and Scalar Components and Length
The names of the components of a vector or scalar are denoted by a single letter. As a notational
convenience, several letters are associated with each component based on common usage of position,
color or texture coordinate vectors. The individual components can be selected by following the variable
name with period ( . ) and then the component name.
The component names supported are:
{x, y, z, w} Useful when accessing vectors that represent points or normals
{r, g, b, a} Useful when accessing vectors that represent colors
{s, t, p, q} Useful when accessing vectors that represent texture coordinates
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The component names x, r, and s are, for example, synonyms for the same (first) component in a vector.
They are also the names of the only component in a scalar.
Note that the third component of the texture coordinate set, r in OpenGL, has been renamed p so as to
avoid the confusion with r (for red) in a color.
Accessing components beyond those declared for the type is a compile-time error so, for example:
vec2 pos;
float height;
pos.x // is legal
pos.z // is illegal
height.x // is legal
height.y // is illegal
The component selection syntax allows multiple components to be selected by appending their names
(from the same name set) after the period ( . ).
vec4 v4;
v4.rgba; // is a vec4 and the same as just using v4,
v4.rgb; // is a vec3,
v4.b; // is a float,
v4.xy; // is a vec2,
v4.xgba; // is illegal - the component names do not come from
// the same set.
The order of the components can be different to swizzle them, or replicated:
vec4 pos = vec4(1.0, 2.0, 3.0, 4.0);
vec4 swiz= pos.wzyx; // swiz = (4.0, 3.0, 2.0, 1.0)
vec4 dup = pos.xxyy; // dup = (1.0, 1.0, 2.0, 2.0)
float f = 1.2;
vec4 dup = f.xxxx; // dup = (1.2, 1.2, 1.2, 1.2)
This notation is more concise than the constructor syntax. To form an r-value, it can be applied to any
expression that results in a vector or scalar r-value. Any resulting vector of any operation must be a valid
vector in the language; hence the following results in a compile-time error:
vec4 f;
vec4 g = pos.xyzwxy.xyzw; // illegal; pos.xyzwxy is non-existent “vec6”
The component group notation can occur on the left hand side of an expression.
vec4 pos = vec4(1.0, 2.0, 3.0, 4.0);
pos.xw = vec2(5.0, 6.0); // pos = (5.0, 2.0, 3.0, 6.0)
pos.wx = vec2(7.0, 8.0); // pos = (8.0, 2.0, 3.0, 7.0)
pos.xx = vec2(3.0, 4.0); // illegal - 'x' used twice
pos.xy = vec3(1.0, 2.0, 3.0); // illegal - mismatch between vec2 and vec3
To form an l-value, swizzling must be applied to an l-value of vector or scalar type, contain no duplicate
components, and it results in an l-value of scalar or vector type, depending on number of components
specified.
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Array subscripting syntax can also be applied to vectors (but not to scalars) to provide numeric indexing.
So in
vec4 pos;
pos[2] refers to the third element of pos and is equivalent to pos.z. This allows variable indexing into a
vector, as well as a generic way of accessing components. Any integer expression can be used as the
subscript. The first component is at index zero. Reading from or writing to a vector using a constant
integral expression with a value that is negative or greater than or equal to the size of the vector results in
a compile-time error. When indexing with non-constant expressions, behavior is undefined if the index is
negative, or greater than or equal to the size of the vector.
The length method may be applied to vectors (but not scalars). The result is the number of components in
the vector. For example,
vec3 v;
const int L = v.length();
sets the constant L to 3. The type returned by .length() on a vector is int, and the value returned is a
constant expression.
5.6 Matrix Components
The components of a matrix can be accessed using array subscripting syntax. Applying a single subscript
to a matrix treats the matrix as an array of column vectors, and selects a single column, whose type is a
vector of the same size as the matrix. The leftmost column is column 0. A second subscript would then
operate on the resulting vector, as defined earlier for vectors. Hence, two subscripts select a column and
then a row.
mat4 m;
m[1] = vec4(2.0); // sets the second column to all 2.0
m[0][0] = 1.0; // sets the upper left element to 1.0
m[2][3] = 2.0; // sets the 4th element of the third column to 2.0
Behavior is undefined when accessing a component outside the bounds of a matrix with a non-constant
expression. It is a compile-time error to access a matrix with a constant expression that is outside the
bounds of the matrix.
The length method may be applied to matrices. The result is the number of columns of the matrix. For
example,
mat3x4 v;
const int L = v.length();
sets the constant L to 3. The type returned by .length() on a matrix is int, and the value returned is a
constant expression.
5.7 Structure and Array Operations
The members of a structure and the length method of an array are selected using the period ( . ).
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In total, only the following operators are allowed to operate on arrays and structures as whole entities:
field selector .
equality == !=
assignment =
indexing (arrays only) [ ]
The equality operators and assignment operator are only allowed if the two operands are same size and
type. Structure types must be of the same declared structure. Both array operands must be explicitly
sized. When using the equality operators, two structures are equal if and only if all the members are
component-wise equal, and two arrays are equal if and only if all the elements are element-wise equal.
Array elements are accessed using the array subscript operator ( [ ] ). An example of accessing an array
element is
diffuseColor += lightIntensity[3] * NdotL;
Array indices start at zero. Array elements are accessed using an expression whose type is int or uint.
Behavior is undefined if a shader subscripts an array with an index less than 0 or greater than or equal to
the size the array was declared with.
Arrays can also be accessed with the method operator ( . ) and the length method to query the size of the
array:
lightIntensity.length() // return the size of the array
5.8 Assignments
Assignments of values to variable names are done with the assignment operator ( = ):
lvalue-expression = rvalue-expression
The lvalue-expression evaluates to an l-value. The assignment operator stores the value of rvalue-
expression into the l-value and returns an r-value with the type and precision of lvalue-expression. The
lvalue-expression and rvalue-expression must have the same type, or the expression must have a type in
the table in section 4.1.10 “Implicit Conversions” that converts to the type of lvalue-expression, in which
case an implicit conversion will be done on the rvalue-expression before the assignment is done. Any
other desired type-conversions must be specified explicitly via a constructor. L-values must be writable.
Variables that are built-in types, entire structures or arrays, structure members, l-values with the field
selector ( . ) applied to select components or swizzles without repeated fields, l-values within parentheses,
and l-values dereferenced with the array subscript operator ( [ ] ) are all l-values. Other binary or unary
expressions, function names, swizzles with repeated fields, and constants cannot be l-values. The ternary
operator (?:) is also not allowed as an l-value. Using an incorrect expression as an l-value results in a
compile-time error.
Expressions on the left of an assignment are evaluated before expressions on the right of the assignment.
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The other assignment operators are
•add into (+=)
•subtract from (-=)
•multiply into (*=)
•divide into (/=)
•modulus into (%=)
•left shift by (<<=)
•right shift by (>>=)
•and into (&=)
•inclusive-or into (|=)
•exclusive-or into (^=)
where the general expression
lvalue op= expression
is equivalent to
lvalue = lvalue op expression
where op is as described below, and the l-value and expression must satisfy the semantic requirements of
both op and equals (=).
Reading a variable before writing (or initializing) it is legal, however the value is undefined.
5.9 Expressions
Expressions in the shading language are built from the following:
•Constants of type bool, all integer types, all floating-point types, all vector types, and all matrix types.
•Constructors of all types.
•Variable names of all types.
•An array, vector, or matrix expression with the length method applied.
•Subscripted array names.
•Function calls that return values.
•Component field selectors and array subscript results.
•Parenthesized expression. Any expression can be parenthesized. Parentheses can be used to group
operations. Operations within parentheses are done before operations across parentheses.
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•The arithmetic binary operators add (+), subtract (-), multiply (*), and divide (/) operate on integer and
floating-point scalars, vectors, and matrices. If the fundamental types in the operands do not match,
then the conversions from section 4.1.10 “Implicit Conversions” are applied to create matching types.
All arithmetic binary operators result in the same fundamental type (signed integer, unsigned integer,
single-precision floating point, or double-precision floating point) as the operands they operate on,
after operand type conversion. After conversion, the following cases are valid
•The two operands are scalars. In this case the operation is applied, resulting in a scalar.
•One operand is a scalar, and the other is a vector or matrix. In this case, the scalar operation is
applied independently to each component of the vector or matrix, resulting in the same size vector
or matrix.
•The two operands are vectors of the same size. In this case, the operation is done component-wise
resulting in the same size vector.
•The operator is add (+), subtract (-), or divide (/), and the operands are matrices with the same
number of rows and the same number of columns. In this case, the operation is done component-
wise resulting in the same size matrix.
•The operator is multiply (*), where both operands are matrices or one operand is a vector and the
other a matrix. A right vector operand is treated as a column vector and a left vector operand as a
row vector. In all these cases, it is required that the number of columns of the left operand is equal
to the number of rows of the right operand. Then, the multiply (*) operation does a linear
algebraic multiply, yielding an object that has the same number of rows as the left operand and the
same number of columns as the right operand. Section 5.10 “Vector and Matrix Operations”
explains in more detail how vectors and matrices are operated on.
All other cases result in a compile-time error.
Dividing by zero does not cause an exception but does result in an unspecified value. Use the built-in
functions dot, cross, matrixCompMult, and outerProduct, to get, respectively, vector dot product,
vector cross product, matrix component-wise multiplication, and the matrix product of a column
vector times a row vector.
•The operator modulus (%) operates on signed or unsigned integer scalars or integer vectors. If the
fundamental types in the operands do not match, then the conversions from section 4.1.10 “Implicit
Conversions” are applied to create matching types. The operands cannot be vectors of differing size;
this is a compile time error. If one operand is a scalar and the other vector, then the scalar is applied
component-wise to the vector, resulting in the same type as the vector. If both are vectors of the same
size, the result is computed component-wise. The resulting value is undefined for any component
computed with a second operand that is zero, while results for other components with non-zero second
operands remain defined. If both operands are non-negative, then the remainder is non-negative.
Results are undefined if one or both operands are negative. The operator modulus (%) is not defined
for any other data types (non-integer types).
•The arithmetic unary operators negate (-), post- and pre-increment and decrement (-- and ++) operate
on integer or floating-point values (including vectors and matrices). All unary operators work
component-wise on their operands. These result with the same type they operated on. For post- and
pre-increment and decrement, the expression must be one that could be assigned to (an l-value). Pre-
increment and pre-decrement add or subtract 1 or 1.0 to the contents of the expression they operate on,
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and the value of the pre-increment or pre-decrement expression is the resulting value of that
modification. Post-increment and post-decrement expressions add or subtract 1 or 1.0 to the contents
of the expression they operate on, but the resulting expression has the expression’s value before the
post-increment or post-decrement was executed.
•The relational operators greater than (>), less than (<), greater than or equal (>=), and less than or
equal (<=) operate only on scalar integer and scalar floating-point expressions. The result is scalar
Boolean. Either the operands’ types must match, or the conversions from section 4.1.10 “Implicit
Conversions” will be applied to obtain matching types. To do component-wise relational comparisons
on vectors, use the built-in functions lessThan, lessThanEqual, greaterThan, and
greaterThanEqual.
•The equality operators equal (==), and not equal (!=) operate on all types. They result in a scalar
Boolean. If the operand types do not match, then there must be a conversion from section 4.1.10
“Implicit Conversions” applied to one operand that can make them match, in which case this
conversion is done. For vectors, matrices, structures, and arrays, all components, members, or
elements of one operand must equal the corresponding components, members, or elements in the other
operand for the operands to be considered equal. To get a vector of component-wise equality results
for vectors, use the built-in functions equal and notEqual.
•The logical binary operators and (&&), or ( | | ), and exclusive or (^^) operate only on two Boolean
expressions and result in a Boolean expression. And (&&) will only evaluate the right hand operand
if the left hand operand evaluated to true. Or ( | | ) will only evaluate the right hand operand if the left
hand operand evaluated to false. Exclusive or (^^) will always evaluate both operands.
•The logical unary operator not (!). It operates only on a Boolean expression and results in a Boolean
expression. To operate on a vector, use the built-in function not.
•The sequence ( , ) operator that operates on expressions by returning the type and value of the right-
most expression in a comma separated list of expressions. All expressions are evaluated, in order,
from left to right.
•The ternary selection operator (?:). It operates on three expressions (exp1 ? exp2 : exp3). This
operator evaluates the first expression, which must result in a scalar Boolean. If the result is true, it
selects to evaluate the second expression, otherwise it selects to evaluate the third expression. Only
one of the second and third expressions is evaluated. The second and third expressions can be any
type, as long their types match, or there is a conversion in section 4.1.10 “Implicit Conversions” that
can be applied to one of the expressions to make their types match. This resulting matching type is the
type of the entire expression.
•The one's complement operator (~). The operand must be of type signed or unsigned integer or integer
vector, and the result is the one's complement of its operand; each bit of each component is
complemented, including any sign bits.
•The shift operators (<<) and (>>). For both operators, the operands must be signed or unsigned
integers or integer vectors. One operand can be signed while the other is unsigned. In all cases, the
resulting type will be the same type as the left operand. If the first operand is a scalar, the second
operand has to be a scalar as well. If the first operand is a vector, the second operand must be a scalar
or a vector, and the result is computed component-wise. The result is undefined if the right operand is
negative, or greater than or equal to the number of bits in the left expression's base type. The value of
E1 << E2 is E1 (interpreted as a bit pattern) left-shifted by E2 bits. The value of E1 >> E2 is E1 right-
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shifted by E2 bit positions. If E1 is a signed integer, the right-shift will extend the sign bit. If E1 is an
unsigned integer, the right-shift will zero-extend.
•The bitwise operators and (&), exclusive-or (^), and inclusive-or (|). The operands must be of type
signed or unsigned integers or integer vectors. The operands cannot be vectors of differing size; this is
a compile-time error. If one operand is a scalar and the other a vector, the scalar is applied
component-wise to the vector, resulting in the same type as the vector. The fundamental types of the
operands (signed or unsigned) must match, and will be the resulting fundamental type. For and (&),
the result is the bitwise-and function of the operands. For exclusive-or (^), the result is the bitwise
exclusive-or function of the operands. For inclusive-or (|), the result is the bitwise inclusive-or
function of the operands.
For a complete specification of the syntax of expressions, see section 9 “Shading Language Grammar.”
5.10 Vector and Matrix Operations
With a few exceptions, operations are component-wise. Usually, when an operator operates on a vector or
matrix, it is operating independently on each component of the vector or matrix, in a component-wise
fashion. For example,
vec3 v, u;
float f;
v = u + f;
will be equivalent to
v.x = u.x + f;
v.y = u.y + f;
v.z = u.z + f;
And
vec3 v, u, w;
w = v + u;
will be equivalent to
w.x = v.x + u.x;
w.y = v.y + u.y;
w.z = v.z + u.z;
and likewise for most operators and all integer and floating-point vector and matrix types. The exceptions
are matrix multiplied by vector, vector multiplied by matrix, and matrix multiplied by matrix. These do
not operate component-wise, but rather perform the correct linear algebraic multiply.
vec3 v, u;
mat3 m;
u = v * m;
is equivalent to
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u.x = dot(v, m[0]); // m[0] is the left column of m
u.y = dot(v, m[1]); // dot(a,b) is the inner (dot) product of a and b
u.z = dot(v, m[2]);
And
u = m * v;
is equivalent to
u.x = m[0].x * v.x + m[1].x * v.y + m[2].x * v.z;
u.y = m[0].y * v.x + m[1].y * v.y + m[2].y * v.z;
u.z = m[0].z * v.x + m[1].z * v.y + m[2].z * v.z;
And
mat3 m, n, r;
r = m * n;
is equivalent to
r[0].x = m[0].x * n[0].x + m[1].x * n[0].y + m[2].x * n[0].z;
r[1].x = m[0].x * n[1].x + m[1].x * n[1].y + m[2].x * n[1].z;
r[2].x = m[0].x * n[2].x + m[1].x * n[2].y + m[2].x * n[2].z;
r[0].y = m[0].y * n[0].x + m[1].y * n[0].y + m[2].y * n[0].z;
r[1].y = m[0].y * n[1].x + m[1].y * n[1].y + m[2].y * n[1].z;
r[2].y = m[0].y * n[2].x + m[1].y * n[2].y + m[2].y * n[2].z;
r[0].z = m[0].z * n[0].x + m[1].z * n[0].y + m[2].z * n[0].z;
r[1].z = m[0].z * n[1].x + m[1].z * n[1].y + m[2].z * n[1].z;
r[2].z = m[0].z * n[2].x + m[1].z * n[2].y + m[2].z * n[2].z;
and similarly for other sizes of vectors and matrices.
5.11 Out-of-Bounds Accesses
In the subsections described above for array, vector, matrix and structure accesses, any out-of-bounds
access produced undefined behavior. However, if robust buffer access is enabled via the OpenGL API,
such accesses will be bound within the memory extent of the active program. It will not be possible to
access memory from other programs, and accesses will not result in abnormal program termination. Out-
of-bounds reads return undefined values, which include values from other variables of the active program
or zero. Out-of-bounds writes may be discarded or overwrite other variables of the active program,
depending on the value of the computed index and how this relates to the extent of the active program's
memory. Applications that require defined behavior for out-of-bounds accesses should range check all
computed indices before dereferencing an array.
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The fundamental building blocks of the OpenGL Shading Language are:
•statements and declarations
•function definitions
•selection (if-else and switch-case-default)
•iteration (for, while, and do-while)
•jumps (discard, return, break, and continue)
The overall structure of a shader is as follows
translation-unit:
global-declaration
translation-unit global-declaration
global-declaration:
function-definition
declaration
That is, a shader is a sequence of declarations and function bodies. Function bodies are defined as
function-definition:
function-prototype { statement-list }
statement-list:
statement
statement-list statement
statement:
compound-statement
simple-statement
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Curly braces are used to group sequences of statements into compound statements.
compound-statement:
{ statement-list }
simple-statement:
declaration-statement
expression-statement
selection-statement
iteration-statement
jump-statement
Simple declaration, expression, and jump statements end in a semi-colon.
This above is slightly simplified, and the complete grammar specified in section 9 “Shading Language
Grammar” should be used as the definitive specification.
Declarations and expressions have already been discussed.
6.1 Function Definitions
As indicated by the grammar above, a valid shader is a sequence of global declarations and function
definitions. A function is declared as the following example shows:
// prototype
returnType functionName (type0 arg0, type1 arg1, ..., typen argn);
and a function is defined like
// definition
returnType functionName (type0 arg0, type1 arg1, ..., typen argn)
{
// do some computation
return returnValue;
}
where returnType must be present and include a type. If the type of returnValue does not match
returnType, there must be an implicit conversion in section 4.1.10 “Implicit Conversions” that converts
the type of returnValue to returnType, or a compile-time error will result.
Each of the typeN must include a type and can optionally include parameter qualifiers. The formal
argument names (args above) in the declarations are optional for both the declaration and definition
forms.
A function is called by using its name followed by a list of arguments in parentheses.
Arrays are allowed as arguments and as the return type. In both cases, the array must be explicitly sized.
An array is passed or returned by using just its name, without brackets, and the size of the array must
match the size specified in the function's declaration.
Structures are also allowed as argument types. The return type can also be structure.
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See section 9 “Shading Language Grammar” for the definitive reference on the syntax to declare and
define functions.
All functions must be either declared with a prototype or defined with a body before they are called. For
example:
float myfunc (float f, // f is an input parameter
out float g); // g is an output parameter
Functions that return no value must be declared as void. A void function can only use return without a
return argument, even if the return argument has void type. Return statements only accept values:
void func1() { }
void func2() { return func1(); } // illegal return statement
Only a precision qualifier is allowed on the return type of a function. Formal parameters can have
parameter, precision, and memory qualifiers, but no other qualifiers.
Functions that accept no input arguments need not use void in the argument list because prototypes (or
definitions) are required and therefore there is no ambiguity when an empty argument list "( )" is declared.
The idiom “(void)” as a parameter list is provided for convenience.
Function names can be overloaded. The same function name can be used for multiple functions, as long
as the parameter types differ. If a function name is declared twice with the same parameter types, then the
return types and all qualifiers must also match, and it is the same function being declared. For example,
vec4 f(in vec4 x, out vec4 y); // (A)
vec4 f(in vec4 x, out uvec4 y); // (B) okay, different argument type
vec4 f(in ivec4 x, out dvec4 y); // (C) okay, different argument type
int f(in vec4 x, out vec4 y); // error, only return type differs
vec4 f(in vec4 x, in vec4 y); // error, only qualifier differs
vec4 f(const in vec4 x, out vec4 y); // error, only qualifier differs
When function calls are resolved, an exact type match for all the arguments is sought. If an exact match is
found, all other functions are ignored, and the exact match is used. If no exact match is found, then the
implicit conversions in section 4.1.10 “Implicit Conversions” will be applied to find a match.
Mismatched types on input parameters (in or inout or default) must have a conversion from the calling
argument type to the formal parameter type. Mismatched types on output parameters (out or inout) must
have a conversion from the formal parameter type to the calling argument type.
If implicit conversions can be used to find more than one matching function, a single best-matching
function is sought. To determine a best match, the conversions between calling argument and formal
parameter types are compared for each function argument and pair of matching functions. After these
comparisons are performed, each pair of matching functions are compared. A function declaration A is
considered a better match than function declaration B if
•for at least one function argument, the conversion for that argument in A is better than the
corresponding conversion in B; and
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•there is no function argument for which the conversion in B is better than the corresponding
conversion in A.
If a single function declaration is considered a better match than every other matching function
declaration, it will be used. Otherwise, a compile-time semantic error for an ambiguous overloaded
function call occurs.
To determine whether the conversion for a single argument in one match is better than that for another
match, the following rules are applied, in order:
1. An exact match is better than a match involving any implicit conversion.
2. A match involving an implicit conversion from float to double is better than a match involving
any other implicit conversion.
3. A match involving an implicit conversion from either int or uint to float is better than a match
involving an implicit conversion from either int or uint to double.
If none of the rules above apply to a particular pair of conversions, neither conversion is considered better
than the other.
For the example function prototypes (A), (B), and (C) above, the following examples show how the rules
apply to different sets of calling argument types:
f(vec4, vec4); // exact match of vec4 f(in vec4 x, out vec4 y)
f(vec4, uvec4); // exact match of vec4 f(in vec4 x, out uvec4 y)
f(vec4, ivec4); // matched to vec4 f(in vec4 x, out vec4 y)
// (C) not relevant, can't convert vec4 to
// ivec4. (A) better than (B) for 2nd
// argument (rule 3), same on first argument.
f(ivec4, vec4); // NOT matched. All three match by implicit
// conversion. (C) is better than (A) and (B)
// on the first argument. (A) is better than
// (B) and (C).
User-defined functions can have multiple declarations, but only one definition. A shader can redefine
built-in functions. If a built-in function is redeclared in a shader (i.e., a prototype is visible) before a call
to it, then the linker will only attempt to resolve that call within the set of shaders that are linked with it.
The function main is used as the entry point to a shader executable. A shader need not contain a function
named main, but one shader in a set of shaders linked together to form a single shader executable must, or
a link-time error results. This function takes no arguments, returns no value, and must be declared as type
void:
void main()
{
...
}
The function main can contain uses of return. See section 6.4 “Jumps” for more details.
It is a compile-time or link-time error to declare or define a function main with any other parameters or
return type.
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6.1.1 Function Calling Conventions
Functions are called by value-return. This means input arguments are copied into the function at call time,
and output arguments are copied back to the caller before function exit. Because the function works with
local copies of parameters, there are no issues regarding aliasing of variables within a function. To
control what parameters are copied in and/or out through a function definition or declaration:
•The keyword in is used as a qualifier to denote a parameter is to be copied in, but not copied out.
•The keyword out is used as a qualifier to denote a parameter is to be copied out, but not copied in.
This should be used whenever possible to avoid unnecessarily copying parameters in.
•The keyword inout is used as a qualifier to denote the parameter is to be both copied in and copied
out. It means the same thing as specifying both in and out.
•A function parameter declared with no such qualifier means the same thing as specifying in.
All arguments are evaluated at call time, exactly once, in order, from left to right. Evaluation of an in
parameter results in a value that is copied to the formal parameter. Evaluation of an out parameter results
in an l-value that is used to copy out a value when the function returns. Evaluation of an inout parameter
results in both a value and an l-value; the value is copied to the formal parameter at call time and the l-
value is used to copy out a value when the function returns.
The order in which output parameters are copied back to the caller is undefined.
If the function matching described in the previous section required argument type conversions, these
conversions are applied at copy-in and copy-out times.
In a function, writing to an input-only parameter is allowed. Only the function’s copy is modified. This
can be prevented by declaring a parameter with the const qualifier.
When calling a function, expressions that do not evaluate to l-values cannot be passed to parameters
declared as out or inout, or a compile-time error results
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function-prototype :
precision-qualifier type function-name(parameter-qualifiers precision-qualifier type name
array-specifier, ... )
type :
any basic type, array type, structure name, or structure definition
parameter-qualifiers :
empty
list of parameter-qualifier
parameter-qualifier :
const
in
out
inout
precise
memory qualifier
precision qualifier
name :
empty
identifier
array-specifier :
empty
[ integral-constant-expression ]
The const qualifier cannot be used with out or inout, or a compile-time error results. The above is used
both for function declarations (i.e., prototypes) and for function definitions. Hence, function definitions
can have unnamed arguments.
Recursion is not allowed, not even statically. Static recursion is present if the static function-call graph of
a program contains cycles. This includes all potential function calls through variables declared as
subroutine uniform (described below). It is a compile-time or link-time error if a single compilation
unit (shader) contains either static recursion or the potential for recursion through subroutine variables.
6.1.2 Subroutines
Subroutines provide a mechanism allowing shaders to be compiled in a manner where the target of one or
more function calls can be changed at run-time without requiring any shader recompilation. For example,
a single shader may be compiled with support for multiple illumination algorithms to handle different
kinds of lights or surface materials. An application using such a shader may switch illumination
algorithms by changing the value of its subroutine uniforms. To use subroutines, a subroutine type is
declared, one or more functions are associated with that subroutine type, and a subroutine variable of that
type is declared. The function currently assigned to the variable function is then called by using function
calling syntax replacing a function name with the name of the subroutine variable. Subroutine variables
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are uniforms, and are assigned to specific functions only through commands (UniformSubroutinesuiv) in
the OpenGL API.
Subroutine types are declared using a statement similar to a function declaration, with the subroutine
keyword, as follows:
subroutine returnType subroutineTypeName(type0 arg0, type1 arg1,
..., typen argn);
As with function declarations, the formal argument names (args above) are optional. Functions are
associated with subroutine types of matching declarations by defining the function with the subroutine
keyword and a list of subroutine types the function matches:
subroutine(subroutineTypeName0, ..., subroutineTypeNameN)
returnType functionName(type0 arg0, type1 arg1, ..., typen argn)
{ ... } // function body
It is a compile-time error if arguments and return type don't match between the function and each
associated subroutine type.
Functions declared with subroutine must include a body. An overloaded function cannot be declared
with subroutine; a program will fail to compile or link if any shader or stage contains two or more
functions with the same name if the name is associated with a subroutine type.
A function declared with subroutine can also be called directly with a static use of functionName, as is
done with non-subroutine function declarations and calls.
Subroutine type variables are required to be subroutine uniforms, and are declared with a specific
subroutine type in a subroutine uniform variable declaration:
subroutine uniform subroutineTypeName subroutineVarName;
Subroutine uniform variables are called the same way functions are called. When a subroutine variable
(or an element of a subroutine variable array) is associated with a particular function, all function calls
through that variable will call that particular function.
Unlike other uniform variables, subroutine uniform variables are scoped to the shader execution stage the
variable is declared in.
Subroutine variables may be declared as explicitly-sized arrays, which can be indexed only with
dynamically uniform expressions.
6.2 Selection
Conditional control flow in the shading language is done by either if, if-else, or switch statements:
selection-statement :
if ( bool-expression ) statement
if ( bool-expression ) statement else statement
switch ( init-expression ) { switch-statement-listopt }
Where switch-statement-list is a list of zero or more switch-statement and other statements defined by the
language, where switch-statement adds some forms of labels. That is
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switch-statement-list :
switch-statement
switch-statement-list switch-statement
switch-statement :
case constant-expression :
default :
statement
If an if-expression evaluates to true, then the first statement is executed. If it evaluates to false and there
is an else part then the second statement is executed.
Any expression whose type evaluates to a Boolean can be used as the conditional expression bool-
expression. Vector types are not accepted as the expression to if.
Conditionals can be nested.
The type of the init-expression value in a switch statement must be a scalar int or uint. The type of the
constant-expression value in a case label also must be a scalar int or uint. When any pair of these values
is tested for "equal value" and the types do not match, an implicit conversion will be done to convert the
int to a uint (see section 4.1.10 “Implicit Conversions”) before the compare is done. If a case label has a
constant-expression of equal value to init-expression, execution will continue after that label. It is a
compile-time error to have two case label constant-expression of equal value. Otherwise, if there is a
default label, execution will continue after that label. Otherwise, execution skips the rest of the switch
statement. It is a compile-time error to have more than one default. A break statement not nested in a
loop or other switch statement (either not nested or nested only in if or if-else statements) will also skip
the rest of the switch statement. Fall through labels are allowed, but it is a compile-time error to have no
statement between a label and the end of the switch statement. No statements are allowed in a switch
statement before the first case statement.
No case or default labels can be nested inside other flow control nested within their corresponding
switch.
6.3 Iteration
For, while, and do loops are allowed as follows:
for (init-expression; condition-expression; loop-expression)
sub-statement
while (condition-expression)
sub-statement
do
statement
while (condition-expression)
See section 9 “Shading Language Grammar” for the definitive specification of loops.
The for loop first evaluates the init-expression, then the condition-expression. If the condition-
expression evaluates to true, then the body of the loop is executed. After the body is executed, a for loop
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will then evaluate the loop-expression, and then loop back to evaluate the condition-expression, repeating
until the condition-expression evaluates to false. The loop is then exited, skipping its body and skipping
its loop-expression. Variables modified by the loop-expression maintain their value after the loop is
exited, provided they are still in scope. Variables declared in init-expression or condition-expression are
only in scope until the end of the sub-statement of the for loop.
The while loop first evaluates the condition-expression. If true, then the body is executed. This is then
repeated, until the condition-expression evaluates to false, exiting the loop and skipping its body.
Variables declared in the condition-expression are only in scope until the end of the sub-statement of the
while loop.
The do-while loop first executes the body, then executes the condition-expression. This is repeated until
condition-expression evaluates to false, and then the loop is exited.
Expressions for condition-expression must evaluate to a Boolean.
Both the condition-expression and the init-expression can declare and initialize a variable, except in the
do-while loop, which cannot declare a variable in its condition-expression. The variable’s scope lasts
only until the end of the sub-statement that forms the body of the loop.
Loops can be nested.
Non-terminating loops are allowed. The consequences of very long or non-terminating loops are platform
dependent.
6.4 Jumps
These are the jumps:
jump_statement:
continue;
break;
return;
return expression;
discard; // in the fragment shader language only
There is no “goto” nor other non-structured flow of control.
The continue jump is used only in loops. It skips the remainder of the body of the inner most loop of
which it is inside. For while and do-while loops, this jump is to the next evaluation of the loop
condition-expression from which the loop continues as previously defined. For for loops, the jump is to
the loop-expression, followed by the condition-expression.
The break jump can also be used only in loops and switch statements. It is simply an immediate exit of
the inner-most loop or switch statements containing the break. No further execution of condition-
expression, loop-expression, or switch-statement is done.
The discard keyword is only allowed within fragment shaders. It can be used within a fragment shader to
abandon the operation on the current fragment. This keyword causes the fragment to be discarded and no
updates to any buffers will occur. Control flow exits the shader, and subsequent implicit or explicit
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derivatives are undefined when this exit is non-uniform. It would typically be used within a conditional
statement, for example:
if (intensity < 0.0)
discard;
A fragment shader may test a fragment’s alpha value and discard the fragment based on that test.
However, it should be noted that coverage testing occurs after the fragment shader runs, and the coverage
test can change the alpha value.
The return jump causes immediate exit of the current function. If it has expression then that is the return
value for the function.
The function main can use return. This simply causes main to exit in the same way as when the end of
the function had been reached. It does not imply a use of discard in a fragment shader. Using return in
main before defining outputs will have the same behavior as reaching the end of main before defining
outputs.
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7.1 Built-In Language Variables
Some OpenGL operations occur in fixed functionality and need to provide values to or receive values
from shader executables. Shaders communicate with fixed-function OpenGL pipeline stages, and
optionally with other shader executables, through the use of built-in input and output variables.
In the compute language, the built-in variables are declared as follows:
// work group dimensions
in uvec3 gl_NumWorkGroups;
const uvec3 gl_WorkGroupSize;
// work group and invocation IDs
in uvec3 gl_WorkGroupID;
in uvec3 gl_LocalInvocationID;
// derived variables
in uvec3 gl_GlobalInvocationID;
in uint gl_LocalInvocationIndex;
In the vertex language, the built-ins are intrinsically declared as:
in int gl_VertexID;
in int gl_InstanceID;
out gl_PerVertex {
vec4 gl_Position;
float gl_PointSize;
float gl_ClipDistance[];
};
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In the geometry language, the built-in variables are intrinsically declared as:
in gl_PerVertex {
vec4 gl_Position;
float gl_PointSize;
float gl_ClipDistance[];
} gl_in[];
in int gl_PrimitiveIDIn;
in int gl_InvocationID;
out gl_PerVertex {
vec4 gl_Position;
float gl_PointSize;
float gl_ClipDistance[];
};
out int gl_PrimitiveID;
out int gl_Layer;
out int gl_ViewportIndex;
In the tessellation control language, built-in variables are intrinsically declared as:
in gl_PerVertex {
vec4 gl_Position;
float gl_PointSize;
float gl_ClipDistance[];
} gl_in[gl_MaxPatchVertices];
in int gl_PatchVerticesIn;
in int gl_PrimitiveID;
in int gl_InvocationID;
out gl_PerVertex {
vec4 gl_Position;
float gl_PointSize;
float gl_ClipDistance[];
} gl_out[];
patch out float gl_TessLevelOuter[4];
patch out float gl_TessLevelInner[2];
In the tessellation evaluation language, built-in variables are intrinsically declared as:
in gl_PerVertex {
vec4 gl_Position;
float gl_PointSize;
float gl_ClipDistance[];
} gl_in[gl_MaxPatchVertices];
in int gl_PatchVerticesIn;
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in int gl_PrimitiveID;
in vec3 gl_TessCoord;
patch in float gl_TessLevelOuter[4];
patch in float gl_TessLevelInner[2];
out gl_PerVertex {
vec4 gl_Position;
float gl_PointSize;
float gl_ClipDistance[];
};
In the fragment language, built-in variables are intrinsically declared as:
in vec4 gl_FragCoord;
in bool gl_FrontFacing;
in float gl_ClipDistance[];
in vec2 gl_PointCoord;
in int gl_PrimitiveID;
in int gl_SampleID;
in vec2 gl_SamplePosition;
in int gl_SampleMaskIn[];
in int gl_Layer;
in int gl_ViewportIndex;
out float gl_FragDepth;
out int gl_SampleMask[];
Each of the above variables is discussed below.
The built-in variable gl_NumWorkGroups is a compute-shader input variable containing the total number
of global work items in each dimension of the work group that will execute the compute shader. Its
content is equal to the values specified in the num_groups_x, num_groups_y, and num_groups_z
parameters passed to the DispatchCompute API entry point.
The built-in constant gl_WorkGroupSize is a compute-shader constant containing the local work-group
size of the shader. The size of the work group in the X, Y, and Z dimensions is stored in the x, y, and z
components. The constants values in gl_WorkGroupSize will match those specified in the required
local_size_x, local_size_y, and local_size_z layout qualifiers for the current shader. This is a constant so
that it can be used to size arrays of memory that can be shared within the local work group. It is a
compile-time error to use gl_WorkGroupSize in a shader that does not declare a fixed local group size, or
before that shader has declared a fixed local group size, using local_size_x, local_size_y, and
local_size_z. When a size is given for some of these identifiers, but not all, the corresponding
gl_WorkGroupSize will have a size of 1.
The built-in variable gl_WorkGroupID is a compute-shader input variable containing the three-
dimensional index of the global work group that the current invocation is executing in. The possible
values range across the parameters passed into DispatchCompute, i.e., from (0, 0, 0) to
(gl_NumWorkGroups.x - 1, gl_NumWorkGroups.y - 1, gl_NumWorkGroups.z -1).
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The built-in variable gl_LocalInvocationID is a compute-shader input variable containing the t-
dimensional index of the local work group within the global work group that the current invocation is
executing in. The possible values for this variable range across the local work group size, i.e., (0,0,0) to
(gl_WorkGroupSize.x - 1, gl_WorkGroupSize.y - 1, gl_WorkGroupSize.z - 1).
The built-in variable gl_GlobalInvocationID is a compute shader input variable containing the global
index of the current work item. This value uniquely identifies this invocation from all other invocations
across all local and global work groups initiated by the current DispatchCompute call. This is computed
as:
gl_GlobalInvocationID =
gl_WorkGroupID * gl_WorkGroupSize + gl_LocalInvocationID;
The built-in variable gl_LocalInvocationIndex is a compute shader input variable that contains the one-
dimensional representation of the gl_LocalInvocationID. This is useful for uniquely identifying a unique
region of shared memory within the local work group for this invocation to use. This is computed as:
gl_LocalInvocationIndex =
gl_LocalInvocationID.z * gl_WorkGroupSize.x * gl_WorkGroupSize.y +
gl_LocalInvocationID.y * gl_WorkGroupSize.x +
gl_LocalInvocationID.x;
The variable gl_VertexID is a vertex language input variable that holds an integer index for the vertex, as
defined under “Shader Inputs” in section 11.1.3.9 “Shader Inputs” in the OpenGL Graphics System
Specification. While the variable gl_VertexID is always present, its value is not always defined.
The variable gl_InstanceID is a vertex language input variable that holds the instance number of the
current primitive in an instanced draw call (see “Shader Inputs” in section 11.1.3.9 “Shader Inputs” in the
OpenGL Graphics System Specification). If the current primitive does not come from an instanced draw
call, the value of gl_InstanceID is zero.
As an output variable, gl_Position is intended for writing the homogeneous vertex position. It can be
written at any time during shader execution. This value will be used by primitive assembly, clipping,
culling, and other fixed functionality operations, if present, that operate on primitives after vertex
processing has occurred. Its value is undefined after the vertex processing stage if the vertex shader
executable does not write gl_Position, and it is undefined after geometry processing if the geometry
executable calls EmitVertex() without having written gl_Position since the last EmitVertex() (or hasn't
written it at all). As an input variable, gl_Position reads the output written in the previous shader stage to
gl_Position.
As an output variable, gl_PointSize is intended for a shader to write the size of the point to be rasterized.
It is measured in pixels. If gl_PointSize is not written to, its value is undefined in subsequent pipe stages.
As an input variable, gl_PointSize reads the output written in the previous shader stage to gl_PointSize .
The variable gl_ClipDistance provides the forward compatible mechanism for controlling user clipping.
The element gl_ClipDistance[i] specifies a clip distance for each plane i. A distance of 0 means the
vertex is on the plane, a positive distance means the vertex is inside the clip plane, and a negative distance
means the point is outside the clip plane. The clip distances will be linearly interpolated across the
primitive and the portion of the primitive with interpolated distances less than 0 will be clipped.
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The gl_ClipDistance array is predeclared as unsized and must be sized by the shader either redeclaring it
with a size or indexing it only with integral constant expressions. This needs to size the array to include
all the clip planes that are enabled via the OpenGL API; if the size does not include all enabled planes,
results are undefined. The size can be at most gl_MaxClipDistances. The number of varying components
(see gl_MaxVaryingComponents) consumed by gl_ClipDistance will match the size of the array, no
matter how many planes are enabled. The shader must also set all values in gl_ClipDistance that have
been enabled via the OpenGL API, or results are undefined. Values written into gl_ClipDistance for
planes that are not enabled have no effect.
As an output variable, gl_ClipDistance provides the place for the shader to write these distances. As an
input in all but the fragment language, it reads the values written in the previous shader stage. In the
fragment language, gl_ClipDistance array contains linearly interpolated values for the vertex values
written by a shader to the gl_ClipDistance vertex output variable. Only elements in this array that have
clipping enabled will have defined values.
The output variable gl_PrimitiveID is available only in the geometry language and provides a single
integer that serves as a primitive identifier. This is then available to fragment shaders as the fragment
input gl_PrimitiveID, which will select the written primitive ID from the provoking vertex in the primitive
being shaded. If a fragment shader using gl_PrimitiveID is active and a geometry shader is also active,
the geometry shader must write to gl_PrimitiveID or the fragment shader input gl_PrimitiveID is
undefined. See section 11.3.4.5 "Geometry Shader Outputs" of the OpenGL Graphics System
Specification for more information.
For tessellation control and evaluation languages the input variable gl_PrimitiveID is filled with the
number of primitives processed by the shader since the current set of rendering primitives was started.
For the fragment language, it is filled with the value written to the gl_PrimitiveID geometry shader output
if a geometry shader is present. Otherwise, it is assigned in the same manner as with tessellation control
and evaluation shaders.
The geometry language input variable gl_PrimitiveIDIn behaves identically to the tessellation control and
evaluation language input variable gl_PrimitiveID.
The input variable gl_InvocationID is available only in the tessellation control and geometry languages.
In the tessellation control shader, it identifies the number of the output patch vertex assigned to the
tessellation control shader invocation. In the geometry shader, it identifies the invocation number
assigned to the geometry shader invocation. In both cases, gl_InvocationID is assigned integer values in
the range [0, N-1], where N is the number of output patch vertices or geometry shader invocations per
primitive.
The variable gl_Layer is available as an output variable in the geometry language and an input variable in
the fragment language. In the geometry language, it is used to select a specific layer (or face and layer of
a cube map) of a multi-layer framebuffer attachment. The actual layer used will come from one of the
vertices in the primitive being shaded. Which vertex the layer comes from is discussed in section 11.3.4.6
“Layer and Viewport Selection” of the OpenGL Specification. It might be undefined, so it is best to write
the same layer value for all vertices of a primitive. If a shader statically assigns a value to gl_Layer,
layered rendering mode is enabled. See section 11.3.4.5 “Geometry Shader Outputs”and section 9.4.9
“Layered Framebuffers” of the OpenGL Graphics System Specification for more information. If a shader
statically assigns a value to gl_Layer, and there is an execution path through the shader that does not set
gl_Layer, then the value of gl_Layer is undefined for executions of the shader that take that path.
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The output variable gl_Layer takes on a special value when used with an array of cube map textures.
Instead of only referring to the layer, it is used to select a cube map face and a layer. Setting gl_Layer to
the value layer*6+face will render to face face of the cube defined in layer layer. The face values are
defined in Table 9.3 of section 9.4.9 “Layered Framebuffers” of the OpenGL Graphics System
Specification, but repeated below for clarity.
Face Value Resulting Target
0TEXTURE_CUBE_MAP_POSITIVE_X
1TEXTURE_CUBE_MAP_NEGATIVE_X
2TEXTURE_CUBE_MAP_POSITIVE_Y
3TEXTURE_CUBE_MAP_NEGATIVE_Y
4TEXTURE_CUBE_MAP_POSITIVE_Z
5TEXTURE_CUBE_MAP_NEGATIVE_Z
For example, to render to the positive y cube map face located in the 5th layer of the cube map array,
gl_Layer should be set to 5*6+2.
The input variable gl_Layer in the fragment language will have the same value that was written to the
output variable gl_Layer in the geometry language. If the geometry stage does not dynamically assign a
value to gl_Layer, the value of gl_Layer in the fragment stage will be undefined. If the geometry stage
makes no static assignment to gl_Layer, the input value in the fragment stage will be zero. Otherwise, the
fragment stage will read the same value written by the geometry stage, even if that value is out of range.
If a fragment shader contains a static access to gl_Layer, it will count against the implementation defined
limit for the maximum number of inputs to the fragment stage.
The variable gl_ViewportIndex is available as an output variable in the geometry language and an input
variable in the fragment language. In the geometry language, it provides the index of the viewport to
which the next primitive emitted from the geometry shader should be drawn. Primitives generated by
the geometry shader will undergo viewport transformation and scissor testing using the viewport
transformation and scissor rectangle selected by the value of gl_ViewportIndex. The viewport index used
will come from one of the vertices in the primitive being shaded. However, which vertex the viewport
index comes from is implementation-dependent, so it is best to use the same viewport index for all
vertices of the primitive. If a geometry shader does not assign a value to gl_ViewportIndex, viewport
transform and scissor rectangle zero will be used. If a geometry shader statically assigns a value to
gl_ViewportIndex and there is a path through the shader that does not assign a value to gl_ViewportIndex,
the value of gl_ViewportIndex is undefined for executions of the shader that take that path. See section
11.3.4.6 “Layer and Viewport Selection” of the OpenGL Graphics System Specification (Core Profile) for
more information.
The input variable gl_ViewportIndex in the fragment stage will have the same value that was written to the
output variable gl_ViewportIndex in the geometry stage. If the geometry stage does not dynamically
assign to gl_ViewportIndex, the value of gl_ViewportIndex in the fragment shader will be undefined. If
the geometry stage makes no static assignment to gl_ViewportIndex, the fragment stage will read zero.
Otherwise, the fragment stage will read the same value written by the geometry stage, even if that value is
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out of range. If a fragment shader contains a static access to gl_ViewportIndex, it will count against the
implementation defined limit for the maximum number of inputs to the fragment stage.
The variable gl_PatchVerticesIn is available only in the tessellation control and evaluation languages. It
is an integer specifying the number of vertices in the input patch being processed by the shader. A single
tessellation control or evaluation shader can read patches of differing sizes, so the value of
gl_PatchVerticesIn may differ between patches.
The output variables gl_TessLevelOuter[] and gl_TessLevelInner[] are available only in the tessellation
control language. The values written to these variables are assigned to the corresponding outer and inner
tessellation levels of the output patch. They are used by the tessellation primitive generator to control
primitive tessellation and may be read by tessellation evaluation shaders.
The variable gl_TessCoord is available only in the tessellation evaluation language. It specifies a three-
component (u,v,w) vector identifying the position of the vertex being processed by the shader relative to
the primitive being tessellated. Its values will obey the properties
gl_TessCoord.x == 1.0 – (1.0 – gl_TessCoord.x) // two operations performed
gl_TessCoord.y == 1.0 – (1.0 – gl_TessCoord.y) // two operations performed
gl_TessCoord.z == 1.0 – (1.0 – gl_TessCoord.z) // two operations performed
to aid in replicating subdivision computations.
The input variables gl_TessLevelOuter[] and gl_TessLevelInner[] are available only in the tessellation
evaluation shader. If a tessellation control shader is active, these variables are filled with corresponding
outputs written by the tessellation control shader. Otherwise, they are assigned with default tessellation
levels specified in section 11.2.3.3 “Tessellation Evaluation Shader Inputs” in the OpenGL Graphics
System Specification.
Fragment shaders output values to the OpenGL pipeline using declared out variables, the built-in
variables gl_FragDepth and gl_SampleMask, unless the discard statement is executed.
The fixed functionality computed depth for a fragment may be obtained by reading gl_FragCoord.z,
described below.
Writing to gl_FragDepth will establish the depth value for the fragment being processed. If depth
buffering is enabled, and no shader writes gl_FragDepth, then the fixed function value for depth will be
used as the fragment’s depth value. If a shader statically assigns a value to gl_FragDepth, and there is an
execution path through the shader that does not set gl_FragDepth, then the value of the fragment’s depth
may be undefined for executions of the shader that take that path. That is, if the set of linked fragment
shaders statically contain a write to gl_FragDepth, then it is responsible for always writing it.
If a shader executes the discard keyword, the fragment is discarded, and the values of any user-defined
fragment outputs, gl_FragDepth, and gl_SampleMask become irrelevant.
The variable gl_FragCoord is available as an input variable from within fragment shaders and it holds the
window relative coordinates (x, y, z, 1/w) values for the fragment. If multi-sampling, this value can be for
any location within the pixel, or one of the fragment samples. The use of centroid does not further
restrict this value to be inside the current primitive. This value is the result of the fixed functionality that
interpolates primitives after vertex processing to generate fragments. The z component is the depth value
that would be used for the fragment’s depth if no shader contained any writes to gl_FragDepth. This is
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useful for invariance if a shader conditionally computes gl_FragDepth but otherwise wants the fixed
functionality fragment depth.
Fragment shaders have access to the input built-in variable gl_FrontFacing, whose value is true if the
fragment belongs to a front-facing primitive. One use of this is to emulate two-sided lighting by selecting
one of two colors calculated by a vertex or geometry shader.
The values in gl_PointCoord are two-dimensional coordinates indicating where within a point primitive
the current fragment is located, when point sprites are enabled. They range from 0.0 to 1.0 across the
point. If the current primitive is not a point, or if point sprites are not enabled, then the values read from
gl_PointCoord are undefined.
For both the input array gl_SampleMaskIn[] and the output array gl_SampleMask[], bit B of mask M
(gl_SampleMaskIn[M] or gl_SampleMask[M]) corresponds to sample 32*M+B. These arrays have
ceil(s/32) elements, where s is the maximum number of color samples supported by the implementation.
The input variable gl_SampleMaskIn indicates the set of samples covered by the primitive generating the
fragment during multisample rasterization. It has a sample bit set if and only if the sample is considered
covered for this fragment shader invocation.
The output array gl_SampleMask[] sets the sample mask for the fragment being processed. Coverage for
the current fragment will become the logical AND of the coverage mask and the output gl_SampleMask.
This array must be sized in the fragment shader either implicitly or explicitly, to be no larger than the
implementation-dependent maximum sample-mask (as an array of 32bit elements), determined by the
maximum number of samples.. If the fragment shader statically assigns a value to gl_SampleMask, the
sample mask will be undefined for any array elements of any fragment shader invocations that fail to
assign a value. If a shader does not statically assign a value to gl_SampleMask, the sample mask has no
effect on the processing of a fragment.
The input variable gl_SampleID is filled with the sample number of the sample currently being processed.
This variable is in the range 0 to gl_NumSamples-1, where gl_NumSamples is the total number of samples
in the framebuffer, or 1 if rendering to a non-multisample framebuffer. Any static use of this variable in a
fragment shader causes the entire shader to be evaluated per-sample.
The input variable gl_SamplePosition contains the position of the current sample within the multi-sample
draw buffer. The x and y components of gl_SamplePosition contain the sub-pixel coordinate of the current
sample and will have values in the range 0.0 to 1.0. Any static use of this variable in a fragment shader
causes the entire shader to be evaluated per sample.
The gl_PerVertex block can be redeclared in a shader to explicitly indicate what subset of the fixed
pipeline interface will be used. This is necessary to establish the interface between multiple programs.
For example:
out gl_PerVertex {
vec4 gl_Position; // will use gl_Position
float gl_PointSize; // will use gl_PointSize
vec4 t; // error, only gl_PerVertex members allowed
}; // no other members of gl_PerVertex will be used
This establishes the output interface the shader will use with the subsequent pipeline stage. It must be a
subset of the built-in members of gl_PerVertex. Such a redeclaration can also add the invariant qualifier,
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interpolation qualifiers, and the layout qualifiers xfb_offset, xfb_buffer, and xfb_stride. It can also add
an array size for unsized arrays. For example:
out layout(xfb_buffer = 1, xfb_stride = 16) gl_PerVertex {
vec4 gl_Position;
layout(xfb_offset = 0) float gl_ClipDistance[4];
};
Other layout qualifiers, like location, cannot be added to such a redeclaration, unless specifically stated.
If a built-in interface block is redeclared, it must appear in the shader before any use of any member
included in the built-in declaration, or a compile-time error will result. It is also a compile-time error to
redeclare the block more than once or to redeclare a built-in block and then use a member from that built-
in block that was not included in the redeclaration. Also, if a built-in interface block is redeclared, no
member of the built-in declaration can be redeclared outside the block redeclaration. If multiple shaders
using members of a built-in block belonging to the same interface are linked together in the same
program, they must all redeclare the built-in block in the same way, as described in section 4.3.9
“Interface Blocks” for interface block matching, or a link-time error will result. It will also be a link-time
error if some shaders in a program redeclare a specific built-in interface block while another shader in that
program does not redeclare that interface block yet still uses a member of that interface block. If a built-
in block interface is formed across shaders in different programs, the shaders must all redeclare the built-
in block in the same way (as described for a single program), or the values passed along the interface are
undefined.
7.1.1 Compatibility Profile Built-In Language Variables
When using the compatibility profile, the GL can provide fixed functionality behavior for the vertex and
fragment programmable pipeline stages. For example, mixing a fixed functionality vertex stage with a
programmable fragment stage.
The following built-in vertex, tessellation control, tessellation evaluation, and geometry output variables
are available to specify inputs for the subsequent programmable shader stage or the fixed functionality
fragment stage. A particular one should be written to if any functionality in a corresponding fragment
shader or fixed pipeline uses it or state derived from it. Otherwise, behavior is undefined. The following
members are added to the output gl_PerVertex block in these languages:
out gl_PerVertex { // part of the gl_PerVertex block described in 7.1
// in addition to other gl_PerVertex members...
vec4 gl_ClipVertex;
vec4 gl_FrontColor;
vec4 gl_BackColor;
vec4 gl_FrontSecondaryColor;
vec4 gl_BackSecondaryColor;
vec4 gl_TexCoord[];
float gl_FogFragCoord;
};
The output variable gl_ClipVertex provides a place for vertex and geometry shaders to write the
coordinate to be used with the user clipping planes. Writing to gl_ClipDistance is the preferred method
for user clipping. It is a compile-time or link-time error for the set of shaders forming a program to
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statically read or write both gl_ClipVertex and gl_ClipDistance. If neither gl_ClipVertex nor
gl_ClipDistance is written, their values are undefined and any clipping against user clip planes is also
undefined.
Similarly to what was previously described for the core profile, the gl_PerVertex block can be redeclared
in a shader to explicitly include these additional members. For example:
out gl_PerVertex {
vec4 gl_Position; // will use gl_Position
vec4 gl_FrontColor; // will consume gl_color in the fragment shader
vec4 gl_BackColor;
vec4 gl_TexCoord[3]; // 3 elements of gl_TexCoord will be used
}; // no other aspects of the fixed interface will be used
The user must ensure the clip vertex and user clipping planes are defined in the same coordinate space.
User clip planes work properly only under linear transform. It is undefined what happens under non-
linear transform.
The output variables gl_FrontColor, glFrontSecondaryColor, gl_BackColor, and glBackSecondaryColor
assign primary and secondary colors for front and back faces of primitives containing the vertex being
processed. The output variable gl_TexCoord assigns texture coordinates for the vertex being processed.
For gl_FogFragCoord, the value written will be used as the “c” value in section 16.4 “Fog” of the
compatibility profile of the OpenGL Graphics System Specification, by the fixed functionality pipeline.
For example, if the z-coordinate of the fragment in eye space is desired as “c”, then that's what the vertex
shader executable should write into gl_FogFragCoord.
As with all arrays, indices used to subscript gl_TexCoord must either be an integral constant expressions,
or this array must be redeclared by the shader with a size. The size can be at most gl_MaxTextureCoords.
Using indexes close to 0 may aid the implementation in preserving varying resources. The redeclaration
of gl_TexCoord can also be done at global scope as, for example:
in vec4 gl_TexCoord[3];
out vec4 gl_TexCoord[4];
(This treatment is a special case for gl_TexCoord[], not a general method for redeclaring members of
blocks.) It is a compile-time error to redeclare gl_TexCoord[] at global scope if there is a redeclaration
of the corresponding built-in block; only one form of redeclaration is allowed within a shader (and hence
within a stage, as block redeclarations must match across all shaders using it).
In the tessellation control, evaluation, and geometry shaders, the outputs of the previous stage described
above are also available in the input gl_PerVertex block in these languages.
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in gl_PerVertex { // part of the gl_PerVertex block described in 7.1
// in addition to other gl_PerVertex members...
vec4 gl_ClipVertex;
vec4 gl_FrontColor;
vec4 gl_BackColor;
vec4 gl_FrontSecondaryColor;
vec4 gl_BackSecondaryColor;
vec4 gl_TexCoord[];
float gl_FogFragCoord;
} gl_in[];
These can be redeclared to establish an explicit pipeline interface, the same way as described above for
the output block gl_PerVertex, and the input redeclaration must match the output redeclaration of the
previous stage. However, when a built-in interface block with an instance name is redeclared (e.g., gl_in),
the instance name must be included in the redeclaration. It is a compile-time error to not include the built-
in instance name or to change its name. For example,
in gl_PerVertex {
vec4 gl_ClipVertex;
vec4 gl_FrontColor;
} gl_in[]; // must be present and must be “gl_in[]”
Treatment of gl_TexCoord[] redeclaration is also identical to that described for the output block
gl_TexCoord[] redeclaration.
The following fragment input block is also available in a fragment shader when using the compatibility
profile:
in gl_PerFragment {
in float gl_FogFragCoord;
in vec4 gl_TexCoord[];
in vec4 gl_Color;
in vec4 gl_SecondaryColor;
};
The values in gl_Color and gl_SecondaryColor will be derived automatically by the system from
gl_FrontColor, gl_BackColor, gl_FrontSecondaryColor, and gl_BackSecondaryColor based on which
face is visible in the primitive producing the fragment. If fixed functionality is used for vertex processing,
then gl_FogFragCoord will either be the z-coordinate of the fragment in eye space, or the interpolation of
the fog coordinate, as described in section 16.4 “Fog” of the compatibility profile of the OpenGL
Graphics System Specification. The gl_TexCoord[] values are the interpolated gl_TexCoord[] values
from a vertex shader or the texture coordinates of any fixed pipeline based vertex functionality.
Indices to the fragment shader gl_TexCoord array are as described above in the vertex shader text.
As described above for the input and output gl_PerVertex blocks, the gl_PerFragment block can be
redeclared to create an explicit interface to another program. When matching these interfaces between
separate programs, members in the gl_PerVertex output block must be declared if and only if the
corresponding fragment-shader members generated from them are present in the gl_PerFragment input
block. These matches are described in detail in section 7.4.1 “Shader Interface Matching” of the OpenGL
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Graphics System Specification. If they don't match within a program, a link-time error will result. If the
mismatch is between two programs, values passed between programs are undefined. Unlike with all other
block matching, the order of declaration within gl_PerFragment does not have to match across shaders
and does not have to correspond with order of declaration in a matching gl_PerVertex redeclaration.
The following fragment output variables are available in a fragment shader when using the compatibility
profile:
out vec4 gl_FragColor;
out vec4 gl_FragData[gl_MaxDrawBuffers];
Writing to gl_FragColor specifies the fragment color that will be used by the subsequent fixed
functionality pipeline. If subsequent fixed functionality consumes fragment color and an execution of the
fragment shader executable does not write a value to gl_FragColor then the fragment color consumed is
undefined.
The variable gl_FragData is an array. Writing to gl_FragData[n] specifies the fragment data that will be
used by the subsequent fixed functionality pipeline for data n. If subsequent fixed functionality consumes
fragment data and an execution of a fragment shader executable does not write a value to it, then the
fragment data consumed is undefined.
If a shader statically assigns a value to gl_FragColor, it may not assign a value to any element of
gl_FragData. If a shader statically writes a value to any element of gl_FragData, it may not assign a
value to gl_FragColor. That is, a shader may assign values to either gl_FragColor or gl_FragData, but
not both. Multiple shaders linked together must also consistently write just one of these variables.
Similarly, if user-declared output variables are in use (statically assigned to), then the built-in variables
gl_FragColor and gl_FragData may not be assigned to. These incorrect usages all generate compile-time
or link-time errors.
If a shader executes the discard keyword, the fragment is discarded, and the values of gl_FragDepth and
gl_FragColor become irrelevant.
7.2 Compatibility Profile Vertex Shader Built-In Inputs
The following predeclared input names can be used from within a vertex shader to access the current
values of OpenGL state when using the compatibility profile.
in vec4 gl_Color;
in vec4 gl_SecondaryColor;
in vec3 gl_Normal;
in vec4 gl_Vertex;
in vec4 gl_MultiTexCoord0;
in vec4 gl_MultiTexCoord1;
in vec4 gl_MultiTexCoord2;
in vec4 gl_MultiTexCoord3;
in vec4 gl_MultiTexCoord4;
in vec4 gl_MultiTexCoord5;
in vec4 gl_MultiTexCoord6;
in vec4 gl_MultiTexCoord7;
in float gl_FogCoord;
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7.3 Built-In Constants
The following built-in constants are provided to all shaders. The actual values used are implementation
dependent, but must be at least the value shown.
//
// Implementation-dependent constants. The example values below
// are the minimum values allowed for these maximums.
//
const ivec3 gl_MaxComputeWorkGroupCount = { 65535, 65535, 65535 };
const ivec3 gl_MaxComputeWorkGroupSize = { 1024, 1024, 64 };
const int gl_MaxComputeUniformComponents = 1024;
const int gl_MaxComputeTextureImageUnits = 16;
const int gl_MaxComputeImageUniforms = 8;
const int gl_MaxComputeAtomicCounters = 8;
const int gl_MaxComputeAtomicCounterBuffers = 8;
const int gl_MaxVertexAttribs = 16;
const int gl_MaxVertexUniformComponents = 1024;
const int gl_MaxVaryingComponents = 60;
const int gl_MaxVertexOutputComponents = 64;
const int gl_MaxGeometryInputComponents = 64;
const int gl_MaxGeometryOutputComponents = 128;
const int gl_MaxFragmentInputComponents = 128;
const int gl_MaxVertexTextureImageUnits = 16;
const int gl_MaxCombinedTextureImageUnits = 96;
const int gl_MaxTextureImageUnits = 16;
const int gl_MaxImageUnits = 8;
const int gl_MaxCombinedImageUnitsAndFragmentOutputs = 8;
const int gl_MaxImageSamples = 0;
const int gl_MaxVertexImageUniforms = 0;
const int gl_MaxTessControlImageUniforms = 0;
const int gl_MaxTessEvaluationImageUniforms = 0;
const int gl_MaxGeometryImageUniforms = 0;
const int gl_MaxFragmentImageUniforms = 8;
const int gl_MaxCombinedImageUniforms = 8;
const int gl_MaxFragmentUniformComponents = 1024;
const int gl_MaxDrawBuffers = 8;
const int gl_MaxClipDistances = 8;
const int gl_MaxGeometryTextureImageUnits = 16;
const int gl_MaxGeometryOutputVertices = 256;
const int gl_MaxGeometryTotalOutputComponents = 1024;
const int gl_MaxGeometryUniformComponents = 1024;
const int gl_MaxGeometryVaryingComponents = 64;
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const int gl_MaxTessControlInputComponents = 128;
const int gl_MaxTessControlOutputComponents = 128;
const int gl_MaxTessControlTextureImageUnits = 16;
const int gl_MaxTessControlUniformComponents = 1024;
const int gl_MaxTessControlTotalOutputComponents = 4096;
const int gl_MaxTessEvaluationInputComponents = 128;
const int gl_MaxTessEvaluationOutputComponents = 128;
const int gl_MaxTessEvaluationTextureImageUnits = 16;
const int gl_MaxTessEvaluationUniformComponents = 1024;
const int gl_MaxTessPatchComponents = 120;
const int gl_MaxPatchVertices = 32;
const int gl_MaxTessGenLevel = 64;
const int gl_MaxViewports = 16;
const int gl_MaxVertexUniformVectors = 256;
const int gl_MaxFragmentUniformVectors = 256;
const int gl_MaxVaryingVectors = 15;
const int gl_MaxVertexAtomicCounters = 0;
const int gl_MaxTessControlAtomicCounters = 0;
const int gl_MaxTessEvaluationAtomicCounters = 0;
const int gl_MaxGeometryAtomicCounters = 0;
const int gl_MaxFragmentAtomicCounters = 8;
const int gl_MaxCombinedAtomicCounters = 8;
const int gl_MaxAtomicCounterBindings = 1;
const int gl_MaxVertexAtomicCounterBuffers = 0;
const int gl_MaxTessControlAtomicCounterBuffers = 0;
const int gl_MaxTessEvaluationAtomicCounterBuffers = 0;
const int gl_MaxGeometryAtomicCounterBuffers = 0;
const int gl_MaxFragmentAtomicCounterBuffers = 1;
const int gl_MaxCombinedAtomicCounterBuffers = 1;
const int gl_MaxAtomicCounterBufferSize = 32;
const int gl_MinProgramTexelOffset = -8;
const int gl_MaxProgramTexelOffset = 7;
const int gl_MaxTransformFeedbackBuffers = 4;
const int gl_MaxTransformFeedbackInterleavedComponents = 64;
The constant gl_MaxVaryingFloats is removed in the core profile, use gl_MaxVaryingComponents
instead.
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7.3.1 Compatibility Profile Built-In Constants
const int gl_MaxTextureUnits = 2;
const int gl_MaxTextureCoords = 8;
const int gl_MaxClipPlanes = 8;
const int gl_MaxVaryingFloats = 60;
7.4 Built-In Uniform State
As an aid to accessing OpenGL processing state, the following uniform variables are built into the
OpenGL Shading Language.
//
// Depth range in window coordinates,
// section 13.6.1 “Controlling the Viewport” in the
// OpenGL Graphics System Specification.
//
// Note: Depth-range state is only for viewport 0.
//
struct gl_DepthRangeParameters {
float near; // n
float far; // f
float diff; // f - n
};
uniform gl_DepthRangeParameters gl_DepthRange;
uniform int gl_NumSamples;
7.4.1 Compatibility Profile State
These variables are present only in the compatibility profile. They are not available to compute shaders,
but are available to all other shaders.
//
// compatibility profile only
//
uniform mat4 gl_ModelViewMatrix;
uniform mat4 gl_ProjectionMatrix;
uniform mat4 gl_ModelViewProjectionMatrix;
uniform mat4 gl_TextureMatrix[gl_MaxTextureCoords];
//
// compatibility profile only
//
uniform mat3 gl_NormalMatrix; // transpose of the inverse of the
// upper leftmost 3x3 of gl_ModelViewMatrix
uniform mat4 gl_ModelViewMatrixInverse;
uniform mat4 gl_ProjectionMatrixInverse;
uniform mat4 gl_ModelViewProjectionMatrixInverse;
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7 Built-in Variables
uniform mat4 gl_TextureMatrixInverse[gl_MaxTextureCoords];
uniform mat4 gl_ModelViewMatrixTranspose;
uniform mat4 gl_ProjectionMatrixTranspose;
uniform mat4 gl_ModelViewProjectionMatrixTranspose;
uniform mat4 gl_TextureMatrixTranspose[gl_MaxTextureCoords];
uniform mat4 gl_ModelViewMatrixInverseTranspose;
uniform mat4 gl_ProjectionMatrixInverseTranspose;
uniform mat4 gl_ModelViewProjectionMatrixInverseTranspose;
uniform mat4 gl_TextureMatrixInverseTranspose[gl_MaxTextureCoords];
//
// compatibility profile only
//
uniform float gl_NormalScale;
//
// compatibility profile only
//
uniform vec4 gl_ClipPlane[gl_MaxClipPlanes];
//
// compatibility profile only
//
struct gl_PointParameters {
float size;
float sizeMin;
float sizeMax;
float fadeThresholdSize;
float distanceConstantAttenuation;
float distanceLinearAttenuation;
float distanceQuadraticAttenuation;
};
uniform gl_PointParameters gl_Point;
//
// compatibility profile only
//
struct gl_MaterialParameters {
vec4 emission; // Ecm
vec4 ambient; // Acm
vec4 diffuse; // Dcm
vec4 specular; // Scm
float shininess; // Srm
};
uniform gl_MaterialParameters gl_FrontMaterial;
uniform gl_MaterialParameters gl_BackMaterial;
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//
// compatibility profile only
//
struct gl_LightSourceParameters {
vec4 ambient; // Acli
vec4 diffuse; // Dcli
vec4 specular; // Scli
vec4 position; // Ppli
vec4 halfVector; // Derived: Hi
vec3 spotDirection; // Sdli
float spotExponent; // Srli
float spotCutoff; // Crli
// (range: [0.0,90.0], 180.0)
float spotCosCutoff; // Derived: cos(Crli)
// (range: [1.0,0.0],-1.0)
float constantAttenuation; // K0
float linearAttenuation; // K1
float quadraticAttenuation;// K2
};
uniform gl_LightSourceParameters gl_LightSource[gl_MaxLights];
struct gl_LightModelParameters {
vec4 ambient; // Acs
};
uniform gl_LightModelParameters gl_LightModel;
//
// compatibility profile only
//
// Derived state from products of light and material.
//
struct gl_LightModelProducts {
vec4 sceneColor; // Derived. Ecm + Acm * Acs
};
uniform gl_LightModelProducts gl_FrontLightModelProduct;
uniform gl_LightModelProducts gl_BackLightModelProduct;
struct gl_LightProducts {
vec4 ambient; // Acm * Acli
vec4 diffuse; // Dcm * Dcli
vec4 specular; // Scm * Scli
};
uniform gl_LightProducts gl_FrontLightProduct[gl_MaxLights];
uniform gl_LightProducts gl_BackLightProduct[gl_MaxLights];
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//
// compatibility profile only
//
uniform vec4 gl_TextureEnvColor[gl_MaxTextureUnits];
uniform vec4 gl_EyePlaneS[gl_MaxTextureCoords];
uniform vec4 gl_EyePlaneT[gl_MaxTextureCoords];
uniform vec4 gl_EyePlaneR[gl_MaxTextureCoords];
uniform vec4 gl_EyePlaneQ[gl_MaxTextureCoords];
uniform vec4 gl_ObjectPlaneS[gl_MaxTextureCoords];
uniform vec4 gl_ObjectPlaneT[gl_MaxTextureCoords];
uniform vec4 gl_ObjectPlaneR[gl_MaxTextureCoords];
uniform vec4 gl_ObjectPlaneQ[gl_MaxTextureCoords];
//
// compatibility profile only
//
struct gl_FogParameters {
vec4 color;
float density;
float start;
float end;
float scale; // Derived: 1.0 / (end - start)
};
uniform gl_FogParameters gl_Fog;
137
8 Built-in Functions
The OpenGL Shading Language defines an assortment of built-in convenience functions for scalar and
vector operations. Many of these built-in functions can be used in more than one type of shader, but some
are intended to provide a direct mapping to hardware and so are available only for a specific type of
shader.
The built-in functions basically fall into three categories:
•They expose some necessary hardware functionality in a convenient way such as accessing a texture
map. There is no way in the language for these functions to be emulated by a shader.
•They represent a trivial operation (clamp, mix, etc.) that is very simple for the user to write, but they
are very common and may have direct hardware support. It is a very hard problem for the compiler to
map expressions to complex assembler instructions.
•They represent an operation graphics hardware is likely to accelerate at some point. The trigonometry
functions fall into this category.
Many of the functions are similar to the same named ones in common C libraries, but they support vector
input as well as the more traditional scalar input.
Applications should be encouraged to use the built-in functions rather than do the equivalent computations
in their own shader code since the built-in functions are assumed to be optimal (e.g., perhaps supported
directly in hardware).
User code can replace built-in functions with their own if they choose, by simply redeclaring and defining
the same name and argument list. Because built-in functions are in a more outer scope than user built-in
functions, doing this will hide all built-in functions with the same name as the redeclared function.
When the built-in functions are specified below, where the input arguments (and corresponding output)
can be float, vec2, vec3, or vec4, genType is used as the argument. Where the input arguments (and
corresponding output) can be int, ivec2, ivec3, or ivec4, genIType is used as the argument. Where the
input arguments (and corresponding output) can be uint, uvec2, uvec3, or uvec4, genUType is used as the
argument. Where the input arguments (or corresponding output) can be bool, bvec2, bvec3, or bvec4,
genBType is used as the argument. Where the input arguments (and corresponding output) can be double,
dvec2, dvec3, dvec4, genDType is used as the argument. For any specific use of a function, the actual
types substituted for genType, genIType, genUType, or genBType have to have the same number of
components for all arguments and for the return type. Similarly, mat is used for any matrix basic type
with single-precision components and dmat is used for any matrix basic type with double-precision
components.
138
8 Built-in Functions
8.1 Angle and Trigonometry Functions
Function parameters specified as angle are assumed to be in units of radians. In no case will any of these
functions result in a divide by zero error. If the divisor of a ratio is 0, then results will be undefined.
These all operate component-wise. The description is per component.
Syntax Description
genType radians (genType degrees)Converts degrees to radians, i.e.,
180 ⋅degrees
genType degrees (genType radians)Converts radians to degrees, i.e.,
180
⋅radians
genType sin (genType angle) The standard trigonometric sine function.
genType cos (genType angle) The standard trigonometric cosine function.
genType tan (genType angle) The standard trigonometric tangent.
genType asin (genType x)Arc sine. Returns an angle whose sine is x. The range
of values returned by this function is
[
−
2,
2
]
Results are undefined if
∣x∣1.
genType acos (genType x)Arc cosine. Returns an angle whose cosine is x. The
range of values returned by this function is [0, p].
Results are undefined if
∣x∣1.
genType atan (genType y, genType x)Arc tangent. Returns an angle whose tangent is y/x. The
signs of x and y are used to determine what quadrant the
angle is in. The range of values returned by this
function is
[−,].
Results are undefined if x and
y are both 0.
genType atan (genType y_over_x)Arc tangent. Returns an angle whose tangent is
y_over_x. The range of values returned by this function
is
[
−
2,
2
]
.
139
8 Built-in Functions
Syntax Description
genType sinh (genType x)Returns the hyperbolic sine function
ex−e−x
2
genType cosh (genType x)Returns the hyperbolic cosine function
exe−x
2
genType tanh (genType x)Returns the hyperbolic tangent function
sinhx
cosh x
genType asinh (genType x)Arc hyperbolic sine; returns the inverse of sinh.
genType acosh (genType x)Arc hyperbolic cosine; returns the non-negative inverse
of cosh. Results are undefined if x < 1.
genType atanh (genType x)Arc hyperbolic tangent; returns the inverse of tanh.
Results are undefined if
∣x∣≥1.
140
8 Built-in Functions
8.2 Exponential Functions
These all operate component-wise. The description is per component.
Syntax Description
genType pow (genType x, genType y)Returns x raised to the y power, i.e.,
xy
Results are undefined if x < 0.
Results are undefined if x = 0 and y <= 0.
genType exp (genType x)Returns the natural exponentiation of x, i.e., ex.
genType log (genType x)Returns the natural logarithm of x, i.e., returns the value
y which satisfies the equation x = ey.
Results are undefined if x <= 0.
genType exp2 (genType x)Returns 2 raised to the x power, i.e.,
2x
genType log2 (genType x)Returns the base 2 logarithm of x, i.e., returns the value
y which satisfies the equation
x=2y
Results are undefined if x <= 0.
genType sqrt (genType x)
genDType sqrt (genDType x)
Returns
√
x
.
Results are undefined if x < 0.
genType inversesqrt (genType x)
genDType inversesqrt (genDType x)Returns
1
√
x
.
Results are undefined if x <= 0.
141
8 Built-in Functions
8.3 Common Functions
These all operate component-wise. The description is per component.
Syntax Description
genType abs (genType x)
genIType abs (genIType x)
genDType abs (genDType x)
Returns x if x >= 0; otherwise it returns –x.
genType sign (genType x)
genIType sign (genIType x)
genDType sign (genDType x)
Returns 1.0 if x > 0, 0.0 if x = 0, or –1.0 if x < 0.
genType floor (genType x)
genDType floor (genDType x)
Returns a value equal to the nearest integer that is less
than or equal to x.
genType trunc (genType x)
genDType trunc (genDType x)
Returns a value equal to the nearest integer to x whose
absolute value is not larger than the absolute value of x.
genType round (genType x)
genDType round (genDType x)
Returns a value equal to the nearest integer to x. The
fraction 0.5 will round in a direction chosen by the
implementation, presumably the direction that is fastest.
This includes the possibility that round(x) returns the
same value as roundEven(x) for all values of x.
genType roundEven (genType x)
genDType roundEven (genDType x)
Returns a value equal to the nearest integer to x. A
fractional part of 0.5 will round toward the nearest even
integer. (Both 3.5 and 4.5 for x will return 4.0.)
genType ceil (genType x)
genDType ceil (genDType x)
Returns a value equal to the nearest integer that is
greater than or equal to x.
genType fract (genType x)
genDType fract (genDType x)
Returns x – floor (x).
genType mod (genType x, float y)
genType mod (genType x, genType y)
genDType mod (genDType x, double y)
genDType mod (genDType x, genDType y)
Modulus. Returns x – y * floor (x/y).
142
8 Built-in Functions
Syntax Description
genType modf (genType x, out genType i)
genDType modf (genDType x,
out genDType i)
Returns the fractional part of x and sets i to the integer
part (as a whole number floating-point value). Both the
return value and the output parameter will have the same
sign as x.
genType min (genType x, genType y)
genType min (genType x, float y)
genDType min (genDType x, genDType y)
genDType min (genDType x, double y)
genIType min (genIType x, genIType y)
genIType min (genIType x, int y)
genUType min (genUType x, genUType y)
genUType min (genUType x, uint y)
Returns y if y < x; otherwise it returns x.
genType max (genType x, genType y)
genType max (genType x, float y)
genDType max (genDType x, genDType y)
genDType max (genDType x, double y)
genIType max (genIType x, genIType y)
genIType max (genIType x, int y)
genUType max (genUType x, genUType y)
genUType max (genUType x, uint y)
Returns y if x < y; otherwise it returns x.
143
8 Built-in Functions
Syntax Description
genType clamp (genType x,
genType minVal,
genType maxVal)
genType clamp (genType x,
float minVal,
float maxVal)
genDType clamp (genDType x,
genDType minVal,
genDType maxVal)
genDType clamp (genDType x,
double minVal,
double maxVal)
genIType clamp (genIType x,
genIType minVal,
genIType maxVal)
genIType clamp (genIType x,
int minVal,
int maxVal)
genUType clamp (genUType x,
genUType minVal,
genUType maxVal)
genUType clamp (genUType x,
uint minVal,
uint maxVal)
Returns min (max (x, minVal), maxVal).
Results are undefined if minVal > maxVal.
genType mix (genType x,
genType y,
genType a)
genType mix (genType x,
genType y,
float a)
genDType mix (genDType x,
genDType y,
genDType a)
genDType mix (genDType x,
genDType y,
double a)
Returns the linear blend of x and y, i.e.,
x⋅(1−a)+ y⋅a
144
8 Built-in Functions
Syntax Description
genType mix (genType x,
genType y,
genBType a)
genDType mix (genDType x,
genDType y,
genBType a)
Selects which vector each returned component comes
from. For a component of a that is false, the
corresponding component of x is returned. For a
component of a that is true, the corresponding
component of y is returned. Components of x and y that
are not selected are allowed to be invalid floating-point
values and will have no effect on the results. Thus, this
provides different functionality than, for example,
genType mix(genType x, genType y, genType(a))
where a is a Boolean vector.
genType step (genType edge, genType x)
genType step (float edge, genType x)
genDType step (genDType edge,
genDType x)
genDType step (double edge, genDType x)
Returns 0.0 if x < edge; otherwise it returns 1.0.
genType smoothstep (genType edge0,
genType edge1,
genType x)
genType smoothstep (float edge0,
float edge1,
genType x)
genDType smoothstep (genDType edge0,
genDType edge1,
genDType x)
genDType smoothstep (double edge0,
double edge1,
genDType x)
Returns 0.0 if x <= edge0 and 1.0 if x >= edge1 and
performs smooth Hermite interpolation between 0 and 1
when edge0 < x < edge1. This is useful in cases where
you would want a threshold function with a smooth
transition. This is equivalent to:
genType t;
t = clamp ((x – edge0) / (edge1 – edge0), 0, 1);
return t * t * (3 – 2 * t);
(And similarly for doubles.)
Results are undefined if edge0 >= edge1.
genBType isnan (genType x)
genBType isnan (genDType x)
Returns true if x holds a NaN. Returns false otherwise.
Always returns false if NaNs are not implemented.
genBType isinf (genType x)
genBType isinf (genDType x)
Returns true if x holds a positive infinity or negative
infinity. Returns false otherwise.
genIType floatBitsToInt (genType value)
genUType floatBitsToUint (genType value)
Returns a signed or unsigned integer value representing
the encoding of a float. The float value's bit-level
representation is preserved.
145
8 Built-in Functions
Syntax Description
genType intBitsToFloat (genIType value)
genType uintBitsToFloat (genUType value)
Returns a float value corresponding to a signed or
unsigned integer encoding of a float. If a NaN is passed
in, it will not signal, and the resulting value is
unspecified. If an Inf is passed in, the resulting value is
the corresponding Inf.
genType fma (genType a, genType b,
genType c)
genDType fma (genDType a, genDType b,
genDType c)
Computes and returns a*b + c.
In uses where the return value is eventually consumed by
a variable declared as precise:
•fma() is considered a single operation, whereas the
expression “a*b + c” consumed by a variable
declared precise is considered two operations.
•The precision of fma() can differ from the precision
of the expression “a*b + c”.
•fma() will be computed with the same precision as
any other fma() consumed by a precise variable,
giving invariant results for the same input values of
a, b, and c.
Otherwise, in the absence of precise consumption, there
are no special constraints on the number of operations or
difference in precision between fma() and the expression
“a*b + c”.
genType frexp (genType x,
out genIType exp)
genDType frexp (genDType x,
out genIType exp)
Splits x into a floating-point significand in the range
[0.5, 1.0) and an integral exponent of two, such that:
x=significand⋅2exponent
The significand is returned by the function and the
exponent is returned in the parameter exp. For a
floating-point value of zero, the significand and
exponent are both zero. For a floating-point value that is
an infinity or is not a number, the results are undefined.
genType ldexp (genType x,
in genIType exp)
genDType ldexp (genDType x,
in genIType exp)
Builds a floating-point number from x and the
corresponding integral exponent of two in exp, returning:
significand⋅2exponent
If this product is too large to be represented in the
floating-point type, the result is undefined.
146
8 Built-in Functions
8.4 Floating-Point Pack and Unpack Functions
These functions do not operate component-wise, rather, as described in each case.
Syntax Description
uint packUnorm2x16 (vec2 v)
uint packSnorm2x16 (vec2 v)
uint packUnorm4x8 (vec4 v)
uint packSnorm4x8 (vec4 v)
First, converts each component of the normalized
floating-point value v into 8- or 16-bit integer values.
Then, the results are packed into the returned 32-bit
unsigned integer.
The conversion for component c of v to fixed point is
done as follows:
packUnorm2x16: round(clamp(c, 0, +1) * 65535.0)
packSnorm2x16: round(clamp(c, -1, +1) * 32767.0)
packUnorm4x8: round(clamp(c, 0, +1) * 255.0)
packSnorm4x8: round(clamp(c, -1, +1) * 127.0)
The first component of the vector will be written to the
least significant bits of the output; the last component
will be written to the most significant bits.
vec2 unpackUnorm2x16 (uint p)
vec2 unpackSnorm2x16 (uint p)
vec4 unpackUnorm4x8 (uint p)
vec4 unpackSnorm4x8 (uint p)
First, unpacks a single 32-bit unsigned integer p into a
pair of 16-bit unsigned integers, a pair of 16-bit signed
integers, four 8-bit unsigned integers, or four 8-bit
signed integers. Then, each component is converted to a
normalized floating-point value to generate the returned
two- or four-component vector.
The conversion for unpacked fixed-point value f to
floating point is done as follows:
unpackUnorm2x16: f / 65535.0
unpackSnorm2x16: clamp(f / 32767.0, -1, +1)
unpackUnorm4x8: f / 255.0
unpackSnorm4x8: clamp(f / 127.0, -1, +1)
The first component of the returned vector will be
extracted from the least significant bits of the input; the
last component will be extracted from the most
significant bits.
147
8 Built-in Functions
Syntax Description
double packDouble2x32 (uvec2 v)Returns a double-precision value obtained by packing
the components of v into a 64-bit value. If an IEEE 754
Inf or NaN is created, it will not signal, and the resulting
floating-point value is unspecified. Otherwise, the bit-
level representation of v is preserved. The first vector
component specifies the 32 least significant bits; the
second component specifies the 32 most significant bits.
uvec2 unpackDouble2x32 (double v)Returns a two-component unsigned integer vector
representation of v. The bit-level representation of v is
preserved. The first component of the vector contains
the 32 least significant bits of the double; the second
component consists of the 32 most significant bits.
uint packHalf2x16 (vec2 v)Returns an unsigned integer obtained by converting the
components of a two-component floating-point vector to
the 16-bit floating-point representation found in the
OpenGL Specification, and then packing these two 16-
bit integers into a 32-bit unsigned integer.
The first vector component specifies the 16 least-
significant bits of the result; the second component
specifies the 16 most-significant bits.
vec2 unpackHalf2x16 (uint v)Returns a two-component floating-point vector with
components obtained by unpacking a 32-bit unsigned
integer into a pair of 16-bit values, interpreting those
values as 16-bit floating-point numbers according to the
OpenGL Specification, and converting them to 32-bit
floating-point values.
The first component of the vector is obtained from the
16 least-significant bits of v; the second component is
obtained from the 16 most-significant bits of v.
148
8 Built-in Functions
8.5 Geometric Functions
These operate on vectors as vectors, not component-wise.
Syntax Description
float length (genType x)
double length (genDType x)
Returns the length of vector x, i.e.,
√
x[0]2+x[1]2+...
float distance (genType p0, genType p1)
double distance (genDType p0,
genDType p1)
Returns the distance between p0 and p1, i.e.,
length (p0 – p1)
float dot (genType x, genType y)
double dot (genDType x, genDType y)
Returns the dot product of x and y, i.e.,
x[0]⋅y[0]+x[1]⋅y[1]+...
vec3 cross (vec3 x, vec3 y)
dvec3 cross (dvec3 x, dvec3 y)
Returns the cross product of x and y, i.e.,
[
x[1]⋅y[2]− y[1]⋅x[2]
x[2]⋅y[0]− y[2]⋅x[0]
x[0]⋅y[1]− y[0]⋅x[1]
]
genType normalize (genType x)
genDType normalize (genDType x)
Returns a vector in the same direction as x but with a
length of 1.
compatibility profile only
vec4 ftransform ()
Available only when using the compatibility profile. For
core OpenGL, use invariant.
For vertex shaders only. This function will ensure that
the incoming vertex value will be transformed in a way
that produces exactly the same result as would be
produced by OpenGL’s fixed functionality transform. It
is intended to be used to compute gl_Position, e.g.,
gl_Position = ftransform()
This function should be used, for example, when an
application is rendering the same geometry in separate
passes, and one pass uses the fixed functionality path to
render and another pass uses programmable shaders.
149
8 Built-in Functions
Syntax Description
genType faceforward (genType N,
genType I,
genType Nref)
genDType faceforward (genDType N,
genDType I,
genDType Nref)
If dot(Nref, I) < 0 return N, otherwise return –N.
genType reflect (genType I, genType N)
genDType reflect (genDType I,
genDType N)
For the incident vector I and surface orientation N,
returns the reflection direction:
I – 2 * dot(N, I) * N
N must already be normalized in order to achieve the
desired result.
genType refract (genType I, genType N,
float eta)
genDType refract (genDType I,
genDType N,
float eta)
For the incident vector I and surface normal N, and the
ratio of indices of refraction eta, return the refraction
vector. The result is computed by
k = 1.0 - eta * eta * (1.0 - dot(N, I) * dot(N, I))
if (k < 0.0)
return genType(0.0) // or genDType(0.0)
else
return eta * I - (eta * dot(N, I) + sqrt(k)) * N
The input parameters for the incident vector I and the
surface normal N must already be normalized to get the
desired results.
150
8 Built-in Functions
8.6 Matrix Functions
For each of the following built-in matrix functions, there is both a single-precision floating-point version,
where all arguments and return values are single precision, and a double-precision floating-point version,
where all arguments and return values are double precision. Only the single-precision floating-point
version is shown.
Syntax Description
mat matrixCompMult (mat x, mat y)Multiply matrix x by matrix y component-wise, i.e.,
result[i][j] is the scalar product of x[i][j] and y[i][j].
Note: to get linear algebraic matrix multiplication, use
the multiply operator (*).
mat2 outerProduct (vec2 c, vec2 r)
mat3 outerProduct (vec3 c, vec3 r)
mat4 outerProduct (vec4 c, vec4 r)
mat2x3 outerProduct (vec3 c, vec2 r)
mat3x2 outerProduct (vec2 c, vec3 r)
mat2x4 outerProduct (vec4 c, vec2 r)
mat4x2 outerProduct (vec2 c, vec4 r)
mat3x4 outerProduct (vec4 c, vec3 r)
mat4x3 outerProduct (vec3 c, vec4 r)
Treats the first parameter c as a column vector (matrix
with one column) and the second parameter r as a row
vector (matrix with one row) and does a linear algebraic
matrix multiply c * r, yielding a matrix whose number of
rows is the number of components in c and whose
number of columns is the number of components in r.
mat2 transpose (mat2 m)
mat3 transpose (mat3 m)
mat4 transpose (mat4 m)
mat2x3 transpose (mat3x2 m)
mat3x2 transpose (mat2x3 m)
mat2x4 transpose (mat4x2 m)
mat4x2 transpose (mat2x4 m)
mat3x4 transpose (mat4x3 m)
mat4x3 transpose (mat3x4 m)
Returns a matrix that is the transpose of m. The input
matrix m is not modified.
float determinant (mat2 m)
float determinant (mat3 m)
float determinant (mat4 m)
Returns the determinant of m.
151
8 Built-in Functions
Syntax Description
mat2 inverse (mat2 m)
mat3 inverse (mat3 m)
mat4 inverse (mat4 m)
Returns a matrix that is the inverse of m. The input
matrix m is not modified. The values in the returned
matrix are undefined if m is singular or poorly-
conditioned (nearly singular).
152
8 Built-in Functions
8.7 Vector Relational Functions
Relational and equality operators (<, <=, >, >=, ==, !=) are defined to operate on scalars and produce
scalar Boolean results. For vector results, use the following built-in functions. Below, the following
placeholders are used for the listed specific types:
Placeholder Specific Types Allowed
bvec bvec2, bvec3, bvec4
ivec ivec2, ivec3, ivec4
uvec uvec2, uvec3, uvec4
vec vec2, vec3, vec4, dvec2, dvec3, dvec4
In all cases, the sizes of all the input and return vectors for any particular call must match.
Syntax Description
bvec lessThan (vec x, vec y)
bvec lessThan (ivec x, ivec y)
bvec lessThan (uvec x, uvec y)
Returns the component-wise compare of x < y.
bvec lessThanEqual (vec x, vec y)
bvec lessThanEqual (ivec x, ivec y)
bvec lessThanEqual (uvec x, uvec y)
Returns the component-wise compare of x <= y.
bvec greaterThan (vec x, vec y)
bvec greaterThan (ivec x, ivec y)
bvec greaterThan (uvec x, uvec y)
Returns the component-wise compare of x > y.
bvec greaterThanEqual (vec x, vec y)
bvec greaterThanEqual (ivec x, ivec y)
bvec greaterThanEqual (uvec x, uvec y)
Returns the component-wise compare of x >= y.
bvec equal (vec x, vec y)
bvec equal (ivec x, ivec y)
bvec equal (uvec x, uvec y)
bvec equal (bvec x, bvec y)
bvec notEqual (vec x, vec y)
bvec notEqual (ivec x, ivec y)
bvec notEqual (uvec x, uvec y)
bvec notEqual (bvec x, bvec y)
Returns the component-wise compare of x == y.
Returns the component-wise compare of x != y.
bool any (bvec x) Returns true if any component of x is true.
153
8 Built-in Functions
Syntax Description
bool all (bvec x) Returns true only if all components of x are true.
bvec not (bvec x) Returns the component-wise logical complement of x.
154
8 Built-in Functions
8.8 Integer Functions
These all operate component-wise. The description is per component. The notation [a, b] means the set
of bits from bit-number a through bit-number b, inclusive. The lowest-order bit is bit 0. “Bit number”
will always refer to counting up from the lowest-order bit as bit 0.
Syntax Description
genUType uaddCarry (genUType x,
genUType y,
out genUType carry)
Adds 32-bit unsigned integer x and y, returning the sum
modulo 232. The value carry is set to 0 if the sum was
less than 232, or to 1 otherwise.
genUType usubBorrow (genUType x,
genUType y,
out genUType
borrow)
Subtracts the 32-bit unsigned integer y from x, returning
the difference if non-negative, or 232 plus the difference
otherwise. The value borrow is set to 0 if x >= y, or to
1 otherwise.
void umulExtended (genUType x,
genUType y,
out genUType msb,
out genUType lsb)
void imulExtended (genIType x,
genIType y,
out genIType msb,
out genIType lsb)
Multiplies 32-bit integers x and y, producing a 64-bit
result. The 32 least-significant bits are returned in lsb.
The 32 most-significant bits are returned in msb.
genIType bitfieldExtract (genIType value,
int offset, int bits)
genUType bitfieldExtract (genUType value,
int offset, int bits)
Extracts bits [offset, offset + bits - 1] from value,
returning them in the least significant bits of the result.
For unsigned data types, the most significant bits of the
result will be set to zero. For signed data types, the
most significant bits will be set to the value of bit offset
+ bits – 1.
If bits is zero, the result will be zero. The result will be
undefined if offset or bits is negative, or if the sum of
offset and bits is greater than the number of bits used
to store the operand.
155
8 Built-in Functions
Syntax Description
genIType bitfieldInsert (genIType base,
genIType insert,
int offset, int bits)
genUType bitfieldInsert (genUType base,
genUType insert,
int offset, int bits)
Returns the insertion of the bits least-significant bits of
insert into base.
The result will have bits [offset, offset + bits - 1] taken
from bits [0, bits – 1] of insert, and all other bits taken
directly from the corresponding bits of base. If bits is
zero, the result will simply be base. The result will be
undefined if offset or bits is negative, or if the sum of
offset and bits is greater than the number of bits used to
store the operand.
genIType bitfieldReverse (genIType value)
genUType bitfieldReverse (genUType value)
Returns the reversal of the bits of value. The bit
numbered n of the result will be taken from bit (bits - 1)
- n of value, where bits is the total number of bits used
to represent value.
genIType bitCount (genIType value)
genIType bitCount (genUType value)
Returns the number of bits set to 1 in the binary
representation of value.
genIType findLSB (genIType value)
genIType findLSB (genUType value)
Returns the bit number of the least significant bit set to
1 in the binary representation of value. If value is zero,
-1will be returned.
genIType findMSB (genIType value)
genIType findMSB (genUType value)
Returns the bit number of the most significant bit in the
binary representation of value.
For positive integers, the result will be the bit number of
the most significant bit set to 1. For negative integers,
the result will be the bit number of the most significant
bit set to 0. For a value of zero or negative one, -1 will
be returned.
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8 Built-in Functions
8.9 Texture Functions
Texture lookup functions are available in all shading stages. However, automatic level of detail is
computed only for fragment shaders. Other shaders operate as though the base level of detail were
computed as zero. The functions in the table below provide access to textures through samplers, as set up
through the OpenGL API. Texture properties such as size, pixel format, number of dimensions, filtering
method, number of mipmap levels, depth comparison, and so on are also defined by OpenGL API calls.
Such properties are taken into account as the texture is accessed via the built-in functions defined below.
Texture data can be stored by the GL as single-precision floating point, unsigned normalized integer,
unsigned integer or signed integer data. This is determined by the type of the internal format of the
texture. Texture lookups on unsigned normalized integer and floating-point data return floating-point
values in the range [0, 1].
Texture lookup functions are provided that can return their result as floating point, unsigned integer or
signed integer, depending on the sampler type passed to the lookup function. Care must be taken to use
the right sampler type for texture access. The following table lists the supported combinations of sampler
types and texture internal formats. Blank entries are unsupported. Doing a texture lookup will return
undefined values for unsupported combinations.
Internal Texture Format Floating Point
Sampler Types
Signed Integer
Sampler Types
Unsigned Integer
Sampler Types
Floating point Supported
Normalized Integer Supported
Signed Integer Supported
Unsigned Integer Supported
If an integer sampler type is used, the result of a texture lookup is an ivec4. If an unsigned integer sampler
type is used, the result of a texture lookup is a uvec4. If a floating-point sampler type is used, the result of
a texture lookup is a vec4, where each component is in the range [0, 1].
In the prototypes below, the “g” in the return type “gvec4” is used as a placeholder for nothing, “i”, or “u”
making a return type of vec4, ivec4, or uvec4. In these cases, the sampler argument type also starts with
“g”, indicating the same substitution done on the return type; it is either a single-precision floating point,
signed integer, or unsigned integer sampler, matching the basic type of the return type, as described
above.
For shadow forms (the sampler parameter is a shadow-type), a depth comparison lookup on the depth
texture bound to sampler is done as described in section 8.22 “Texture Comparison Modes” of the
OpenGL Graphics System Specification. See the table below for which component specifies Dref. The
texture bound to sampler must be a depth texture, or results are undefined. If a non-shadow texture call is
made to a sampler that represents a depth texture with depth comparisons turned on, then results are
undefined. If a shadow texture call is made to a sampler that represents a depth texture with depth
comparisons turned off, then results are undefined. If a shadow texture call is made to a sampler that does
not represent a depth texture, then results are undefined.
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8 Built-in Functions
In all functions below, the bias parameter is optional for fragment shaders. The bias parameter is not
accepted in any other shader stage. For a fragment shader, if bias is present, it is added to the implicit
level of detail prior to performing the texture access operation. No bias or lod parameters for rectangle
textures, multi-sample textures, or texture buffers are supported because mipmaps are not allowed for
these types of textures.
The implicit level of detail is selected as follows: For a texture that is not mipmapped, the texture is used
directly. If it is mipmapped and running in a fragment shader, the LOD computed by the implementation
is used to do the texture lookup. If it is mipmapped and running on the vertex shader, then the base
texture is used.
Some texture functions (non-“Lod” and non-“Grad” versions) may require implicit derivatives. Implicit
derivatives are undefined within non-uniform control flow and for non-fragment-shader texture fetches.
For Cube forms, the direction of P is used to select which face to do a 2-dimensional texture lookup in, as
described in section 8.13 “Cube Map Texture Selection” in the OpenGL Graphics System Specification.
For Array forms, the array layer used will be
max (0,min(d−1, floor(layer+0.5)))
where d is the depth of the texture array and layer comes from the component indicated in the tables
below.
For depth/stencil textures, the sampler type should match the component being accessed as set through the
OpenGL API. When the depth/stencil texture mode is set to DEPTH_COMPONENT, a floating-point
sampler type should be used. When the depth/stencil texture mode is set to STENCIL_INDEX, an
unsigned integer sampler type should be used. Doing a texture lookup with an unsupported combination
will return undefined values.
8.9.1 Texture Query Functions
The textureSize functions query the dimensions of a specific texture level for a sampler.
The textureQueryLod functions are available only in a fragment shader. They take the components of P
and compute the level of detail information that the texture pipe would use to access that texture through a
normal texture lookup. The level of detail λ' (equation 3.18 in the OpenGL Graphics System
Specification) is obtained after any LOD bias, but prior to clamping to [TEXTURE_MIN_LOD,
TEXTURE_MAX_LOD]. The mipmap array(s) that would be accessed are also computed. If a single
level of detail would be accessed, the level-of-detail number relative to the base level is returned. If
multiple levels of detail would be accessed, a floating-point number between the two levels is returned,
with the fractional part equal to the fractional part of the computed and clamped level of detail.
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8 Built-in Functions
The algorithm used is given by the following pseudo-code:
float ComputeAccessedLod(float computedLod)
{
// Clamp the computed LOD according to the texture LOD clamps.
if (computedLod < TEXTURE_MIN_LOD) computedLod = TEXTURE_MIN_LOD;
if (computedLod > TEXTURE_MAX_LOD) computedLod = TEXTURE_MAX_LOD;
// Clamp the computed LOD to the range of accessible levels.
if (computedLod < 0.0)
computedLod = 0.0;
if (computedLod > (float)
maxAccessibleLevel) computedLod = (float) maxAccessibleLevel;
// Return a value according to the min filter.
if (TEXTURE_MIN_FILTER is LINEAR or NEAREST) {
return 0.0;
} else if (TEXTURE_MIN_FILTER is NEAREST_MIPMAP_NEAREST
or LINEAR_MIPMAP_NEAREST) {
return ceil(computedLod + 0.5) - 1.0;
} else {
return computedLod;
}
}
The value maxAccessibleLevel is the level number of the smallest accessible level of the mipmap array
(the value q in section 8.14.3 “Mipmapping” of the OpenGL Graphics System Specification) minus the
base level.
Syntax Description
int textureSize (gsampler1D sampler, int lod)
ivec2 textureSize (gsampler2D sampler, int lod)
ivec3 textureSize (gsampler3D sampler, int lod)
ivec2 textureSize (gsamplerCube sampler, int lod)
int textureSize (sampler1DShadow sampler, int lod)
ivec2 textureSize (sampler2DShadow sampler, int lod)
ivec2 textureSize (samplerCubeShadow sampler, int lod)
ivec3 textureSize (gsamplerCubeArray sampler, int lod)
ivec3 textureSize (samplerCubeArrayShadow sampler, int lod)
ivec2 textureSize (gsampler2DRect sampler)
ivec2 textureSize (sampler2DRectShadow sampler)
ivec2 textureSize (gsampler1DArray sampler, int lod)
ivec3 textureSize (gsampler2DArray sampler, int lod)
ivec2 textureSize (sampler1DArrayShadow sampler, int lod)
ivec3 textureSize (sampler2DArrayShadow sampler, int lod)
int textureSize (gsamplerBuffer sampler)
ivec2 textureSize (gsampler2DMS sampler)
ivec3 textureSize (gsampler2DMSArray sampler)
Returns the dimensions of level
lod (if present) for the texture
bound to sampler, as described
in section 11.1.3.4 “Texture
Queries” of the OpenGL
Graphics System Specification.
The components in the return
value are filled in, in order, with
the width, height, and depth of
the texture.
For the array forms, the last
component of the return value is
the number of layers in the
texture array, or the number of
cubes in the texture cube map
array.
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8 Built-in Functions
Syntax Description
vec2 textureQueryLod(gsampler1D sampler, float P)
vec2 textureQueryLod(gsampler2D sampler, vec2 P)
vec2 textureQueryLod(gsampler3D sampler, vec3 P)
vec2 textureQueryLod(gsamplerCube sampler, vec3 P)
vec2 textureQueryLod(gsampler1DArray sampler, float P)
vec2 textureQueryLod(gsampler2DArray sampler, vec2 P)
vec2 textureQueryLod(gsamplerCubeArray sampler, vec3 P)
vec2 textureQueryLod(sampler1DShadow sampler, float P)
vec2 textureQueryLod(sampler2DShadow sampler, vec2 P)
vec2 textureQueryLod(samplerCubeShadow sampler, vec3 P)
vec2 textureQueryLod(sampler1DArrayShadow sampler, float P)
vec2 textureQueryLod(sampler2DArrayShadow sampler, vec2 P)
vec2 textureQueryLod(samplerCubeArrayShadow sampler, vec3 P)
Returns the mipmap array(s)
that would be accessed in the x
component of the return value.
Returns the computed level of
detail relative to the base level
in the y component of the return
value.
If called on an incomplete
texture, the results are
undefined.
int textureQueryLevels(gsampler1D sampler)
int textureQueryLevels(gsampler2D sampler)
int textureQueryLevels(gsampler3D sampler)
int textureQueryLevels(gsamplerCube sampler)
int textureQueryLevels(gsampler1DArray sampler)
int textureQueryLevels(gsampler2DArray sampler)
int textureQueryLevels(gsamplerCubeArray sampler)
int textureQueryLevels(sampler1DShadow sampler)
int textureQueryLevels(sampler2DShadow sampler)
int textureQueryLevels(samplerCubeShadow sampler)
int textureQueryLevels(sampler1DArrayShadow sampler)
int textureQueryLevels(sampler2DArrayShadow sampler)
int textureQueryLevels(samplerCubeArrayShadow sampler)
Returns the number of mipmap
levels accessible in the texture
associated with sampler, as
defined in the OpenGL
Specification.
The value zero will be returned
if no texture or an incomplete
texture is associated with
sampler.
Available in all shader stages.
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8 Built-in Functions
8.9.2 Texel Lookup Functions
Syntax Description
gvec4 texture (gsampler1D sampler, float P [, float bias] )
gvec4 texture (gsampler2D sampler, vec2 P [, float bias] )
gvec4 texture (gsampler3D sampler, vec3 P [, float bias] )
gvec4 texture (gsamplerCube sampler, vec3 P [, float bias] )
float texture (sampler1DShadow sampler, vec3 P [, float bias] )
float texture (sampler2DShadow sampler, vec3 P [, float bias] )
float texture (samplerCubeShadow sampler, vec4 P [, float bias] )
gvec4 texture (gsampler1DArray sampler, vec2 P [, float bias] )
gvec4 texture (gsampler2DArray sampler, vec3 P [, float bias] )
gvec4 texture (gsamplerCubeArray sampler, vec4 P [, float bias] )
float texture (sampler1DArrayShadow sampler, vec3 P
[, float bias] )
float texture (sampler2DArrayShadow sampler, vec4 P)
gvec4 texture (gsampler2DRect sampler, vec2 P)
float texture (sampler2DRectShadow sampler, vec3 P)
float texture (gsamplerCubeArrayShadow sampler, vec4 P,
float compare)
Use the texture coordinate P to
do a texture lookup in the
texture currently bound to
sampler.
For shadow forms: When
compare is present, it is used as
Dref and the array layer comes
from P.w. When compare is not
present, the last component of
P is used as Dref and the array
layer comes from the second to
last component of P. (The
second component of P is
unused for 1D shadow lookups.)
For non-shadow forms: the array
layer comes from the last
component of P.
gvec4 textureProj (gsampler1D sampler, vec2 P [, float bias] )
gvec4 textureProj (gsampler1D sampler, vec4 P [, float bias] )
gvec4 textureProj (gsampler2D sampler, vec3 P [, float bias] )
gvec4 textureProj (gsampler2D sampler, vec4 P [, float bias] )
gvec4 textureProj (gsampler3D sampler, vec4 P [, float bias] )
float textureProj (sampler1DShadow sampler, vec4 P
[, float bias] )
float textureProj (sampler2DShadow sampler, vec4 P
[, float bias] )
gvec4 textureProj (gsampler2DRect sampler, vec3 P)
gvec4 textureProj (gsampler2DRect sampler, vec4 P)
float textureProj (sampler2DRectShadow sampler, vec4 P)
Do a texture lookup with
projection. The texture
coordinates consumed from P,
not including the last component
of P, are divided by the last
component of P. The resulting
3rd component of P in the
shadow forms is used as Dref.
After these values are computed,
texture lookup proceeds as in
texture.
gvec4 textureLod (gsampler1D sampler, float P, float lod)
gvec4 textureLod (gsampler2D sampler, vec2 P, float lod)
gvec4 textureLod (gsampler3D sampler, vec3 P, float lod)
gvec4 textureLod (gsamplerCube sampler, vec3 P, float lod)
float textureLod (sampler1DShadow sampler, vec3 P, float lod)
float textureLod (sampler2DShadow sampler, vec3 P, float lod)
gvec4 textureLod (gsampler1DArray sampler, vec2 P, float lod)
gvec4 textureLod (gsampler2DArray sampler, vec3 P, float lod)
float textureLod (sampler1DArrayShadow sampler, vec3 P,
float lod)
gvec4 textureLod (gsamplerCubeArray sampler, vec4 P, float lod)
Do a texture lookup as in
texture but with explicit LOD;
lod specifies λbase and sets the
partial derivatives as follows.
(See section 8.14“Texture
Minification” and equations 8.4-
8.6 in the OpenGL Graphics
System Specification.)
∂u
∂x=0∂v
∂x=0∂w
∂x=0
∂u
∂y=0∂v
∂y=0∂w
∂y=0
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8 Built-in Functions
Syntax Description
gvec4 textureOffset (gsampler1D sampler, float P,
int offset [, float bias] )
gvec4 textureOffset (gsampler2D sampler, vec2 P,
ivec2 offset [, float bias] )
gvec4 textureOffset (gsampler3D sampler, vec3 P,
ivec3 offset [, float bias] )
gvec4 textureOffset (gsampler2DRect sampler, vec2 P,
ivec2 offset )
float textureOffset (sampler2DRectShadow sampler, vec3 P,
ivec2 offset )
float textureOffset (sampler1DShadow sampler, vec3 P,
int offset [, float bias] )
float textureOffset (sampler2DShadow sampler, vec3 P,
ivec2 offset [, float bias] )
gvec4 textureOffset (gsampler1DArray sampler, vec2 P,
int offset [, float bias] )
gvec4 textureOffset (gsampler2DArray sampler, vec3 P,
ivec2 offset [, float bias] )
float textureOffset (sampler1DArrayShadow sampler, vec3 P,
int offset [, float bias] )
float textureOffset (sampler2DArrayShadow sampler, vec4 P,
ivec2 offset )
Do a texture lookup as in
texture but with offset added to
the (u,v,w) texel coordinates
before looking up each texel.
The offset value must be a
constant expression. A limited
range of offset values are
supported; the minimum and
maximum offset values are
implementation-dependent and
given by
gl_MinProgramTexelOffset and
gl_MaxProgramTexelOffset,
respectively.
Note that offset does not apply
to the layer coordinate for
texture arrays. This is explained
in detail in section 8.14.2
“Coordinate Wrapping and
Texel Selection” of the OpenGL
Graphics System Specification,
where offset is
(δu,δv,δw).
Note that texel offsets are also
not supported for cube maps.
gvec4 texelFetch (gsampler1D sampler, int P, int lod)
gvec4 texelFetch (gsampler2D sampler, ivec2 P, int lod)
gvec4 texelFetch (gsampler3D sampler, ivec3 P, int lod)
gvec4 texelFetch (gsampler2DRect sampler, ivec2 P)
gvec4 texelFetch (gsampler1DArray sampler, ivec2 P, int lod)
gvec4 texelFetch (gsampler2DArray sampler, ivec3 P, int lod)
gvec4 texelFetch (gsamplerBuffer sampler, int P)
gvec4 texelFetch (gsampler2DMS sampler, ivec2 P, int sample)
gvec4 texelFetch (gsampler2DMSArray sampler, ivec3 P,
int sample)
Use integer texture coordinate P
to lookup a single texel from
sampler. The array layer comes
from the last component of P for
the array forms. The level-of-
detail lod (if present) is as
described in sections 11.1.3.2
“Texel Fetches” and 8.14.1
“Scale Factor and Level of
Detail” of the OpenGL Graphics
System Specification.
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8 Built-in Functions
Syntax Description
gvec4 texelFetchOffset (gsampler1D sampler, int P, int lod,
int offset)
gvec4 texelFetchOffset (gsampler2D sampler, ivec2 P, int lod,
ivec2 offset)
gvec4 texelFetchOffset (gsampler3D sampler, ivec3 P, int lod,
ivec3 offset)
gvec4 texelFetchOffset (gsampler2DRect sampler, ivec2 P,
ivec2 offset)
gvec4 texelFetchOffset (gsampler1DArray sampler, ivec2 P, int lod,
int offset)
gvec4 texelFetchOffset (gsampler2DArray sampler, ivec3 P, int lod,
ivec2 offset)
Fetch a single texel as in
texelFetch offset by offset as
described in textureOffset.
gvec4 textureProjOffset (gsampler1D sampler, vec2 P,
int offset [, float bias] )
gvec4 textureProjOffset (gsampler1D sampler, vec4 P,
int offset [, float bias] )
gvec4 textureProjOffset (gsampler2D sampler, vec3 P,
ivec2 offset [, float bias] )
gvec4 textureProjOffset (gsampler2D sampler, vec4 P,
ivec2 offset [, float bias] )
gvec4 textureProjOffset (gsampler3D sampler, vec4 P,
ivec3 offset [, float bias] )
gvec4 textureProjOffset (gsampler2DRect sampler, vec3 P,
ivec2 offset )
gvec4 textureProjOffset (gsampler2DRect sampler, vec4 P,
ivec2 offset )
float textureProjOffset (sampler2DRectShadow sampler, vec4 P,
ivec2 offset )
float textureProjOffset (sampler1DShadow sampler, vec4 P,
int offset [, float bias] )
float textureProjOffset (sampler2DShadow sampler, vec4 P,
ivec2 offset [, float bias] )
Do a projective texture lookup
as described in textureProj
offset by offset as described in
textureOffset.
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8 Built-in Functions
Syntax Description
gvec4 textureLodOffset (gsampler1D sampler, float P,
float lod, int offset)
gvec4 textureLodOffset (gsampler2D sampler, vec2 P,
float lod, ivec2 offset)
gvec4 textureLodOffset (gsampler3D sampler, vec3 P,
float lod, ivec3 offset)
float textureLodOffset (sampler1DShadow sampler, vec3 P,
float lod, int offset)
float textureLodOffset (sampler2DShadow sampler, vec3 P,
float lod, ivec2 offset)
gvec4 textureLodOffset (gsampler1DArray sampler, vec2 P,
float lod, int offset)
gvec4 textureLodOffset (gsampler2DArray sampler, vec3 P,
float lod, ivec2 offset)
float textureLodOffset (sampler1DArrayShadow sampler, vec3 P,
float lod, int offset)
Do an offset texture lookup with
explicit LOD. See textureLod
and textureOffset.
gvec4 textureProjLod (gsampler1D sampler, vec2 P, float lod)
gvec4 textureProjLod (gsampler1D sampler, vec4 P, float lod)
gvec4 textureProjLod (gsampler2D sampler, vec3 P, float lod)
gvec4 textureProjLod (gsampler2D sampler, vec4 P, float lod)
gvec4 textureProjLod (gsampler3D sampler, vec4 P, float lod)
float textureProjLod (sampler1DShadow sampler, vec4 P, float lod)
float textureProjLod (sampler2DShadow sampler, vec4 P, float lod)
Do a projective texture lookup
with explicit LOD. See
textureProj and textureLod.
gvec4 textureProjLodOffset (gsampler1D sampler, vec2 P,
float lod, int offset)
gvec4 textureProjLodOffset (gsampler1D sampler, vec4 P,
float lod, int offset)
gvec4 textureProjLodOffset (gsampler2D sampler, vec3 P,
float lod, ivec2 offset)
gvec4 textureProjLodOffset (gsampler2D sampler, vec4 P,
float lod, ivec2 offset)
gvec4 textureProjLodOffset (gsampler3D sampler, vec4 P,
float lod, ivec3 offset)
float textureProjLodOffset (sampler1DShadow sampler, vec4 P,
float lod, int offset)
float textureProjLodOffset (sampler2DShadow sampler, vec4 P,
float lod, ivec2 offset)
Do an offset projective texture
lookup with explicit LOD. See
textureProj, textureLod, and
textureOffset.
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8 Built-in Functions
Syntax Description
gvec4 textureGrad (gsampler1D sampler, float P,
float dPdx, float dPdy)
gvec4 textureGrad (gsampler2D sampler, vec2 P,
vec2 dPdx, vec2 dPdy)
gvec4 textureGrad (gsampler3D sampler, vec3 P,
vec3 dPdx, vec3 dPdy)
gvec4 textureGrad (gsamplerCube sampler, vec3 P,
vec3 dPdx, vec3 dPdy)
gvec4 textureGrad (gsampler2DRect sampler, vec2 P,
vec2 dPdx, vec2 dPdy)
float textureGrad (sampler2DRectShadow sampler, vec3 P,
vec2 dPdx, vec2 dPdy)
float textureGrad (sampler1DShadow sampler, vec3 P,
float dPdx, float dPdy)
float textureGrad (sampler2DShadow sampler, vec3 P,
vec2 dPdx, vec2 dPdy)
float textureGrad (samplerCubeShadow sampler, vec4 P,
vec3 dPdx, vec3 dPdy)
gvec4 textureGrad (gsampler1DArray sampler, vec2 P,
float dPdx, float dPdy)
gvec4 textureGrad (gsampler2DArray sampler, vec3 P,
vec2 dPdx, vec2 dPdy)
float textureGrad (sampler1DArrayShadow sampler, vec3 P,
float dPdx, float dPdy)
float textureGrad (sampler2DArrayShadow sampler, vec4 P,
vec2 dPdx, vec2 dPdy)
gvec4 textureGrad (gsamplerCubeArray sampler, vec4 P,
vec3 dPdx, vec3 dPdy)
Do a texture lookup as in
texture but with explicit
gradients. The partial
derivatives of P are with respect
to window x and window y. Set
∂s
∂x=
{
∂P
∂xfor a 1D texture
∂P.s
∂xotherwise
∂s
∂y=
{
∂P
∂yfor a 1D texture
∂P.s
∂yotherwise
∂t
∂x=
{
0.0 for a 1D texture
∂P.t
∂xotherwise
∂t
∂y=
{
0.0 for a 1D texture
∂P.t
∂yotherwise
∂r
∂x=
{
0.0 for 1D or 2D
∂P.p
∂xcube, other
∂r
∂y=
{
0.0 for 1D or 2D
∂P.p
∂ycube, other
For the cube version, the partial
derivatives of P are assumed to
be in the coordinate system used
before texture coordinates are
projected onto the appropriate
cube face.
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8 Built-in Functions
Syntax Description
gvec4 textureGradOffset (gsampler1D sampler, float P,
float dPdx, float dPdy, int offset)
gvec4 textureGradOffset (gsampler2D sampler, vec2 P,
vec2 dPdx, vec2 dPdy, ivec2 offset)
gvec4 textureGradOffset (gsampler3D sampler, vec3 P,
vec3 dPdx, vec3 dPdy, ivec3 offset)
gvec4 textureGradOffset (gsampler2DRect sampler, vec2 P,
vec2 dPdx, vec2 dPdy, ivec2 offset)
float textureGradOffset (sampler2DRectShadow sampler, vec3 P,
vec2 dPdx, vec2 dPdy, ivec2 offset)
float textureGradOffset (sampler1DShadow sampler, vec3 P,
float dPdx, float dPdy, int offset )
float textureGradOffset (sampler2DShadow sampler, vec3 P,
vec2 dPdx, vec2 dPdy, ivec2 offset)
gvec4 textureGradOffset (gsampler1DArray sampler, vec2 P,
float dPdx, float dPdy, int offset)
gvec4 textureGradOffset (gsampler2DArray sampler, vec3 P,
vec2 dPdx, vec2 dPdy, ivec2 offset)
float textureGradOffset (sampler1DArrayShadow sampler, vec3 P,
float dPdx, float dPdy, int offset)
float textureGradOffset (sampler2DArrayShadow sampler, vec4 P,
vec2 dPdx, vec2 dPdy, ivec2 offset)
Do a texture lookup with both
explicit gradient and offset, as
described in textureGrad and
textureOffset.
gvec4 textureProjGrad (gsampler1D sampler, vec2 P,
float dPdx, float dPdy)
gvec4 textureProjGrad (gsampler1D sampler, vec4 P,
float dPdx, float dPdy)
gvec4 textureProjGrad (gsampler2D sampler, vec3 P,
vec2 dPdx, vec2 dPdy)
gvec4 textureProjGrad (gsampler2D sampler, vec4 P,
vec2 dPdx, vec2 dPdy)
gvec4 textureProjGrad (gsampler3D sampler, vec4 P,
vec3 dPdx, vec3 dPdy)
gvec4 textureProjGrad (gsampler2DRect sampler, vec3 P,
vec2 dPdx, vec2 dPdy)
gvec4 textureProjGrad (gsampler2DRect sampler, vec4 P,
vec2 dPdx, vec2 dPdy)
float textureProjGrad (sampler2DRectShadow sampler, vec4 P,
vec2 dPdx, vec2 dPdy)
float textureProjGrad (sampler1DShadow sampler, vec4 P,
float dPdx, float dPdy)
float textureProjGrad (sampler2DShadow sampler, vec4 P,
vec2 dPdx, vec2 dPdy)
Do a texture lookup both
projectively, as described in
textureProj, and with explicit
gradient as described in
textureGrad. The partial
derivatives dPdx and dPdy are
assumed to be already projected.
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Syntax Description
gvec4 textureProjGradOffset (gsampler1D sampler, vec2 P,
float dPdx, float dPdy, int offset)
gvec4 textureProjGradOffset (gsampler1D sampler, vec4 P,
float dPdx, float dPdy, int offset)
gvec4 textureProjGradOffset (gsampler2D sampler, vec3 P,
vec2 dPdx, vec2 dPdy, ivec2 offset)
gvec4 textureProjGradOffset (gsampler2D sampler, vec4 P,
vec2 dPdx, vec2 dPdy, ivec2 offset)
gvec4 textureProjGradOffset (gsampler2DRect sampler, vec3 P,
vec2 dPdx, vec2 dPdy, ivec2 offset)
gvec4 textureProjGradOffset (gsampler2DRect sampler, vec4 P,
vec2 dPdx, vec2 dPdy, ivec2 offset)
float textureProjGradOffset (sampler2DRectShadow sampler,
vec4 P,
vec2 dPdx, vec2 dPdy, ivec2 offset)
gvec4 textureProjGradOffset (gsampler3D sampler, vec4 P,
vec3 dPdx, vec3 dPdy, ivec3 offset)
float textureProjGradOffset (sampler1DShadow sampler, vec4 P,
float dPdx, float dPdy, int offset)
float textureProjGradOffset (sampler2DShadow sampler, vec4 P,
vec2 dPdx, vec2 dPdy, ivec2 offset)
Do a texture lookup projectively
and with explicit gradient as
described in textureProjGrad,
as well as with offset, as
described in textureOffset.
8.9.3 Texture Gather Functions
The texture gather functions take components of a single floating-point vector operand as a texture
coordinate, determine a set of four texels to sample from the base level of detail of the specified texture
image, and return one component from each texel in a four-component result vector.
When performing a texture gather operation, the minification and magnification filters are ignored, and
the rules for LINEAR filtering in the OpenGL Specification are applied to the base level of the texture
image to identify the four texels i0j1, i1j1, i1j0, and i0j0. The texels are then converted to texture base colors
(Rs, Gs, Bs, As) according to table 15.1, followed by application of the texture swizzle as described in
section 15.2.1 “Texture Access” of the OpenGL Graphics System Specification. A four-component
vector is assembled by taking the selected component from each of the post-swizzled texture source colors
in the order (i0j1, i1j1, i1j0, i0j0).
For texture gather functions using a shadow sampler type, each of the four texel lookups perform a depth
comparison against the depth reference value passed in (refZ), and returns the result of that comparison in
the appropriate component of the result vector.
As with other texture lookup functions, the results of a texture gather are undefined for shadow samplers if
the texture referenced is not a depth texture or has depth comparisons disabled; or for non-shadow
samplers if the texture referenced is a depth texture with depth comparisons enabled.
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8 Built-in Functions
Syntax Description
gvec4 textureGather (gsampler2D sampler, vec2 P
[, int comp])
gvec4 textureGather (gsampler2DArray sampler,
vec3 P [, int comp])
gvec4 textureGather (gsamplerCube sampler,
vec3 P [, int comp])
gvec4 textureGather (gsamplerCubeArray sampler,
vec4 P[, int comp])
gvec4 textureGather (gsampler2DRect sampler,
vec2 P[, int comp])
vec4 textureGather (sampler2DShadow sampler,
vec2 P, float refZ)
vec4 textureGather (sampler2DArrayShadow sampler,
vec3 P, float refZ)
vec4 textureGather (samplerCubeShadow sampler,
vec3 P, float refZ)
vec4 textureGather (samplerCubeArrayShadow
sampler,
vec4 P, float refZ)
vec4 textureGather (sampler2DRectShadow sampler,
vec2 P, float refZ)
Returns the value
vec4(Sample_i0_j1(P, base).comp,
Sample_i1_j1(P, base).comp,
Sample_i1_j0(P, base).comp,
Sample_i0_j0(P, base).comp)
If specified, the value of comp must be a
constant integer expression with a value of 0,
1, 2, or 3, identifying the x, y, z, or w post-
swizzled component of the four-component
vector lookup result for each texel,
respectively. If comp is not specified, it is
treated as 0, selecting the x component of
each texel to generate the result.
gvec4 textureGatherOffset (
gsampler2D sampler,
vec2 P, ivec2 offset
[, int comp])
gvec4 textureGatherOffset (
gsampler2DArray sampler,
vec3 P, ivec2 offset
[, int comp])
gvec4 textureGatherOffset (
gsampler2DRect sampler,
vec2 P, ivec2 offset
[, int comp])
vec4 textureGatherOffset (
sampler2DShadow sampler,
vec2 P, float refZ, ivec2 offset)
vec4 textureGatherOffset (
sampler2DArrayShadow sampler,
vec3 P, float refZ, ivec2 offset)
vec4 textureGatherOffset (
sampler2DRectShadow sampler,
vec2 P, float refZ, ivec2 offset)
Perform a texture gather operation as in
textureGather by offset as described in
textureOffset except that the offset can be
variable (non constant) and the
implementation-dependent minimum and
maximum offset values are given by
MIN_PROGRAM_TEXTURE_GATHER_OFFSET
and
MAX_PROGRAM_TEXTURE_GATHER_OFFSET,
respectively.
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Syntax Description
gvec4 textureGatherOffsets (
gsampler2D sampler,
vec2 P, ivec2 offsets[4]
[, int comp])
gvec4 textureGatherOffsets (
gsampler2DArray sampler,
vec3 P, ivec2 offsets[4]
[, int comp])
gvec4 textureGatherOffsets (
gsampler2DRect sampler,
vec2 P, ivec2 offsets[4]
[, int comp])
vec4 textureGatherOffsets (
sampler2DShadow sampler,
vec2 P, float refZ, ivec2
offsets[4])
vec4 textureGatherOffsets (
sampler2DArrayShadow sampler,
vec3 P, float refZ, ivec2
offsets[4])
vec4 textureGatherOffsets (
sampler2DRectShadow sampler,
vec2 P, float refZ, ivec2
offsets[4])
Operate identically to textureGatherOffset
except that offsets is used to determine the
location of the four texels to sample. Each
of the four texels is obtained by applying the
corresponding offset in offsets as a (u, v)
coordinate offset to P, identifying the four-
texel LINEAR footprint, and then selecting
the texel i0j0 of that footprint. The specified
values in offsets must be set with constant
integral expressions.
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8 Built-in Functions
8.9.4 Compatibility Profile Texture Functions
The following texture functions are only in the compatibility profile.
Syntax (deprecated) Description (deprecated)
vec4 texture1D (sampler1D sampler,
float coord [, float bias] )
vec4 texture1DProj (sampler1D sampler,
vec2 coord [, float bias] )
vec4 texture1DProj (sampler1D sampler,
vec4 coord [, float bias] )
vec4 texture1DLod (sampler1D sampler,
float coord, float lod)
vec4 texture1DProjLod (sampler1D sampler,
vec2 coord, float lod)
vec4 texture1DProjLod (sampler1D sampler,
vec4 coord, float lod)
See corresponding signature above without
“1D” in the name.
vec4 texture2D (sampler2D sampler,
vec2 coord [, float bias] )
vec4 texture2DProj (sampler2D sampler,
vec3 coord [, float bias] )
vec4 texture2DProj (sampler2D sampler,
vec4 coord [, float bias] )
vec4 texture2DLod (sampler2D sampler,
vec2 coord, float lod)
vec4 texture2DProjLod (sampler2D sampler,
vec3 coord, float lod)
vec4 texture2DProjLod (sampler2D sampler,
vec4 coord, float lod)
See corresponding signature above without
“2D” in the name.
vec4 texture3D (sampler3D sampler,
vec3 coord [, float bias] )
vec4 texture3DProj (sampler3D sampler,
vec4 coord [, float bias] )
vec4 texture3DLod (sampler3D sampler,
vec3 coord, float lod)
vec4 texture3DProjLod (sampler3D sampler,
vec4 coord, float lod)
See corresponding signature above without
“3D” in the name.
Use the texture coordinate coord to do a
texture lookup in the 3D texture currently
bound to sampler. For the projective
(“Proj”) versions, the texture coordinate is
divided by coord.q.
vec4 textureCube (samplerCube sampler,
vec3 coord [, float bias] )
vec4 textureCubeLod (samplerCube sampler,
vec3 coord, float lod)
See corresponding signature above without
“Cube” in the name.
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8 Built-in Functions
Syntax (deprecated) Description (deprecated)
vec4 shadow1D (sampler1DShadow sampler,
vec3 coord [, float bias] )
vec4 shadow2D (sampler2DShadow sampler,
vec3 coord [, float bias] )
vec4 shadow1DProj (sampler1DShadow sampler,
vec4 coord [, float bias] )
vec4 shadow2DProj (sampler2DShadow sampler,
vec4 coord [, float bias] )
vec4 shadow1DLod (sampler1DShadow sampler,
vec3 coord, float lod)
vec4 shadow2DLod (sampler2DShadow sampler,
vec3 coord, float lod)
vec4 shadow1DProjLod(sampler1DShadow sampler,
vec4 coord, float lod)
vec4 shadow2DProjLod(sampler2DShadow sampler,
vec4 coord, float lod)
Same functionality as the “texture” based
names above with the same signature.
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8 Built-in Functions
8.10 Atomic-Counter Functions
The atomic-counter operations in this section operate atomically with respect to each other. They are
atomic for any single counter, meaning any of these operations on a specific counter in one shader
instantiation will be indivisible by any of these operations on the same counter from another shader
instantiation. There is no guarantee that these operations are atomic with respect to other forms of access
to the counter or that they are serialized when applied to separate counters. Such cases would require
additional use of fences, barriers, or other forms of synchronization, if atomicity or serialization is desired.
The value returned by an atomic-counter function is the value of an atomic counter, which may be
•returned and incremented in an atomic operation, or
•decremented and returned in an atomic operation, or
•simply returned.
The underlying counter is a 32-bit unsigned integer. Increments and decrements at the limit of the range
will wrap to [0, 232-1].
Syntax Description
uint atomicCounterIncrement (atomic_uint c)Atomically
1. increments the counter for c, and
2. returns its value prior to the increment
operation.
These two steps are done atomically with respect to
the atomic counter functions in this table.
uint atomicCounterDecrement (atomic_uint c)Atomically
1. decrements the counter for c, and
2. returns the value resulting from the
decrement operation.
These two steps are done atomically with respect to
the atomic counter functions in this table.
uint atomicCounter (atomic_uint c)Returns the counter value for c.
8.11 Atomic Memory Functions
Atomic memory functions perform atomic operations on an individual signed or unsigned integer stored in
buffer-object or shared-variable storage. All of the atomic memory operations read a value from memory,
compute a new value using one of the operations described below, write the new value to memory, and
return the original value read. The contents of the memory being updated by the atomic operation are
guaranteed not to be modified by any other assignment or atomic memory function in any shader
invocation between the time the original value is read and the time the new value is written.
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8 Built-in Functions
Atomic memory functions are supported only for a limited set of variables. A shader will fail to compile
if the value passed to the mem argument of an atomic memory function does not correspond to a buffer or
shared variable. It is acceptable to pass an element of an array or a single component of a vector to the
mem argument of an atomic memory function, as long as the underlying array or vector is a buffer or
shared variable.
Syntax Description
uint atomicAdd (inout uint mem, uint data)
int atomicAdd (inout int mem, int data)
Computes a new value by adding the value of data to
the contents mem.
uint atomicMin (inout uint mem, uint data)
int atomicMin (inout int mem, int data)
Computes a new value by taking the minimum of the
value of data and the contents of mem.
uint atomicMax (inout uint mem, uint data)
int atomicMax (inout int mem, int data)
Computes a new value by taking the maximum of the
value of data and the contents of mem.
uint atomicAnd (inout uint mem, uint data)
int atomicAnd (inout int mem, int data)
Computes a new value by performing a bit-wise
AND of the value of data and the contents of mem.
uint atomicOr (inout uint mem, uint data)
int atomicOr (inout int mem, int data)
Computes a new value by performing a bit-wise OR
of the value of data and the contents of mem.
uint atomicXor (inout uint mem, uint data)
int atomicXor (inout int mem, int data)
Computes a new value by performing a bit-wise
EXCLUSIVE OR of the value of data and the
contents of mem.
uint atomicExchange (inout uint mem, uint data)
int atomicExchange (inout int mem, int data)
Computes a new value by simply copying the value
of data.
uint atomicCompSwap (inout uint mem,
uint compare, uint data)
int atomicCompSwap (inout int mem,
int compare, int data)
Compares the value of compare and the contents of
mem. If the values are equal, the new value is given
by data; otherwise, it is taken from the original
contents of mem.
8.12 Image Functions
Variables using one of the image basic types may be used by the built-in shader image memory functions
defined in this section to read and write individual texels of a texture. Each image variable references an
image unit, which has a texture image attached.
When image memory functions below access memory, an individual texel in the image is identified using
an (i), (i, j), or (i, j, k) coordinate corresponding to the values of P. For image2DMS and
image2DMSArray variables (and the corresponding int/unsigned int types) corresponding to multi-
sample textures, each texel may have multiple samples and an individual sample is identified using the
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8 Built-in Functions
integer sample parameter. The coordinates and sample number are used to select an individual texel in
the manner described in section 8.25 “Texture Image Loads and Stores” of the OpenGL specification.
Loads and stores support float, integer, and unsigned integer types. The data types below starting
“gimage” serve as placeholders meaning types starting either “image”, “iimage”, or “uimage” in the same
way as gvec or gsampler in earlier sections.
The IMAGE_PARAMS in the prototypes below is a placeholder representing 33 separate functions, each
for a different type of image variable. The IMAGE_PARAMS placeholder is replaced by one of the
following parameter lists:
gimage1D image, int P
gimage2D image, ivec2 P
gimage3D image, ivec3 P
gimage2DRect image, ivec2 P
gimageCube image, ivec3 P
gimageBuffer image, int P
gimage1DArray image, ivec2 P
gimage2DArray image, ivec3 P
gimageCubeArray image, ivec3 P
gimage2DMS image, ivec2 P, int sample
gimage2DMSArray image, ivec3 P, int sample
where each of the lines represents one of three different image variable types, and image, P, and sample
specify the individual texel to operate on. The method for identifying the individual texel operated on
from image, P, and sample, and the method for reading and writing the texel are specified in section 8.25
“Texture Image Loads and Stores” of the OpenGL specification.
The atomic functions perform atomic operations on individual texels or samples of an image variable.
Atomic memory operations read a value from the selected texel, compute a new value using one of the
operations described below, write the new value to the selected texel, and return the original value read.
The contents of the texel being updated by the atomic operation are guaranteed not to be modified by any
other image store or atomic function between the time the original value is read and the time the new
value is written.
Atomic memory operations are supported on only a subset of all image variable types; image must be
either:
•a signed integer image variable (type starts “iimage”) and a format qualifier of r32i, used with a
data argument of type int, or
•an unsigned image variable (type starts “uimage”) and a format qualifier of r32ui, used with a
data argument of type uint.
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8 Built-in Functions
Syntax Description
int imageSize (readonly writeonly
gimage1D image)
ivec2 imageSize (readonly writeonly
gimage2D image)
ivec3 imageSize (readonly writeonly
gimage3D image)
ivec2 imageSize (readonly writeonly
gimageCube image)
ivec3 imageSize (readonly writeonly
gimageCubeArray image)
ivec2 imageSize (readonly writeonly
gimageRect image)
ivec2 imageSize (readonly writeonly
gimage1DArray image)
ivec3 imageSize (readonly writeonly
gimage2DArray image)
int imageSize (readonly writeonly
gimageBuffer image)
ivec2 imageSize (readonly writeonly
gimage2DMS image)
ivec3 imageSize (readonly writeonly
gimage2DMSArray image)
Returns the dimensions of the image or images
bound to image. For arrayed images, the last
component of the return value will hold the size of
the array. Cube images only return the dimensions
of one face, and the number of cubes in the cube map
array, if arrayed.
Note: The qualification readonly writeonly accepts
a variable qualified with readonly, writeonly, both,
or neither. It means the formal argument will be
used for neither reading nor writing to the underlying
memory.
gvec4 imageLoad (readonly IMAGE_PARAMS)Loads the texel at the coordinate P from the image
unit image (in IMAGE_PARAMS). For multi-sample
loads, the sample number is given by sample. When
image, P, sample identify a valid texel, the bits used
to represent the selected texel in memory are
converted to a vec4, ivec4, or uvec4 in the manner
described in section 8.25 “Texture Image Loads and
Stores” of the OpenGL Specification and returned.
void imageStore (writeonly IMAGE_PARAMS,
gvec4 data)
Stores data into the texel at the coordinate P from
the image specified by image. For multi-sample
stores, the sample number is given by sample. When
image, P, and sample identify a valid texel, the bits
used to represent data are converted to the format of
the image unit in the manner described in section
8.25 “Texture Image Loads and Stores” of the
OpenGL Specification and stored to the specified
texel.
uint imageAtomicAdd (IMAGE_PARAMS,
uint data)
int imageAtomicAdd (IMAGE_PARAMS,
int data)
Computes a new value by adding the value of data
to the contents of the selected texel.
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8 Built-in Functions
Syntax Description
uint imageAtomicMin (IMAGE_PARAMS,
uint data)
int imageAtomicMin (IMAGE_PARAMS,
int data)
Computes a new value by taking the minimum of the
value of data and the contents of the selected texel.
uint imageAtomicMax (IMAGE_PARAMS,
uint data)
int imageAtomicMax (IMAGE_PARAMS,
int data)
Computes a new value by taking the maximum of the
value data and the contents of the selected texel.
uint imageAtomicAnd (IMAGE_PARAMS,
uint data)
int imageAtomicAnd (IMAGE_PARAMS,
int data)
Computes a new value by performing a bit-wise
AND of the value of data and the contents of the
selected texel.
uint imageAtomicOr (IMAGE_PARAMS,
uint data)
int imageAtomicOr (IMAGE_PARAMS,
int data)
Computes a new value by performing a bit-wise OR
of the value of data and the contents of the selected
texel.
uint imageAtomicXor (IMAGE_PARAMS,
uint data)
int imageAtomicXor (IMAGE_PARAMS,
int data)
Computes a new value by performing a bit-wise
EXCLUSIVE OR of the value of data and the
contents of the selected texel.
uint imageAtomicExchange (IMAGE_PARAMS,
uint data)
int imageAtomicExchange (IMAGE_PARAMS,
int data)
Computes a new value by simply copying the value
of data.
uint imageAtomicCompSwap
(IMAGE_PARAMS,
uint compare,
uint data)
int imageAtomicCompSwap
(IMAGE_PARAMS,
int compare,
int data)
Compares the value of compare and the contents of
the selected texel. If the values are equal, the new
value is given by data; otherwise, it is taken from the
original value loaded from the texel.
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8.13 Fragment Processing Functions
Fragment processing functions are only available in fragment shaders.
8.13.1 Derivative Functions
Derivatives may be computationally expensive and/or numerically unstable. Therefore, an OpenGL
implementation may approximate the true derivatives by using a fast but not entirely accurate derivative
computation. Derivatives are undefined within non-uniform control flow.
The expected behavior of a derivative is specified using forward/backward differencing.
Forward differencing:
F(x+dx)−F(x) ∼ dFdx(x)⋅dx
1a
dFdx (x) ∼ F(x+dx)−F(x)
dx
1b
Backward differencing:
F(x−dx)−F(x) ∼−dFdx(x)⋅dx
2a
dFdx (x) ∼ F(x)−F(x−dx)
dx
2b
With single-sample rasterization, dx <= 1.0 in equations 1b and 2b. For multi-sample rasterization, dx <
2.0 in equations 1b and 2b.
dFdy is approximated similarly, with y replacing x.
A GL implementation may use the above or other methods to perform the calculation, subject to the
following conditions:
1. The method may use piecewise linear approximations. Such linear approximations imply that higher
order derivatives, dFdx(dFdx(x)) and above, are undefined.
2. The method may assume that the function evaluated is continuous. Therefore derivatives within non-
uniform control flow are undefined.
3. The method may differ per fragment, subject to the constraint that the method may vary by window
coordinates, not screen coordinates. The invariance requirement described in section 14.2
“Invariance” of the OpenGL Graphics System Specification, is relaxed for derivative calculations,
because the method may be a function of fragment location.
Other properties that are desirable, but not required, are:
4. Functions should be evaluated within the interior of a primitive (interpolated, not extrapolated).
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8 Built-in Functions
5. Functions for dFdx should be evaluated while holding y constant. Functions for dFdy should be
evaluated while holding x constant. However, mixed higher order derivatives, like dFdx(dFdy(y))
and dFdy(dFdx(x)) are undefined.
6. Derivatives of constant arguments should be 0.
In some implementations, varying degrees of derivative accuracy may be obtained by providing GL hints
(section 21.4 “Hints” of the OpenGL Graphics System Specification), allowing a user to make an image
quality versus speed trade off.
Syntax Description
genType dFdx (genType p)Returns the derivative in x using local differencing for
the input argument p.
genType dFdy (genType p)Returns the derivative in y using local differencing for
the input argument p.
These two functions are commonly used to estimate the
filter width used to anti-alias procedural textures. We
are assuming that the expression is being evaluated in
parallel on a SIMD array so that at any given point in
time the value of the function is known at the grid points
represented by the SIMD array. Local differencing
between SIMD array elements can therefore be used to
derive dFdx, dFdy, etc.
genType fwidth (genType p)Returns the sum of the absolute derivative in x and y
using local differencing for the input argument p, i.e.,
abs (dFdx (p)) + abs (dFdy (p));
8.13.2 Interpolation Functions
Built-in interpolation functions are available to compute an interpolated value of a fragment shader input
variable at a shader-specified (x, y) location. A separate (x, y) location may be used for each invocation of
the built-in function, and those locations may differ from the default (x, y) location used to produce the
default value of the input.
For all of the interpolation functions, interpolant must be an input variable or an element of an input
variable declared as an array. Component selection operators (e.g., .xy) may be used when specifying
interpolant. Arrayed inputs can be indexed with general (nonuniform) integer expressions. If interpolant
is declared with the flat qualifier, the interpolated value will have the same value everywhere for a single
primitive, so the location used for interpolation has no effect and the functions just return that same value.
If interpolant is declared with the centroid qualifier, the value returned by interpolateAtSample() and
interpolateAtOffset() will be evaluated at the specified location, ignoring the location normally used
with the centroid qualifier. If interpolant is declared with the noperspective qualifier, the interpolated
value will be computed without perspective correction.
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8 Built-in Functions
Syntax Description
float interpolateAtCentroid (float interpolant)
vec2 interpolateAtCentroid (vec2 interpolant)
vec3 interpolateAtCentroid (vec3 interpolant)
vec4 interpolateAtCentroid (vec4 interpolant)
Returns the value of the input interpolant sampled at
a location inside both the pixel and the primitive
being processed. The value obtained would be the
same value assigned to the input variable if declared
with the centroid qualifier.
float interpolateAtSample (float interpolant,
int sample)
vec2 interpolateAtSample (vec2 interpolant,
int sample)
vec3 interpolateAtSample (vec3 interpolant,
int sample)
vec4 interpolateAtSample (vec4 interpolant,
int sample)
Returns the value of the input interpolant variable at
the location of sample number sample. If
multisample buffers are not available, the input
variable will be evaluated at the center of the pixel.
If sample sample does not exist, the position used to
interpolate the input variable is undefined.
float interpolateAtOffset (float interpolant,
vec2 offset)
vec2 interpolateAtOffset (vec2 interpolant,
vec2 offset)
vec3 interpolateAtOffset (vec3 interpolant,
vec2 offset)
vec4 interpolateAtOffset (vec4 interpolant,
vec2 offset)
Returns the value of the input interpolant variable
sampled at an offset from the center of the pixel
specified by offset. The two floating-point
components of offset, give the offset in pixels in the x
and y directions, respectively. An offset of (0, 0)
identifies the center of the pixel. The range and
granularity of offsets supported by this function is
implementation-dependent.
8.14 Noise Functions
The noise functions noise1, noise2, noise3, and noise4 have been deprecated starting with version 4.4 of
GLSL. They are defined to return the value 0.0 or a vector whose components are all 0.0. However, as in
previous releases, they are not semantically considered to be compile-time constant expressions.
Syntax (deprecated) Description (deprecated)
float noise1 (genType x)Returns a 1D noise value based on the input value x.
vec2 noise2 (genType x)Returns a 2D noise value based on the input value x.
vec3 noise3 (genType x)Returns a 3D noise value based on the input value x.
vec4 noise4 (genType x)Returns a 4D noise value based on the input value x.
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8.15 Geometry Shader Functions
These functions are only available in geometry shaders. They are described in more depth following the
table.
Syntax Description
void EmitStreamVertex (int stream)Emits the current values of output variables to the current
output primitive on stream stream. The argument to stream
must be a constant integral expression. On return from this
call, the values of all output variables are undefined.
Can only be used if multiple output streams are supported.
void EndStreamPrimitive (int stream)Completes the current output primitive on stream stream and
starts a new one. The argument to stream must be a constant
integral expression. No vertex is emitted.
Can only be used if multiple output streams are supported.
void EmitVertex () Emits the current values of output variables to the current
output primitive. On return from this call, the values of
output variables are undefined.
When multiple output streams are supported, this is
equivalent to calling EmitStreamVertex(0).
void EndPrimitive () Completes the current output primitive and starts a new one.
No vertex is emitted.
When multiple output streams are supported, this is
equivalent to calling EndStreamPrimitive(0).
The function EmitStreamVertex() specifies that a vertex is completed. A vertex is added to the current
output primitive in vertex stream stream using the current values of all built-in and user-defined output
variables associated with stream. The values of all output variables for all output streams are undefined
after a call to EmitStreamVertex(). If a geometry shader invocation has emitted more vertices than
permitted by the output layout qualifier max_vertices, the results of calling EmitStreamVertex() are
undefined.
The function EndStreamPrimitive() specifies that the current output primitive for vertex stream stream is
completed and a new output primitive (of the same type) will be started by any subsequent
EmitStreamVertex(). This function does not emit a vertex. If the output layout is declared to be
“points”, calling EndStreamPrimitive() is optional.
A geometry shader starts with an output primitive containing no vertices for each stream. When a
geometry shader terminates, the current output primitive for each stream is automatically completed. It is
not necessary to call EndStreamPrimitive() if the geometry shader writes only a single primitive.
180
8 Built-in Functions
Multiple output streams are supported only if the output primitive type is declared to be points. A
program will fail to link if it contains a geometry shader calling EmitStreamVertex() or
EndStreamPrimitive() if its output primitive type is not points.
181
8 Built-in Functions
8.16 Shader Invocation Control Functions
The shader invocation control function is available only in tessellation control shaders and compute
shaders. It is used to control the relative execution order of multiple shader invocations used to process a
patch (in the case of tessellation control shaders) or a local work group (in the case of compute shaders),
which are otherwise executed with an undefined relative order.
Syntax Description
void barrier () For any given static instance of barrier(), all tessellation control shader
invocations for a single input patch must enter it before any will be
allowed to continue beyond it, or all invocations for a single work group
must enter it before any will continue beyond it.
The function barrier() provides a partially defined order of execution between shader invocations. This
ensures that values written by one invocation prior to a given static instance of barrier() can be safely
read by other invocations after their call to the same static instance barrier(). Because invocations may
execute in undefined order between these barrier calls, the values of a per-vertex or per-patch output
variable or shared variables for compute shaders will be undefined in a number of cases enumerated in
section 4.3.6 “Output Variables” (for tessellation control shaders) and section 4.3.8 "Shared Variables"
(for compute shaders).
For tessellation control shaders, the barrier() function may only be placed inside the function main() of
the tessellation control shader and may not be called within any control flow. Barriers are also disallowed
after a return statement in the function main(). Any such misplaced barriers result in a compile-time
error.
For compute shaders, the barrier() function may be placed within flow control, but that flow control must
be uniform flow control. That is, all the controlling expressions that lead to execution of the barrier must
be dynamically uniform expressions. This ensures that if any shader invocation enters a conditional
statement, then all invocations will enter it. While compilers are encouraged to give warnings if they can
detect this might not happen, compilers cannot completely determine this. Hence, it is the author's
responsibility to ensure barrier() only exists inside uniform flow control. Otherwise, some shader
invocations will stall indefinitely, waiting for a barrier that is never reached by other invocations.
182
8 Built-in Functions
8.17 Shader Memory Control Functions
Shaders of all types may read and write the contents of textures and buffer objects using image variables.
While the order of reads and writes within a single shader invocation is well-defined, the relative order of
reads and writes to a single shared memory address from multiple separate shader invocations is largely
undefined. The order of memory accesses performed by one shader invocation, as observed by other
shader invocations, is also largely undefined but can be controlled through memory control functions.
Syntax Description
void memoryBarrier () Control the ordering of memory transactions issued by a
single shader invocation.
void memoryBarrierAtomicCounter () Control the ordering of accesses to atomic-counter variables
issued by a single shader invocation.
void memoryBarrierBuffer () Control the ordering of memory transactions to buffer
variables issued within a single shader invocation.
void memoryBarrierShared () Control the ordering of memory transactions to shared
variables issued within a single shader invocation.
Only available in compute shaders.
void memoryBarrierImage () Control the ordering of memory transactions to images
issued within a single shader invocation.
void groupMemoryBarrier () Control the ordering of all memory transactions issued within
a single shader invocation, as viewed by other invocations in
the same work group.
Only available in compute shaders.
The memory barrier built-in functions can be used to order reads and writes to variables stored in memory
accessible to other shader invocations. When called, these functions will wait for the completion of all
reads and writes previously performed by the caller that access selected variable types, and then return
with no other effect. The built-in functions memoryBarrierAtomicCounter(), memoryBarrierBuffer(),
memoryBarrierImage(), and memoryBarrierShared() wait for the completion of accesses to atomic
counter, buffer, image, and shared variables, respectively. The built-in functions memoryBarrier() and
groupMemoryBarrier() wait for the completion of accesses to all of the above variable types. The
functions memoryBarrierShared() and groupMemoryBarrier() are available only in compute shaders;
the other functions are available in all shader types.
183
8 Built-in Functions
When these functions return, the results of any memory stores performed using coherent variables
performed prior to the call will be visible to any future coherent access to the same memory performed by
any other shader invocation. In particular, the values written this way in one shader stage are guaranteed
to be visible to coherent memory accesses performed by shader invocations in subsequent stages when
those invocations were triggered by the execution of the original shader invocation (e.g., fragment shader
invocations for a primitive resulting from a particular geometry shader invocation).
Additionally, memory barrier functions order stores performed by the calling invocation, as observed by
other shader invocations. Without memory barriers, if one shader invocation performs two stores to
coherent variables, a second shader invocation might see the values written by the second store prior to
seeing those written by the first. However, if the first shader invocation calls a memory barrier function
between the two stores, selected other shader invocations will never see the results of the second store
before seeing those of the first. When using the function groupMemoryBarrier(), this ordering
guarantee applies only to other shader invocations in the same compute shader work group; all other
memory barrier functions provide the guarantee to all other shader invocations. No memory barrier is
required to guarantee the order of memory stores as observed by the invocation performing the stores; an
invocation reading from a variable that it previously wrote will always see the most recently written value
unless another shader invocation also wrote to the same memory.
184
9 Shading Language Grammar for Core Profile
9 Shading Language Grammar for Core
Profile
The grammar is fed from the output of lexical analysis. The tokens returned from lexical analysis are
CONST BOOL FLOAT DOUBLE INT UINT
BREAK CONTINUE DO ELSE FOR IF DISCARD RETURN SWITCH CASE DEFAULT SUBROUTINE
BVEC2 BVEC3 BVEC4 IVEC2 IVEC3 IVEC4 UVEC2 UVEC3 UVEC4 VEC2 VEC3 VEC4
MAT2 MAT3 MAT4 CENTROID IN OUT INOUT
UNIFORM PATCH SAMPLE BUFFER SHARED
COHERENT VOLATILE RESTRICT READONLY WRITEONLY
DVEC2 DVEC3 DVEC4 DMAT2 DMAT3 DMAT4
NOPERSPECTIVE FLAT SMOOTH LAYOUT
MAT2X2 MAT2X3 MAT2X4
MAT3X2 MAT3X3 MAT3X4
MAT4X2 MAT4X3 MAT4X4
DMAT2X2 DMAT2X3 DMAT2X4
DMAT3X2 DMAT3X3 DMAT3X4
DMAT4X2 DMAT4X3 DMAT4X4
ATOMIC_UINT
SAMPLER1D SAMPLER2D SAMPLER3D SAMPLERCUBE SAMPLER1DSHADOW SAMPLER2DSHADOW
SAMPLERCUBESHADOW SAMPLER1DARRAY SAMPLER2DARRAY SAMPLER1DARRAYSHADOW
SAMPLER2DARRAYSHADOW ISAMPLER1D ISAMPLER2D ISAMPLER3D ISAMPLERCUBE
ISAMPLER1DARRAY ISAMPLER2DARRAY USAMPLER1D USAMPLER2D USAMPLER3D
USAMPLERCUBE USAMPLER1DARRAY USAMPLER2DARRAY
SAMPLER2DRECT SAMPLER2DRECTSHADOW ISAMPLER2DRECT USAMPLER2DRECT
SAMPLERBUFFER ISAMPLERBUFFER USAMPLERBUFFER
SAMPLERCUBEARRAY SAMPLERCUBEARRAYSHADOW
ISAMPLERCUBEARRAY USAMPLERCUBEARRAY
SAMPLER2DMS ISAMPLER2DMS USAMPLER2DMS
SAMPLER2DMSARRAY ISAMPLER2DMSARRAY USAMPLER2DMSARRAY
IMAGE1D IIMAGE1D UIMAGE1D IMAGE2D IIMAGE2D
UIMAGE2D IMAGE3D IIMAGE3D UIMAGE3D
IMAGE2DRECT IIMAGE2DRECT UIMAGE2DRECT
IMAGECUBE IIMAGECUBE UIMAGECUBE
IMAGEBUFFER IIMAGEBUFFER UIMAGEBUFFER
IMAGE1DARRAY IIMAGE1DARRAY UIMAGE1DARRAY
IMAGE2DARRAY IIMAGE2DARRAY UIMAGE2DARRAY
IMAGECUBEARRAY IIMAGECUBEARRAY UIMAGECUBEARRAY
IMAGE2DMS IIMAGE2DMS UIMAGE2DMS
IMAGE2DMSARRAY IIMAGE2DMSARRAY UIMAGE2DMSARRAY
185
9 Shading Language Grammar for Core Profile
STRUCT VOID WHILE
IDENTIFIER TYPE_NAME
FLOATCONSTANT DOUBLECONSTANT INTCONSTANT UINTCONSTANT BOOLCONSTANT
FIELD_SELECTION
LEFT_OP RIGHT_OP
INC_OP DEC_OP LE_OP GE_OP EQ_OP NE_OP
AND_OP OR_OP XOR_OP MUL_ASSIGN DIV_ASSIGN ADD_ASSIGN
MOD_ASSIGN LEFT_ASSIGN RIGHT_ASSIGN AND_ASSIGN XOR_ASSIGN OR_ASSIGN
SUB_ASSIGN
LEFT_PAREN RIGHT_PAREN LEFT_BRACKET RIGHT_BRACKET LEFT_BRACE RIGHT_BRACE DOT
COMMA COLON EQUAL SEMICOLON BANG DASH TILDE PLUS STAR SLASH PERCENT
LEFT_ANGLE RIGHT_ANGLE VERTICAL_BAR CARET AMPERSAND QUESTION
INVARIANT PRECISE
HIGH_PRECISION MEDIUM_PRECISION LOW_PRECISION PRECISION
The following describes the grammar for the OpenGL Shading Language in terms of the above tokens.
The starting rule is translation_unit. An empty shader (one having no tokens to parse, after pre-
processing) is valid, resulting in no compile-time errors, even though the grammar below does not have a
rule to accept an empty token stream.
variable_identifier:
IDENTIFIER
primary_expression:
variable_identifier
INTCONSTANT
UINTCONSTANT
FLOATCONSTANT
BOOLCONSTANT
DOUBLECONSTANT
LEFT_PAREN expression RIGHT_PAREN
postfix_expression:
primary_expression
postfix_expression LEFT_BRACKET integer_expression RIGHT_BRACKET
function_call
postfix_expression DOT FIELD_SELECTION
postfix_expression INC_OP
postfix_expression DEC_OP
186
9 Shading Language Grammar for Core Profile
integer_expression:
expression
function_call:
function_call_or_method
function_call_or_method:
function_call_generic
function_call_generic:
function_call_header_with_parameters RIGHT_PAREN
function_call_header_no_parameters RIGHT_PAREN
function_call_header_no_parameters:
function_call_header VOID
function_call_header
function_call_header_with_parameters:
function_call_header assignment_expression
function_call_header_with_parameters COMMA assignment_expression
function_call_header:
function_identifier LEFT_PAREN
// Grammar Note: Constructors look like functions, but lexical analysis recognized most of them as
// keywords. They are now recognized through “type_specifier”.
// Methods (.length), subroutine array calls, and identifiers are recognized through postfix_expression.
function_identifier:
type_specifier
postfix_expression
unary_expression:
postfix_expression
INC_OP unary_expression
DEC_OP unary_expression
unary_operator unary_expression
// Grammar Note: No traditional style type casts.
187
9 Shading Language Grammar for Core Profile
unary_operator:
PLUS
DASH
BANG
TILDE
// Grammar Note: No '*' or '&' unary ops. Pointers are not supported.
multiplicative_expression:
unary_expression
multiplicative_expression STAR unary_expression
multiplicative_expression SLASH unary_expression
multiplicative_expression PERCENT unary_expression
additive_expression:
multiplicative_expression
additive_expression PLUS multiplicative_expression
additive_expression DASH multiplicative_expression
shift_expression:
additive_expression
shift_expression LEFT_OP additive_expression
shift_expression RIGHT_OP additive_expression
relational_expression:
shift_expression
relational_expression LEFT_ANGLE shift_expression
relational_expression RIGHT_ANGLE shift_expression
relational_expression LE_OP shift_expression
relational_expression GE_OP shift_expression
equality_expression:
relational_expression
equality_expression EQ_OP relational_expression
equality_expression NE_OP relational_expression
and_expression:
equality_expression
and_expression AMPERSAND equality_expression
188
9 Shading Language Grammar for Core Profile
exclusive_or_expression:
and_expression
exclusive_or_expression CARET and_expression
inclusive_or_expression:
exclusive_or_expression
inclusive_or_expression VERTICAL_BAR exclusive_or_expression
logical_and_expression:
inclusive_or_expression
logical_and_expression AND_OP inclusive_or_expression
logical_xor_expression:
logical_and_expression
logical_xor_expression XOR_OP logical_and_expression
logical_or_expression:
logical_xor_expression
logical_or_expression OR_OP logical_xor_expression
conditional_expression:
logical_or_expression
logical_or_expression QUESTION expression COLON assignment_expression
assignment_expression:
conditional_expression
unary_expression assignment_operator assignment_expression
assignment_operator:
EQUAL
MUL_ASSIGN
DIV_ASSIGN
MOD_ASSIGN
ADD_ASSIGN
SUB_ASSIGN
LEFT_ASSIGN
RIGHT_ASSIGN
AND_ASSIGN
XOR_ASSIGN
189
9 Shading Language Grammar for Core Profile
OR_ASSIGN
expression:
assignment_expression
expression COMMA assignment_expression
constant_expression:
conditional_expression
declaration:
function_prototype SEMICOLON
init_declarator_list SEMICOLON
PRECISION precision_qualifier type_specifier SEMICOLON
type_qualifier IDENTIFIER LEFT_BRACE struct_declaration_list RIGHT_BRACE SEMICOLON
type_qualifier IDENTIFIER LEFT_BRACE struct_declaration_list RIGHT_BRACE
IDENTIFIER SEMICOLON
type_qualifier IDENTIFIER LEFT_BRACE struct_declaration_list RIGHT_BRACE
IDENTIFIER array_specifier SEMICOLON
type_qualifier SEMICOLON
type_qualifier IDENTIFIER SEMICOLON
type_qualifier IDENTIFIER identifier_list SEMICOLON
identifier_list:
COMMA IDENTIFIER
identifier_list COMMA IDENTIFIER
function_prototype:
function_declarator RIGHT_PAREN
function_declarator:
function_header
function_header_with_parameters
function_header_with_parameters:
function_header parameter_declaration
function_header_with_parameters COMMA parameter_declaration
function_header:
fully_specified_type IDENTIFIER LEFT_PAREN
190
9 Shading Language Grammar for Core Profile
parameter_declarator:
type_specifier IDENTIFIER
type_specifier IDENTIFIER array_specifier
parameter_declaration:
type_qualifier parameter_declarator
parameter_declarator
type_qualifier parameter_type_specifier
parameter_type_specifier
parameter_type_specifier:
type_specifier
init_declarator_list:
single_declaration
init_declarator_list COMMA IDENTIFIER
init_declarator_list COMMA IDENTIFIER array_specifier
init_declarator_list COMMA IDENTIFIER array_specifier EQUAL initializer
init_declarator_list COMMA IDENTIFIER EQUAL initializer
single_declaration:
fully_specified_type
fully_specified_type IDENTIFIER
fully_specified_type IDENTIFIER array_specifier
fully_specified_type IDENTIFIER array_specifier EQUAL initializer
fully_specified_type IDENTIFIER EQUAL initializer
// Grammar Note: No 'enum', or 'typedef'.
fully_specified_type:
type_specifier
type_qualifier type_specifier
invariant_qualifier:
INVARIANT
interpolation_qualifier:
191
9 Shading Language Grammar for Core Profile
SMOOTH
FLAT
NOPERSPECTIVE
layout_qualifier:
LAYOUT LEFT_PAREN layout_qualifier_id_list RIGHT_PAREN
layout_qualifier_id_list:
layout_qualifier_id
layout_qualifier_id_list COMMA layout_qualifier_id
layout_qualifier_id:
IDENTIFIER
IDENTIFIER EQUAL constant_expression
SHARED
precise_qualifier:
PRECISE
type_qualifier:
single_type_qualifier
type_qualifier single_type_qualifier
single_type_qualifier:
storage_qualifier
layout_qualifier
precision_qualifier
interpolation_qualifier
invariant_qualifier
precise_qualifier
storage_qualifier:
CONST
INOUT
IN
OUT
CENTROID
PATCH
SAMPLE
192
9 Shading Language Grammar for Core Profile
UNIFORM
BUFFER
SHARED
COHERENT
VOLATILE
RESTRICT
READONLY
WRITEONLY
SUBROUTINE
SUBROUTINE LEFT_PAREN type_name_list RIGHT_PAREN
type_name_list:
TYPE_NAME
type_name_list COMMA TYPE_NAME
type_specifier:
type_specifier_nonarray
type_specifier_nonarray array_specifier
array_specifier:
LEFT_BRACKET RIGHT_BRACKET
LEFT_BRACKET constant_expression RIGHT_BRACKET
array_specifier LEFT_BRACKET RIGHT_BRACKET
array_specifier LEFT_BRACKET constant_expression RIGHT_BRACKET
type_specifier_nonarray:
VOID
FLOAT
DOUBLE
INT
UINT
BOOL
VEC2
VEC3
VEC4
DVEC2
DVEC3
193
9 Shading Language Grammar for Core Profile
DVEC4
BVEC2
BVEC3
BVEC4
IVEC2
IVEC3
IVEC4
UVEC2
UVEC3
UVEC4
MAT2
MAT3
MAT4
MAT2X2
MAT2X3
MAT2X4
MAT3X2
MAT3X3
MAT3X4
MAT4X2
MAT4X3
MAT4X4
DMAT2
DMAT3
DMAT4
DMAT2X2
DMAT2X3
DMAT2X4
DMAT3X2
DMAT3X3
DMAT3X4
DMAT4X2
DMAT4X3
DMAT4X4
ATOMIC_UINT
SAMPLER1D
SAMPLER2D
194
9 Shading Language Grammar for Core Profile
SAMPLER3D
SAMPLERCUBE
SAMPLER1DSHADOW
SAMPLER2DSHADOW
SAMPLERCUBESHADOW
SAMPLER1DARRAY
SAMPLER2DARRAY
SAMPLER1DARRAYSHADOW
SAMPLER2DARRAYSHADOW
SAMPLERCUBEARRAY
SAMPLERCUBEARRAYSHADOW
ISAMPLER1D
ISAMPLER2D
ISAMPLER3D
ISAMPLERCUBE
ISAMPLER1DARRAY
ISAMPLER2DARRAY
ISAMPLERCUBEARRAY
USAMPLER1D
USAMPLER2D
USAMPLER3D
USAMPLERCUBE
USAMPLER1DARRAY
USAMPLER2DARRAY
USAMPLERCUBEARRAY
SAMPLER2DRECT
SAMPLER2DRECTSHADOW
ISAMPLER2DRECT
USAMPLER2DRECT
SAMPLERBUFFER
ISAMPLERBUFFER
USAMPLERBUFFER
SAMPLER2DMS
ISAMPLER2DMS
USAMPLER2DMS
SAMPLER2DMSARRAY
ISAMPLER2DMSARRAY
195
9 Shading Language Grammar for Core Profile
USAMPLER2DMSARRAY
IMAGE1D
IIMAGE1D
UIMAGE1D
IMAGE2D
IIMAGE2D
UIMAGE2D
IMAGE3D
IIMAGE3D
UIMAGE3D
IMAGE2DRECT
IIMAGE2DRECT
UIMAGE2DRECT
IMAGECUBE
IIMAGECUBE
UIMAGECUBE
IMAGEBUFFER
IIMAGEBUFFER
UIMAGEBUFFER
IMAGE1DARRAY
IIMAGE1DARRAY
UIMAGE1DARRAY
IMAGE2DARRAY
IIMAGE2DARRAY
UIMAGE2DARRAY
IMAGECUBEARRAY
IIMAGECUBEARRAY
UIMAGECUBEARRAY
IMAGE2DMS
IIMAGE2DMS
UIMAGE2DMS
IMAGE2DMSARRAY
IIMAGE2DMSARRAY
UIMAGE2DMSARRAY
struct_specifier
TYPE_NAME
precision_qualifier:
HIGH_PRECISION
196
9 Shading Language Grammar for Core Profile
MEDIUM_PRECISION
LOW_PRECISION
struct_specifier:
STRUCT IDENTIFIER LEFT_BRACE struct_declaration_list RIGHT_BRACE
STRUCT LEFT_BRACE struct_declaration_list RIGHT_BRACE
struct_declaration_list:
struct_declaration
struct_declaration_list struct_declaration
struct_declaration:
type_specifier struct_declarator_list SEMICOLON
type_qualifier type_specifier struct_declarator_list SEMICOLON
struct_declarator_list:
struct_declarator
struct_declarator_list COMMA struct_declarator
struct_declarator:
IDENTIFIER
IDENTIFIER array_specifier
initializer:
assignment_expression
LEFT_BRACE initializer_list RIGHT_BRACE
LEFT_BRACE initializer_list COMMA RIGHT_BRACE
initializer_list:
initializer
initializer_list COMMA initializer
declaration_statement:
declaration
statement:
compound_statement
simple_statement
// Grammar Note: labeled statements for SWITCH only; 'goto' is not supported.
197
9 Shading Language Grammar for Core Profile
simple_statement:
declaration_statement
expression_statement
selection_statement
switch_statement
case_label
iteration_statement
jump_statement
compound_statement:
LEFT_BRACE RIGHT_BRACE
LEFT_BRACE statement_list RIGHT_BRACE
statement_no_new_scope:
compound_statement_no_new_scope
simple_statement
compound_statement_no_new_scope:
LEFT_BRACE RIGHT_BRACE
LEFT_BRACE statement_list RIGHT_BRACE
statement_list:
statement
statement_list statement
expression_statement:
SEMICOLON
expression SEMICOLON
selection_statement:
IF LEFT_PAREN expression RIGHT_PAREN selection_rest_statement
selection_rest_statement:
statement ELSE statement
statement
condition:
expression
fully_specified_type IDENTIFIER EQUAL initializer
198
9 Shading Language Grammar for Core Profile
switch_statement:
SWITCH LEFT_PAREN expression RIGHT_PAREN LEFT_BRACE switch_statement_list
RIGHT_BRACE
switch_statement_list:
/* nothing */
statement_list
case_label:
CASE expression COLON
DEFAULT COLON
iteration_statement:
WHILE LEFT_PAREN condition RIGHT_PAREN statement_no_new_scope
DO statement WHILE LEFT_PAREN expression RIGHT_PAREN SEMICOLON
FOR LEFT_PAREN for_init_statement for_rest_statement RIGHT_PAREN
statement_no_new_scope
for_init_statement:
expression_statement
declaration_statement
conditionopt:
condition
/* empty */
for_rest_statement:
conditionopt SEMICOLON
conditionopt SEMICOLON expression
jump_statement:
CONTINUE SEMICOLON
BREAK SEMICOLON
RETURN SEMICOLON
RETURN expression SEMICOLON
DISCARD SEMICOLON // Fragment shader only.
// Grammar Note: No 'goto'. Gotos are not supported.
translation_unit:
external_declaration
translation_unit external_declaration
199
9 Shading Language Grammar for Core Profile
external_declaration:
function_definition
declaration
function_definition:
function_prototype compound_statement_no_new_scope
200
10 Normative References
10 Normative References
1. International Standard ISO/IEC 14882:1998(E). Programming Languages – C++.
Referenced for preprocessor only.
201

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