Name

    ARB_shading_language_420pack

Name Strings

    GL_ARB_shading_language_420pack

Contact

    John Kessenich (cepheus 'at' frii.com)

Contributors

    Pat Brown, NVIDIA
    Jeff Bolz, NVIDIA
    Pierre Boudier, AMD
    Piers Daniell, NVIDIA
    Patrick Doane, Blizzard
    John Kessenich
    Daniel Koch, Transgaming
    Bill Licea-Kane, AMD
    Barthold Lichtenbelt, NVIDIA
    Ian Romanick, Intel
    Bruce Merry, ARM
    Graham Sellers, AMD
    Robert Simpson, Qualcomm
    
Notice

    Copyright (c) 2011-2013 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

Specification Update Policy

    Khronos-approved extension specifications are updated in response to
    issues and bugs prioritized by the Khronos OpenGL Working Group. For
    extensions which have been promoted to a core Specification, fixes will
    first appear in the latest version of that core Specification, and will
    eventually be backported to the extension document. This policy is
    described in more detail at
        https://www.khronos.org/registry/OpenGL/docs/update_policy.php

Status

    Complete.
    Approved by the ARB 25-Jul-2011.
    Approved by the Khronos Promoters on 2011/07/29.

Version

    Last Modified Date: 11-Sep-2014
    Revision: 4

Number

    ARB Extension #108

Dependencies

    GLSL 1.3 is required for all features.  GLSL 1.4 is required 
    declaring a binding for a uniform block, as uniform blocks were not 
    present until version 1.4.

    This extension interacts with the ARB_shader_image_load_store 
    extension, for assigning locations to image variables.

    References to *patch* are not valid unless tessellation stages are
    present and enabled.

    While this document is self-contained, clarifying context for how to 
    add the following changes can be seen by looking at version 4.20 of 
    the GLSL specification.

Overview

    This is a language feature only extension formed from changes made
    to version 4.20 of GLSL.  It includes:

    * Add line-continuation using '\', as in C++.

    * Change from ASCII to UTF-8 for the language character set and also 
      allow any characters inside comments.

    * Allow implicit conversions of return values to the declared type of 
      the function.

    * The *const* keyword can be used to declare variables within a function 
      body with initializer expressions that are not constant expressions.

    * Qualifiers on variable declarations no longer have to follow a strict 
      order. The layout qualifier can be used multiple times, and multiple 
      parameter qualifiers can be used.  However, this is not as
      straightforward as saying declarations have arbitrary lists of 
      initializers.  Typically, one qualifier from each class of qualifiers
      is allowed, so care is now taken to classify them and say so.  Then, 
      of these, order restrictions are removed.

    * Add layout qualifier identifier "binding" to bind the location of a
      uniform block.  This requires version 1.4 of GLSL.  If this extension
      is used with an earlier version than 1.4, this feature is not present.
    
    * Add layout qualifier identifier "binding" to bind units to sampler 
      and image variable declarations.
    
    * Add C-style curly brace initializer lists syntax for initializers. 
      Full initialization of aggregates is required when these are used.

    * Allow ".length()" to be applied to vectors and matrices, returning 
      the number of components or columns.

    * Allow swizzle operations on scalars.

    * Built-in constants for gl_MinProgramTexelOffset and 
      gl_MaxProgramTexelOffset.

IP Status

    No known IP claims.

Additions to Chapter 3 of the OpenGL Shading Language

    Change the opening of 3.1 Character Set to

    "The source character set used for the OpenGL shading languages, outside of 
    comments, is a subset of UTF-8. It includes the following characters:    

        "The backslash ( \ ) as the line-continuation character when used as 
         the last character of a line, just before a new line."

    and after the list, referring to all the characters:

    "An error will be given if any other character is used outside a comment.

    "There are no trigraphs. There are no escape sequences or other uses of the 
    backslash beyond use as the line-continuation character."

    At the end of section 3.2, add

    "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."

        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)

    Add to the preprocessor:

    "Preprocessing takes places after new lines have been removed by the 
    line-continuation character."

    "Including the following line in a shader will enable module import and
    related extended language features described in this extension:

        #extension GL_ARB_shading_language_420pack : <behavior>

    where <behavior> is as specified in section 3.3 for the #extension
    directive."

    A new preprocessor macro is added to the OpenGL Shading Language:

      #define GL_ARB_shading_language_420pack    1

    Add to the section on comments

    "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
    
Additions to Chapter 4 of the OpenGL Shading Language

    Add to the syntax description of integer literals:

    "When tokenizing, the maximal token matching the above will be recognized
    before a new token is started."

    Add to the syntax description of floating-point literals:

    "When tokenizing, the maximal token matching the above will be recognized 
    before a new token is started."

    Add to the description of .length():

    "This returns a type int."

    Add a new section for initializers

    "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

    "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 are illegal.

        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. All of the following declarations are illegal.

        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 an 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.

    Change the definition of the storage qualifier *const* in the table:

        const    |     a variable whose value cannot be changed

    Change the definition of storage qualifiers to only include the following 
    rows

        const
        in
        out
        attribute
        uniform
        varying

    (Removing "centroid in", "sample in", "centroid out", "sample out", 
    "centroid varying", "patch in", and "patch out".)

    and with this introduction:

    "Variable declarations may have at most one storage qualifier specified in 
    front of the type."

    and make a new class "Auxiliary Storage Qualifier" to contain
 
        Auxiliary Storage | Meaning
        Qualifier         |
        -----------------   -------
        centroid          | centroid-based interpolation
        sample            | per-sample interpolation
        patch             | per-tessellation-patch attributes

    add that
    
    "Some input and output qualified variables can be qualified with at most one 
    additional auxiliary storage qualifier.

    "Not all combinations of qualification are allowed. Which variable types can 
    have which qualifiers are specifically defined in upcoming sections."

    Removing

    "These interpolation qualifiers may only precede the qualifiers in, 
    centroid in, sample in, out, centroid out, or sample out in a 
    declaration."

    Change the last paragraph in this section to say

    "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...."

    Change the Constant Qualifier section to instead say

    "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 an 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..."

    In the Constant Expressions section, change the *const* bullet to say

        * a variable declared with the const qualifier and an initializer, 
          where the initializer is a constant expression

    and add the bullet

        * valid use of the length() method on a sized object, whether or not the 
          object itself is constant

    Add to "Inputs" section

    "It is an error to use any auxiliary or interpolation qualifiers on a vertex
    shader input."

    "Applying the *patch* qualifier to inputs can only be done in tessellation
    evaluation shaders."

    "The auxiliary storage qualifiers *centroid* and *sample* can also be 
    applied, as well as the interpolation qualifiers *flat*, *noperspective*,
    and *smooth*."

    Change all occurances of "patch in qualifier" to "patch qualifier" or 
    "patch in qualifiers". Similarly for any occurances of "patch out", 
    "centroid in", "sample in", "centroid out", and "sample out", through-out 
    the specification.

    Also change
    
    "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 centroid in storage qualifier, ..."
    
    to
    
    "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..."
    
    Change
    
    "They are declared in fragment shaders with the in storage qualifier, the 
    centroid in storage qualifier, or the deprecated varying and centroid 
    varying storage qualifiers."
        
    to
    
    "They are declared in fragment shaders with the in storage qualifier or the 
    deprecated varying storage qualifier."

    Add to the "Outputs" section

    "A variable also cannot be declared with both the *in* and the *out* 
    qualifiers."

    Change
    
    "Vertex, tessellation evaluation, and geometry output variables output 
    per-vertex data and are declared using the out, centroid out, or sample out 
    storage qualifiers"
    
    to
    
    "Vertex, tessellation evaluation, and geometry output variables output 
    per-vertex data and are declared using the out storage qualifier"

    Change
    
    "It is an error to use patch out in a vertex, tessellation evaluation, 
    or geometry shader."
    
    to
    
    "Applying *patch* to an output can only be done in a tessellation control
    shader."

    Change
    
    "Per vertex output variables are arrayed (see arrayed under 4.3.4 Inputs) 
    and declared using out or centroid out storage qualifiers."
    
    to

    "Per-vertex output variables are arrayed and declared using the *out*
    qualifier without the *patch* qualifier.  Per-patch output variables are
    declared using the *patch* and *out* storage qualifiers."

    Add

    "It is an error to use auxiliary storage qualifiers or interpolation 
    qualifiers on an output in a fragment shader."

    Modify Interface Blocks with new relaxed qualifier rules:

    "If optional qualifiers are used, they can include interpolation qualifiers,
    auxiliary storage qualifiers, and storage qualifiers..."

    Add to Layout Qualifiers:

    "More than one layout qualifier may appear in a single declaration. If the 
    same layout-qualifier-name occurs in multiple layout qualifiers for the same 
    declaration, the last one overrides the former ones."

    Add to Uniform Block Layout Qualifiers, if the base language version is 1.4 
    or above:

        layout-qualifier-id
            binding = integer-constant

    "The binding identifier specifies the uniform buffer binding point 
    corresponding to the uniform block, which will be used to obtain the 
    values of the member variables of the block. It is an error to specify the
    binding identifier for the global scope or for block member declarations. 
    Any uniform 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 blocks declared with or without a binding 
    identifier can be updated by the OpenGL API.

    "If the binding identifier is used with a uniform block instanced as an 
    array then 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 block instance is less than zero, or 
    greater than or equal to the implementation-dependent maximum number of 
    uniform buffer bindings, a compilation error will occur.  When the binding 
    identifier is used with a uniform block instanced as an array of size N, 
    all elements of the array from binding through binding + N - 1 must be
    within this range."

    Add for sampler and image layout qualifiers:

    Note: The "image" variable aspect of this interacts with
    ARB_shader_image_load_store and only applies if the extension 
    ARB_shader_image_load_store is enabled.

    "Uniform layout qualifiers can be used to bind opaque uniform variables to 
    specific buffers or units.  Texture image units can be bound to samplers 
    and image units can be bound to images.

        layout-qualifier-id
            binding = integer-constant

    "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 compilation 
    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."

    Change the "Interpolation" section to be the "Interplotation Qualifiers"
    section and change the first paragraph to now be the following.

    "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 an error to use interpolation qualifiers with
    *patch*.

    Add the following parameter qualifier

        Qualifier          Meaning
        ---------          -------
        const              for function parameters that cannot be written to

    and note that precision qualifiers are also allowed on parameters.

    Delete the following sentence in the "Invariant Qualifier" section:

    "The invariant qualifier must appear before any interpolation qualifiers 
    or storage qualifiers when combined with a declaration."

    Replace the "Order of Qualification" section with the following

    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."

Additions to Chapter 5 of the OpenGL Shading Language

    Change the section title "Vector Components" to be "Vector and Scalar
    Components and Length".  In that section do the following.

    Change first sentence to "The names of the components of a vector or
    scalar are denoted by a single letter."

    Add the following last sentence to its paragraph:

    "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."

    Add to the example

        float height;
        height.x // is legal
        height.y // is illegal

    and to the third example

        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 is illegal:"

        vec4 f;
        vec4 g = pos.xyzwxy.xyzw; // illegal; pos.xyzwxy is non-existent "vec6"

    Add to the end of this section:

    "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*."

    Add to the Matrix Components section:

    "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*."

Additions to Chapter 6 of the OpenGL Shading Language

    Add to the "Function Definitions" section the following functionality 
    regarding function return values:

    "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 error will 
    result."

    and the clarifations:

    "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."

    Under the section "Function Calling Conventions" change to the *inout*
    bullet:

        * "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."

    and the following grammar changes:  Delete the const-qualifier parts of
    the existing grammar and instead use

        parameter-qualifiers :
            empty
            list of parameter-qualifier

        parameter-qualifier :
            empty
            const
            in
            out
            inout
            memory qualifier
            precision qualifier

Additions to Chapter 7 of the OpenGL Shading Language

    Add to the built-in constants

        const int gl_MinProgramTexelOffset = -7;
        const int gl_MaxProgramTexelOffset = 8;

Additions to Chapter 9 of the OpenGL Shading Language

    TBD.

Conformance Tests

    Verify that the following two vectors are identical:

        vec4 a = vec4(1.0);
        vec4 b = (1.0).xxxx;

    Verify that the following generates a compilation error:

        vec2 a = 1.xx;

    Verify that the following two vectors are identical:

        vec2 a = (1).xx;
        vec2 b = vec2(1.0);

    Verify that the following two arrays are identical:

        vec4 a[] = vec4 [] (vec4(1.2), vec4(0.0), vec4(0.0), vec4(0.0),
                            vec4(0.0), vec4(0.0), vec4(0.0), vec4(2.4),
                            vec4(0.0), vec4(0.0), vec4(0.0), vec4(0.0),
                            vec4(0.0), vec4(0.0), vec4(0.0), vec4(3.6));
        vec4 b[] =        { vec4(1.2), vec4(0.0), vec4(0.0), vec4(0.0),
                            vec4(0.0), vec4(0.0), vec4(0.0), vec4(2.4),
                            vec4(0.0), vec4(0.0), vec4(0.0), vec4(0.0),
                            vec4(0.0), vec4(0.0), vec4(0.0), vec4(3.6) };

    Verify that the following two arrays are identical:

        vec4 a[12] = vec4 [] (vec4(1.2), vec4(0.0), vec4(0.0), vec4(0.0),
                              vec4(0.0), vec4(0.0), vec4(0.0), vec4(2.4),
                              vec4(0.0), vec4(0.0), vec4(0.0), vec4(0.0));
        vec4 b[12] =        { vec4(1.2), vec4(0.0), vec4(0.0), vec4(0.0),
                              vec4(0.0), vec4(0.0), vec4(0.0), vec4(2.4) };

    Verify that the following three matrices are identical:

        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 } };

    Verify that the following generates a compliation error:

        float a[2] = { 1.0, 2.0, 3.0 };

    Verify that the following generates a compliation error:

        mat2x2 d = { 1.0, 0.0, 0.0, 1.0 };


    Verify that the output of the following fragment shader is (0, 1, 0, 1):

    #version 410
    #extension GL_ARB_shading_language_420pack: require
    out vec4 color;
    void main()
    {
        color = (vec2(0).length() == 2) ? vec4(0, 1, 0, 1) : vec4(1, 0, 0, 1);
    }

    Repeat the above test with vec2(0).length() replaced by ivec2(0).length().

    Repeat the above test with vec2(0).length() replaced by uvec2(0).length().

    Repeat the above test with vec2(0).length() replaced by bvec2(0).length().

    Repeat the above test with vec2(0).length() replaced by dvec2(0).length().


    Verify that the output of the following fragment shader is (0, 1, 0, 1):

    #version 410
    #extension GL_ARB_shading_language_420pack: require
    out vec4 color;
    void main()
    {
        color = (vec3(0).length() == 3) ? vec4(0, 1, 0, 1) : vec4(1, 0, 0, 1);
    }

    Repeat the above test with vec3(0).length() replaced by ivec3(0).length().

    Repeat the above test with vec3(0).length() replaced by uvec3(0).length().

    Repeat the above test with vec3(0).length() replaced by bvec3(0).length().

    Repeat the above test with vec3(0).length() replaced by dvec3(0).length().


    Verify that the output of the following fragment shader is (0, 1, 0, 1):

    #version 410
    #extension GL_ARB_shading_language_420pack: require
    out vec4 color;
    void main()
    {
        color = (vec4(0).length() == 4) ? vec4(0, 1, 0, 1) : vec4(1, 0, 0, 1);
    }

    Repeat the above test with vec4(0).length() replaced by ivec4(0).length().

    Repeat the above test with vec4(0).length() replaced by uvec4(0).length().

    Repeat the above test with vec4(0).length() replaced by bvec4(0).length().

    Repeat the above test with vec4(0).length() replaced by dvec4(0).length().


    Verify that the output of the following fragment shader is (0, 1, 0, 1):

    #version 410
    #extension GL_ARB_shading_language_420pack: require
    out vec4 color;
    void main()
    {
        color = (mat2(0).length() == 2) ? vec4(0, 1, 0, 1) : vec4(1, 0, 0, 1);
    }

    Repeat the above test with mat2(0).length() replaced by mat2x2(0).length().

    Repeat the above test with mat2(0).length() replaced by dmat2x2(0).length().

    Repeat the above test with mat2(0).length() replaced by mat2x3(0).length().

    Repeat the above test with mat2(0).length() replaced by dmat2x3(0).length().

    Repeat the above test with mat2(0).length() replaced by mat2x4(0).length().

    Repeat the above test with mat2(0).length() replaced by dmat2x4(0).length().


    Verify that the output of the following fragment shader is (0, 1, 0, 1):

    #version 410
    #extension GL_ARB_shading_language_420pack: require
    out vec4 color;
    void main()
    {
        color = (mat3(0).length() == 3) ? vec4(0, 1, 0, 1) : vec4(1, 0, 0, 1);
    }

    Repeat the above test with mat3(0).length() replaced by mat3x2(0).length().

    Repeat the above test with mat3(0).length() replaced by dmat3x2(0).length().

    Repeat the above test with mat3(0).length() replaced by mat3x3(0).length().

    Repeat the above test with mat3(0).length() replaced by dmat3x3(0).length().

    Repeat the above test with mat3(0).length() replaced by mat3x4(0).length().

    Repeat the above test with mat3(0).length() replaced by dmat3x4(0).length().


    Verify that the output of the following fragment shader is (0, 1, 0, 1):

    #version 410
    #extension GL_ARB_shading_language_420pack: require
    out vec4 color;
    void main()
    {
        color = (mat4(0).length() == 4) ? vec4(0, 1, 0, 1) : vec4(1, 0, 0, 1);
    }

    Repeat the above test with mat4(0).length() replaced by mat4x2(0).length().

    Repeat the above test with mat4(0).length() replaced by dmat4x2(0).length().

    Repeat the above test with mat4(0).length() replaced by mat4x3(0).length().

    Repeat the above test with mat4(0).length() replaced by dmat4x3(0).length().

    Repeat the above test with mat4(0).length() replaced by mat4x4(0).length().

    Repeat the above test with mat4(0).length() replaced by dmat4x4(0).length().


    Verify that the output of the following fragment shader is (0, 1, 0, 1):

    #version 410
    #extension GL_ARB_shading_language_420pack: require
    out vec4 color;
    void main()
    {
        color = (mat2(0)[0].length() == 2)
            ? vec4(0, 1, 0, 1) : vec4(1, 0, 0, 1);
    }

    Repeat the above test with mat2(0)[0].length() replaced by
    mat2x2(0)[0].length().

    Repeat the above test with mat2(0)[0].length() replaced by
    dmat2x2(0)[0].length().

    Repeat the above test with mat2(0)[0].length() replaced by
    mat3x2(0)[0].length().

    Repeat the above test with mat2(0)[0].length() replaced by
    dmat3x2(0)[0].length().

    Repeat the above test with mat2(0)[0].length() replaced by
    mat4x2(0)[0].length().

    Repeat the above test with mat2(0)[0].length() replaced by
    dmat4x2(0)[0].length().


    Verify that the output of the following fragment shader is (0, 1, 0, 1):

    #version 410
    #extension GL_ARB_shading_language_420pack: require
    out vec4 color;
    void main()
    {
        color = (mat3(0)[0].length() == 3)
            ? vec4(0, 1, 0, 1) : vec4(1, 0, 0, 1);
    }

    Repeat the above test with mat3(0)[0].length() replaced by
    mat2x3(0)[0].length().

    Repeat the above test with mat3(0)[0].length() replaced by
    dmat2x3(0)[0].length().

    Repeat the above test with mat3(0)[0].length() replaced by
    mat3x3(0)[0].length().

    Repeat the above test with mat3(0)[0].length() replaced by
    dmat3x3(0)[0].length().

    Repeat the above test with mat3(0)[0].length() replaced by
    mat4x3(0)[0].length().

    Repeat the above test with mat3(0)[0].length() replaced by
    dmat4x3(0)[0].length().


    Verify that the output of the following fragment shader is (0, 1, 0, 1):

    #version 410
    #extension GL_ARB_shading_language_420pack: require
    out vec4 color;
    void main()
    {
        color = (mat4(0)[0].length() == 4)
            ? vec4(0, 1, 0, 1) : vec4(1, 0, 0, 1);
    }

    Repeat the above test with mat4(0)[0].length() replaced by
    mat2x4(0)[0].length().

    Repeat the above test with mat4(0)[0].length() replaced by
    dmat2x4(0)[0].length().

    Repeat the above test with mat4(0)[0].length() replaced by
    mat3x4(0)[0].length().

    Repeat the above test with mat4(0)[0].length() replaced by
    dmat3x4(0)[0].length().

    Repeat the above test with mat4(0)[0].length() replaced by
    mat4x4(0)[0].length().

    Repeat the above test with mat4(0)[0].length() replaced by
    dmat4x4(0)[0].length().


    Verify that the following shader generates a compilation error:

    #version 410
    #extension GL_ARB_shading_language_420pack: require
    out vec4 color;
    void main()
    {
        float v;
        int l = v.length();
        color = vec4(1, 0, 0, 1);
    }

    Repeat the above test with "float v" replaced by "int v".

    Repeat the above test with "float v" replaced by "uint v".

    Repeat the above test with "float v" replaced by "bool v".

    Repeat the above test with "float v" replaced by "double v".


    Verify that the following shader compiles without error:

    #version 410
    #extension GL_ARB_shading_language_420pack: require
    void main()
    {
        float b[vec2(0).length()];
    }

    Repeat the above test with vec2(0).length() replaced by ivec2(0).length().

    Repeat the above test with vec2(0).length() replaced by uvec2(0).length().

    Repeat the above test with vec2(0).length() replaced by bvec2(0).length().

    Repeat the above test with vec2(0).length() replaced by dvec2(0).length().

    Repeat the above test with vec2(0).length() replaced by ivec3(0).length().

    Repeat the above test with vec2(0).length() replaced by uvec3(0).length().

    Repeat the above test with vec2(0).length() replaced by bvec3(0).length().

    Repeat the above test with vec2(0).length() replaced by dvec3(0).length().

    Repeat the above test with vec2(0).length() replaced by ivec4(0).length().

    Repeat the above test with vec2(0).length() replaced by uvec4(0).length().

    Repeat the above test with vec2(0).length() replaced by bvec4(0).length().

    Repeat the above test with vec2(0).length() replaced by dvec4(0).length().

    Repeat the above test with vec2(0).length() replaced by mat2(0).length().

    Repeat the above test with vec2(0).length() replaced by mat2x2(0).length().

    Repeat the above test with vec2(0).length() replaced by mat2x3(0).length().

    Repeat the above test with vec2(0).length() replaced by mat2x4(0).length().

    Repeat the above test with vec2(0).length() replaced by mat3(0).length().

    Repeat the above test with vec2(0).length() replaced by mat3x2(0).length().

    Repeat the above test with vec2(0).length() replaced by mat3x3(0).length().

    Repeat the above test with vec2(0).length() replaced by mat3x4(0).length().

    Repeat the above test with vec2(0).length() replaced by mat4(0).length().

    Repeat the above test with vec2(0).length() replaced by mat4x2(0).length().

    Repeat the above test with vec2(0).length() replaced by mat4x3(0).length().

    Repeat the above test with vec2(0).length() replaced by mat4x4(0).length().

    Repeat the above test with vec2(0).length() replaced by dmat2(0).length().

    Repeat the above test with vec2(0).length() replaced by dmat2x2(0).length().

    Repeat the above test with vec2(0).length() replaced by dmat2x3(0).length().

    Repeat the above test with vec2(0).length() replaced by dmat2x4(0).length().

    Repeat the above test with vec2(0).length() replaced by dmat3(0).length().

    Repeat the above test with vec2(0).length() replaced by dmat3x2(0).length().

    Repeat the above test with vec2(0).length() replaced by dmat3x3(0).length().

    Repeat the above test with vec2(0).length() replaced by dmat3x4(0).length().

    Repeat the above test with vec2(0).length() replaced by dmat4(0).length().

    Repeat the above test with vec2(0).length() replaced by dmat4x2(0).length().

    Repeat the above test with vec2(0).length() replaced by dmat4x3(0).length().

    Repeat the above test with vec2(0).length() replaced by dmat4x4(0).length().

    More TBD.

Issues

    1) How is the ambiguity of swizzling an integer literal resolved?

        RESOLVED: Greedy lexing. The sequence "5.x" will be interpreted
        as the floating-point literal 5.0 followed by the identifier "x".

    2) Should the array subscript operator be applicable to scalars as it is
       to vectors?

        RESOLVED: NO.  In conduction with the rest of the features of this
        extension, this would allow writing generic code such as:

            #define SUM_COMPONENTS(value, result) \
                do {
                    for (int i = 0; i < (value).length(); i++)
                        result += (value)[i];
                } while (0)

        However, using macros in this fashion is a poor approximation
        of generic functions.  Until a better mechanism is added to
        GLSL, there is little utility in array accesses to scalars or
        single component vectors.

    3) Should vectors and matrices also be able to use initializer lists?

        RESOLVED: YES.  Many places in the language treat vectors as 2, 3, or
        4 element arrays of a scalar type.  Matrices are often treated as
        arrays of vector types.  Continuing that treatment here is logical.

    4) What happens when an explicitly sized array is initialized with too few
       elements?  Is it allowed?

        RESOLVED: DISALLOWED.  In C/C++ this is allowed.  The uninitialized
        elements are implicitly initialized to the appropriate reprensentation
        of zero.  However, the belief is that this enables a variety of
        programming errors that are difficult to debug on a GPU.

    5) Can scalars be initialized using an initializer list?

        RESOLVED: NO. In C/C++ this is allowed.  The ISO C99 specification
        specifically say, "The initializer for a scalar shall be a single
        expression, optionally enclosed in braces."  However, this may
        pose problems if / when a vec1 type is added.

    6) Are initializer lists for vectors flexible in the same way that vector
       constructors are?  Specifically, is the initialzier list for 'c',
       below, legal?

        vec2 a = vec2(2, 3);
        vec4 b = vec4(1, a, 4);   // b = { 1, 2, 3, 4 }
        vec4 c = { 1, a, 4 };     // Legal?

        RESOLVED: NO.  In C++ there is a distinction between a constructor
        and an initializer list.  If you had vec2 and vec4 classes in C++, a
        constructor 'vec4(float, vec2, float)' might exist, but the
        initializer list for 'c' would not be legal.

    7) Does adding *const* for read-only variables effect array sizing?
    
    RESOLVED.  No. Array sizing changes are not part of this proposal.  Array 
    sizing rules stay the same, where sizes must be compile-time constants, 
    whether or not the keyword 'const' was involved, just like it always has 
    been.  The following was and still is disallowed:  
    
        "void foo(const s) { int a[s]; }", 
    
    as will be 
    
        "void foo() { const s = exp; int a[s]; }".
    
    8)  Should we allow non-compile-time constant initializers for const at 
    global scope as well?
    
        RESOLVED.  No.  Would make linking more difficult.  
    
    9) Do we want to add the new C++ constexpr for declaring constant 
    functions?
    
        RESOLVED.  No, not yet, could be a potential performance benefit in the 
        future, but treat as a separate extension.  Note that most built-in 
        functions are already understood to be "constexpr".  This has been
        submitted into bugzilla.
    
    10) What combinations of interpolation qualifiers are allowed?  Before, 
    you could take one of smooth, flat, and noperspective and combine that with 
    one of centroid, sample, or patch.  What do combinations do we actually 
    want?  Do we want multiple intances of a single qualifier allowed?  
    (E.g. two layout qualifiers, or "in out"?)
    
        RESOLVED.  Allow multiple "layout".  Not anything else.

    11) Should we allow multiple block declarations to use the same
    binding point?  For example:
           layout(binding = 3) uniform Block1 { ... };
           layout(binding = 3) uniform Block2 { ... };
           
       RESOLVED: There is no reason to disallow this usage; in particular,
       existing UBO shaders effectively declare all blocks with
       "layout(block = 0)".
       
    12) Should the qualifier be called "location", "block" or "binding"?
    
       RESOLVED: The qualifier is called "binding" because it makes it more
       clear what parameter of UniformBlockBinding that this qualifier
       affects. "block" could be confused with either the
       <uniformBlockIndex> or <uniformBlockBinding> and "location" might be
       confused with the current uses of the qualifier "location" for
       attribute locations or it could be confused with uses of "location"
       for uniform variable location, which blocks don't have.

    13) Should sampler initialization use a layout(binding) qualifier
       instead of specifying a uniform initializer?
       
       RESOLVED: Revision #4 of the sampler initialization spec uses a 
       layout(binding) qualifier for sampler uniforms. This is more 
       forward looking and makes the spec more consistent with 
       ARB_uniform_block_binding_initialization and
       ARB_shader_atomic_counters.       

Revision History

    Revision 4, 11-Sep-2014 (pbrown)
      - Fix minor typo.
    Revision 3, 13-May-2014 (John Kessenich)
      - Resolve bug 10593.
    Revision 2, 25-Jul-2011 (John Kessenich)
      - Mark as complete and approved.
      - Tidy up some formating and non-raw-text quote marks.
      - Pull in the initializers for images, to completely subsume
        GL_ARB_uniform_sampler_initialization.
      - Point out image and patch dependencies.
    Revision 1, 5-Jul-2011 (John Kessenich)
      - Initial version.  Formed by combing the content of the
        4.20 draft specification and the content of draft
        extensions:
        * ARB_shading_language_vec1
        * ARB_shading_language_relaxed_qualifiers
        * ARB_shading_language_length
        * ARB_shading_language_initializer_list
        * GL_ARB_uniform_block_binding_initialization
        * GL_ARB_uniform_sampler_initialization (just sampler binding)
        Issues were carried across and renumbered.
        Conformance tests were carried over.