xref: /third_party/openGLES/extensions/ARB/ARB_provoking_vertex.txt (revision 5bd8dead)

Name

    ARB_provoking_vertex

Name Strings

    GL_ARB_provoking_vertex

Contributors

    Cynthia Allison, NVIDIA
    Gregory Roth, NVIDIA
    Daniel Koch, TransGaming
    Gavriel State, TransGaming
    Jason Green, TransGaming
    Ian Romanick, Intel
    Marcus Steyer, NVIDIA
    Pat Brown, NVIDIA
    Stefan Dosinger, CodeWeavers
    Henri Verbeet, CodeWeavers

Contact

    Mark Kilgard, NVIDIA (mjk 'at' nvidia.com)
    Daniel Koch, TransGaming (daniel 'at' transgaming.com)

Notice

    Copyright (c) 2009-2013 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 on July 3, 2009.
    Implemented by NVIDIA, March 2009

Version

    Last Modified Date:  August 2, 2009
    Version:             3

Number

    ARB Extension #64

Dependencies

    This extension is written against the OpenGL 2.1 Specification but
    can apply to any prior specification.

    ARB_geometry_shader4, EXT_geometry_shader4, NV_geometry_shader4,
    and NV_gpu_program4 interact with this extension

    EXT_transform_feedback, NV_transform_feedback, and the transform
    feedback functionality made core by OpenGL 3.0 are clarified by
    this extension.

Overview

    This extension provides an alternative provoking vertex convention
    for rendering lines, triangles, and (optionally depending on the
    implementation) quads.

    The provoking vertex of a primitive is the vertex that determines the
    constant primary and secondary colors when flat shading is enabled.

    In OpenGL, the provoking vertex for triangle, quad, line, and
    (trivially) point primitives is the last vertex used to assemble
    the primitive.  The polygon primitive is an exception in OpenGL where
    the first vertex of a polygon primitive determines the color of the
    polygon, even if actually broken into triangles and/or quads.

    See section 2.14.7 (Flatshading) of the OpenGL 2.1 specification,
    particularly Table 2.12 for more details.

    Alternatively the provoking vertex could be the first vertex of
    the primitive.  Other APIs with flat-shading functionality such
    as Reality Lab and Direct3D have adopted the "first vertex of the
    primitive" convention to determine the provoking vertex.  However,
    these APIs lack quads so do not have a defined provoking vertex
    convention for quads.

    The motivation for this extension is to allow applications developed
    for APIs with a "first vertex of the primitive" provoking vertex to
    be easily converted to OpenGL.

New Procedures and Functions

    void ProvokingVertex(enum mode);

New Tokens

    Accepted by the <mode> parameter of ProvokingVertex:

        FIRST_VERTEX_CONVENTION                   0x8E4D
        LAST_VERTEX_CONVENTION                    0x8E4E

    Accepted by the <pname> parameter of GetBooleanv, GetIntegerv,
    GetFloatv, and GetDoublev:

        PROVOKING_VERTEX                          0x8E4F
        QUADS_FOLLOW_PROVOKING_VERTEX_CONVENTION  0x8E4C

Additions to Chapter 2 of the OpenGL 2.1 Specification (OpenGL Operation)

 -- Section 2.14.7 "Flatshading" (page 69)

    Replace the entire section with:

    "For fixed-function vertex processing, flatshading a primitive means
    to assign all vertices of the primitive the same primary and
    secondary colors (in RGBA mode) or the same color index (in color
    index mode). If a vertex shader is active, flatshading a varying
    output means to assign all vertices of the primitive the same value
    for that output.

    The color and/or varying output values assigned are those of the
    provoking vertex of the primitive. The provoking vertex is
    controlled with the command

       void ProvokingVertex(enum provokeMode);

    provokeMode must be either FIRST_VERTEX_CONVENTION or
    LAST_VERTEX_CONVENTION, and controls selection of the vertex whose
    values are assigned to the flatshaded colors and varying outputs, as
    shown in Table 2.12.

    The provoking vertex behavior of quad primitives is implementation
    dependent, and may be determined by calling GetBooleanv with the
    symbolic constant QUADS_FOLLOW_PROVOKING_VERTEX. A return value of
    TRUE indicates that the provoking vertex mode is respected for quad
    primitives, while a return value of FALSE indicates that the
    implementation always behaves as though the provoking vertex mode
    were LAST_VERTEX_CONVENTION.

    Flatshading of colors in fixed-function vertex processing, and of
    the built-in varying variables gl_FrontColor, gl_BackColor,
    gl_FrontSecondaryColor and gl_BackSecondaryColor when a vertex
    shader is active, is controlled with the command

       void ShadeModel(enum mode);

    <mode> must be SMOOTH or FLAT. If <mode> is SMOOTH, vertex colors
    are treated individually. If <mode> is FLAT, flatshading is enabled
    and colors are taken from the provoking vertex of the primitive. The
    colors selected are those derived from current values, generated by
    lighting, or generated by vertex shading, if lighting is disabled,
    enabled, or a vertex shader is in use, respectively.

    If a vertex or geometry shader is active, user-defined varying
    outputs may be flatshaded by using the <flat> qualifer when
    declaring the ouput, as described in section 4.3.6 of the OpenGL
    Shading Language Specification (version 1.40.06).

    The state required for flatshading is one bit for the shade mode,
    one bit for the provoking vertex mode, and one
    implementation-dependent bit for the provoking vertex behavior of
    quad primitives. The initial value of the shade mode is SMOOTH and
    the initial value of the provoking vertex mode is
    LAST_VERTEX_CONVENTION.

                                     First vertex  Last vertex
        Primitive type of polygon i  convention    convention
        ===========================  ============  ==================================================
        point                             i        i                                               <- same

        independent line                  2i-1     2i
        line loop                         i        i+1, if i<n
                                                   1,   if i==n
        line strip                        i        i+1

        independent triangle              3i-2     3i
        triangle strip                    i        i+2
        triangle fan                      i+1      i+2

        independent quad                  4i-3     4i,   if QUADS_FOLLOW_PROVOKING_VERTEX = true
                                          4i       4i,   if QUADS_FOLLOW_PROVOKING_VERTEX = false  <- same
        quad strip                        2i-1     2i+2, if QUADS_FOLLOW_PROVOKING_VERTEX = true
                                          2i+2     2i+2, if QUADS_FOLLOW_PROVOKING_VERTEX = false  <- same

        single polygon (i=1)              1        1                                               <- same

        line adjacency                    4i-2     4i-1
        line strip adjacency              i+1      i+2
        triangle adjacency                6i-5     6i-1
        triangle strip adjacency          2i-1     2i+3

    Table 2.12: Provoking vertex selection. The vertex colors and/or
    varying values used for flatshading the <i>th primitive generated by
    the drawing commands with the indicated primitive type are derived
    from the corresponding values of the vertex whose index is shown in
    the table. Vertices are numbered 1 through n, where n is the number
    of vertices drawn."

Additions to Chapter 3 of the OpenGL 2.1 Specification (Rasterization)

    None

Additions to Chapter 4 of the OpenGL 2.1 Specification (Per-Fragment
Operations and the Frame Buffer)

    None

Additions to Chapter 5 of the OpenGL 2.1 Specification (Special
Functions)

    None

Additions to Chapter 6 of the OpenGL 2.1 Specification (State and
State Requests)

    None

Additions to the AGL/GLX/WGL Specifications

    None

Additions to the OpenGL Shading Language

    None

Dependencies on ARB_geometry_shader4, EXT_geometry_shader4,
NV_geometry_shader4, NV_gpu_program4, and/or the geometry shader
functionality integrated into the core by OpenGL 3.2:

    If none of ARB_geometry_shader4, EXT_geometry_shader4,
    NV_geometry_shader4, NV_gpu_program4, or OpenGL 3.2 (or later) are
    supported:
       * ignore the rows of table 2.12 for line adjacency, line strip
         adjacency, triangle adjacency, and triangle strip adjacency.
       * ignore any references to geometry shaders.

Dependencies on EXT_gpu_shader4 and/or the flat shading functionality
integrated into the core by OpenGL 3.0:

    If neither EXT_gpu_shader4 or OpenGL 3.0 (or later) are supported,
    ignore the references to the <flat> qualifier in the shading
    language.

Dependencies on EXT_transform_feedback, NV_transform_feedback, and/or
the transform feedback functionality integrated into the core by OpenGL
3.0:

    Clarify the statement describing when transform feedback occurs
    (in section 2.18 of the OpenGL 3.1 specification) to read:

    "The vertices are fed back after vertex color clamping, but before
    flatshading and clipping."

    (The "flatshading and" phrase is newly clarifying.)

Dependencies on ARB_compatibility and OpenGL 3.1 (or later):

    If using OpenGL 3.1 (or later) and ARB_compatibility is not
    supported ignore all references to the following:

       * fixed function vertex processing,
       * primary and secondary vertex colors and related built-in
         varyings,
       * color-index mode,
       * shade mode, ShadeModel and the associated state,
       * quad primitives, the QUADS_FOLLOW_PROVOKING_VERTEX query, and
         the associated state, and
       * the rows in table 2.12 for independent quad, quad strip and
         single polygon.

Dependencies on OpenGL 3.1:

    When OpenGL 3.1 is supported make the following changes:

   Modify the last sentence of Section 3.5.1 "Basic Line Segment
   Rasterization" to read:

   "When the flat keyword is specified, no interpolation is performed,
   and varying outputs are taken from the corresponding generic
   attribute value of the provoking vertex corresponding to that
   primitive (see section 2.14.7)."

   Modify the sentence in Section 3.6.1 "Basic Polygon Rasterization"
   describing the <flat> keyword semantics to read:

   "When the flat keyword is specified, no interpolation is performed,
   and varying outputs are taken from the corresponding generic
   attribute value of the provoking vertex corresponding to that
   primitive (see section 2.14.7)."

   Modify the paragraph in Section 3.6.5 "Polygon Multisample
   Rasterization" describing the <noperspective> and <flat> qualifiers
   semantices to read:

   "When using a vertex shader, the <noperspective> and <flat>
   qualifiers affect how varying shader outputs are interpolated in the
   same fashion as described for basic polygon rasterization in section
   3.6.1"

GLX Protocol

    A new GL rendering command is added. The following command is sent
    to the server as part of a glXRender request:

        ProvokingVertex
            2           8          rendering command length
            2           4227       rendering command opcode
            4           ENUM       provokeMode

Errors

    INVALID_ENUM is generated when ProvokingVertex is called with a
    <provokeMode> that is not either FIRST_VERTEX_CONVENTION or
    LAST_VERTEX_CONVENTION.

New State

(table 6.11, page 276) add the following entry:

Get Value         Type  Get Command  Initial Value           Description       Sec     Attribute
----------------  ----  -----------  ----------------------  ----------------  ------  ---------
PROVOKING_VERTEX  Z2    GetIntegerv  LAST_VERTEX_CONVENTION  Provoking vertex  2.14.7  lighting
                                                                convention

New Implementation Dependent State

(table 6.36, page 301) add the following entry:

Get Value                      Type  Get Command  Initial Value  Description        Sec     Attribute
-----------------------------  ----  -----------  -------------  -----------------  ------  ---------
QUADS_FOLLOW_PROVOKING_VERTEX  B     GetBooleanv  -              True if quad       2.14.7  -
                                                                 primitives follow
                                                                 provoking vertex
                                                                 convention

NVIDIA Implementation Details

    GeForce 8 (G8x, G9x, GT1xx) and up GPUs report true for
    QUADS_FOLLOW_PROVOKING_VERTEX.

    GeForce 6 and 7 (NV4x, G7x) GPUs report false for
    QUADS_FOLLOW_PROVOKING_VERTEX.

Issues

    1.  What should this extension be called?

        RESOLVED:  ARB_provoking_vertex

        The phrase "provoking vertex" is not used in the core OpenGL
        specification but it is the accepted jargon for the
        functionality in question. This is the ARB-ratified version of
        the EXT_provoking_vertext extension.

    2.  How should quads be handled?

        RESOLVED:  Ideally, quadrilateral primitives (GL_QUADS and
        GL_QUAD_STRIP) would follow the provoking vertex mode.

        Other existing APIs with different flatshading conventions do
        not support quads.

        Rather than force support for both the first and last convention
        for quads (which no other API supports), instead this extension
        provides implementations the flexibility to advertise whether
        or not quads respect the provoking vertex or not.

        This resolution ensures more hardware vendors can support
        this extension.  Hardware vendors which support both OpenGL and
        Direct3D's provoking vertex conventions must have support for
        "first vertex" for triangles and lines because Direct3D demands
        these conventions.  Direct3D does not demand a convention for
        quads.  However every vendor supporting OpenGL can support the
        "last vertex" mode for quads.  Leaving the quad behavior up
        to the OpenGL implementation means hardware can simply always
        switch to the OpenGL quad behavior when emitting quads.

        See issue #12 for more details about how the
        implementation-dependent handling of quads is advertised.

    3.  How should the specification language be written for this
        extension?

        RESOLVED:  Update table 2.12 for all supported primitives.

        The current language describes how points and lines are handled
        in prose but the behavior for triangles, quads, and polygons is
        specified in table 2.12.

        Put all the Begin/End batch types in a single table with two
        columns specifying the "first vertex convention" and "last vertex
        convention" provoking vertex modes respectively.

        A unified table is less ambiguous than a combination of a table
        and prose.

    4.  What token names for the provoking vertex conventions should
        be used?

        RESOLVED:  GL_FIRST_VERTEX_CONVENTION and
        GL_LAST_VERTEX_CONVENTION (the initial state, consistent
        with OpenGL's unextended operation).

        The word "convention" is used because (see issue #2), the "first
        vertex" or "last vertex" rules are not iron-clad as they may or
        may do not apply to quads.

        The provoking vertex behavior for polygons and triangle fans
        also isn't strictly first or last vertex:  Polygons always use
        the first vertex (no matter the provoking vertex convention).
        Triangle fans don't really use the first vertex (the spoke vertex)
        when using the "first vertex" provoking vertex rule; see issue #7.

    5.  IRIS GL had a provoking vertex convention for polygons where the
        last vertex of a polygon primitive determined the flat shaded
        color of the polygon.  Should we support this convention?

        RESOLVED:  No.

        Interesting IRIS GL applications relying on this convention
        are assuredly non-existent at this point.  This convention
        also requires waiting until all the vertices for a polygon
        (which OpenGL does not bound) are specified before the polygon
        can begin being rasterized.  The IRIS GL convention was dubious
        for this reason and OpenGL's designers were correct to abandon
        IRIS GL's polygon provoking vertex convention.

    6.  How should line loops behave?

        RESOLVED:  Line loops in GL_FIRST_VERTEX_CONVENTION mode
        should behave as though it were a line strip with the first
        vertex repeated at the end.  In other words, the first vertex
        of the line loop provokes the color for the line primitive that
        closes the line loop.

        Direct3D does not support line loops.

    7.  How are triangle fans handled?

        RESOLVED:  The first vertex of a triangle fan is the spoke vertex.
        Triangle fans in GL_FIRST_VERTEX_CONVENTION mode should use
        the first non-spoke vertex of the primitive as the provoking
        vertex.  In other words, the spoke vertex is never the provoking
        vertex (in either convention).

        The rationale for this is to match DirectX 9's triangle
        fan behavior.  The rationale for the DirectX 9 behavior is
        (presumably) that if the spoke vertex was considered the "first
        vertex" of every primitive in a triangle fan, every flat shaded
        primitive in a triangle fan would necessarily have the spoke
        vertex's color, which isn't very interesting.

        (DirectX 10 does not support triangle fans.)

    8.  How does the provoking vertex convention affect primitives
        generated by a geometry shader?

        RESOLVED:  The provoking vertex convention affects primitives
        whether they are generated by geometry shaders or conventional
        (non-geometry shader) primitive assembly.

        Geometry shaders only generate point, line strips, and triangle
        strips (not line loops, triangle fans, polygons, or quads).
        (So the GL_QUADS_FOLLOW_PROVOKING_VERTEX_CONVENTION is
        irrelevant when a geometry program or shader is active.)

        This makes the supporting the first and last vertex conventions
        for primitives generated by geometry shaders "simple" because in
        the points, line strip, and triangle strip cases, the convention
        really is to use either first or last vertex to define the
        provoking vertex (no caveats).

        There's no special specification language to support the fact that
        the provoking vertex convention applies to primitives generated
        by geometry shaders because flat shading behavior is described
        in Chapter 3's rasterization discussion which is all subsequent
        to the geometry shader processing inserted into Chapter 2.

        DirectX 10 geometry shaders can output flat attributes according
        to Direct3D's "first vertex provokes" convention for line and
        triangle output primitives from a geometry shader.  So matching
        the DirectX 10 geometry shader behavior for flat shading requires
        setting the provoking vertex to GL_FIRST_VERTEX_CONVENTION.

        This said, the OpenGL default convention of "last vertex" for the
        provoking vertex tends to be more useful for geometry shaders.
        By deferring the computation of the flat shaded color to the
        last vertex of every primitive, that tends to give the geometry
        shader compiler the maximum allowance for scheduling computation
        and texturing operations required to compute the flat shaded
        color as long as possible (that is, until the last vertex of
        the primitive).

    9.  Should there be an OPTION or #pragma for geometry shader assembly
        and GLSL respectively to request the specific provoking vertex
        convention for the shader?

        RESOLVED:  No.

        The provoking vertex is context state that doesn't belong within
        a shader as a pragma anymore than the stencil state belongs
        within the shader.  Overriding context state based on a pragma
        in a shader introduces unfortunate validation interactions that
        will slow down shader binds.

        Geometry shaders written for DirectX 10 and using flat attributes
        expect the "first vertex" provoking vertex convention but the
        application is better off specifying the proper provoking vertex
        convention for shaders just as is done with other context state.

        TransGaming supports this resolution to not support a pragma.

    10. How do geometry shader input primitives interact with this
        extension?

        RESOLVED:  Table 2.12 includes the new primitives types
        introduced by geometry shaders (GL_LINES_ADJACENCY_ARB,
        GL_LINE_STRIP_ADJACENCY_ARB, GL_TRIANGLES_ADJACENCY_ARB, and
        GL_TRIANGLE_STRIP_ADJACENCY_ARB).  However the entries for these
        primitive types are only relevant when these new primitive types
        are used with NO geometry shader enabled.

        When a geometry shader is enabled, the only primitive output types
        are points, line strips, and triangle strips.

    11. To what attribute set should the provoking vertex belong?

        RESOLVED:  Lighting (GL_LIGHTING_BIT).

        This is because the provoking vertex bit is described in the
        same context as the shade model (GL_SHADE_MODEL) setting, and
        the shade model state is part of the lighting attribute set.

    12. How should the allowance for handling quadrilateral primitives
        be advertised?

        RESOLVED:  Because this extension is intended to facilitate
        supporting Direct3D content that depends on the Direct3D's
        provoking vertex convention yet Direct3D does not support quad
        primitives (as OpenGL provides with GL_QUAD_STRIP and GL_QUADS),
        the particular provoking vertex behavior of quads is not crucial
        to this extension's intended application.

        In the interest of making this extension's functionality for
        triangle and line primitives broadly available (the primitives
        Direct3D does support with a first vertex provoking vertex
        convention), this extension does not mandate a single uniform
        behavior for quad primitives.  Mandating a particular behavior
        for quad primitives would, for some implementations, encumber the
        performance of this extension in the non-quad case or make this
        implementation of this extension needlessly complex to implement.

        Instead the GL_QUADS_FOLLOW_PROVOKING_VERTEX_CONVENTION
        implementation-dependent boolean indicates whether or not quads
        (generated by GL_QUADS or GL_QUAD_STRIP) should abide by the
        provoking vertex convention or not.

        Whether or not the GL_QUADS_FOLLOW_PROVOKING_VERTEX_CONVENTION
        state is true or false, the provoking vertex behavior of quads
        is well-defined in either case.

        The recommended, though implementation-dependent, value for
        GL_QUADS_FOLLOW_PROVOKING_VERTEX_CONVENTION is true because
        this means quads, will like lines and triangles, follow the
        GL_PROVOKING_VERTEX state as indicated.

    13. How does the provoking vertex state interact with primitive
        restart?

        RESOLVED:  Orthogonally so no specific specification language
        describing the interaction is required.

        Specifically a primitive restart acts as a glEnd/glBegin
        sequence so it restarts the primitive numbering to 1 for the
        vertex immediately following the restart index.

    14. Should the provoking vertex behavior apply to both the primary
        and secondary color?

        RESOLVED:  Yes, the provoking vertex decides both the primary and
        secondary color of a flat-shaded primitive.  That's consistent
        with Direct3D's provoking vertex convention as well as OpenGL's
        current behavior.

    15. Should the provoking vertex behavior be specified with a
        glEnable/glDisable token instead of glProvokingVertex?

        RESOLVED:  The provoking vertex API is closely related
        to glShadeModel which uses an enumerated mode rather than
        glEnable/glDisable to specify flat or smooth shading so the API
        mimics the glShadeModel approach.

        This results in a fairly readable API usage that is more easily
        groaked by unfamiliar programmers:

            glProvokingVertex(GL_FIRST_VERTEX_CONVENTION);

        instead of:

             glEnable(GL_FIRST_VERTEX_CONVENTION);

        It is also not clear that the provoking vertex convention is
        really a single enable.  The convention behaves differently
        depending on the primitive type.  For example, GL_POLYGON always
        uses the first vertex as the provoking vertex regardless of the
        provoking vertex state.

    16. Does the OpenGL Shading Language (GLSL) 1.30 "flat" varying
        qualifier respect the provoking vertex state?

        RESOLVED:  Yes.

        The GLSL 1.30 specification says "This variable [qualified as
        flat] will come from a single provoking vertex, as described by
        the OpenGL Graphics System Specification."  This extension amends
        how the provoking vertex is described so no GLSL specification
        update is required.  This does imply that user-declared varyings
        in a GLSL shader declared with "flat" will have the provoking
        vertex convention applied to determine their value.

    17. How does the provoking vertex apply to Direct3D 10?

        RESOLVED:  Direct3D 10 has deprecated the D3DSHADEMODE state for
        controlling flat or smooth (Gouraud) shading.  However there is
        still the concept of a provoking vertex (called the "leading
        vertex" by Direct3D 10) which still corresponds to this
        extension's "first vertex" convention.

        Use of the leading (provoking) vertex for constant (flat)
        interpolation is indicated by Direct3D 10's "nointerpolation"
        variable storage class (sometimes called an interpolation
        modifier).

    18. Does the NV_gpu_program4 "FLAT" attribute modifier respect the
        provoking vertex state?

        RESOLVED:  Yes.  NVIDIA's NV_gpu_program4 extension, describing
        an OpenGL assembly for Shader Model 4.0, allows a FLAT modifier
        to be specified for fragment program inputs.  The NV_gpu_program4
        specification says "If an attribute is flat-shaded, it will be
        taken from the output attribute of the provoking vertex of the
        primitive using the same data type." This extension amends
        how the provoking vertex is described so no NV_gpu_program4
        specification update is required.

    19. How does this extension interact with transform feedback?

        RESOLVED:  Attribute components written out by transform feedback
        are NOT affected by the flatshading or provoking vertex state.

        While this specification is written against OpenGL 2.1, transform
        feedback was made core functionality with OpenGL 3.0 and then
        the order of the transform feedback was moved in the OpenGL
        3.1 specification.  Therefore the subsequent discussion uses
        the more recent 3.1 sectioning.

        Specifically the OpenGL 3.1 specification (section 2.18: Transform
        Feedback) says "The vertices are fed back after vertex color
        clamping, but before clipping."

        This statement is unclear because flatshading (section 2.13.7:
        Flatshading) happens inbetween vertex color clamping (section
        2.13.6: Clamping or Masking) and primitive clipping (section 2.20:
        Primitive Clipping).

        Base on this issue the sentence is clarified to read: "The
        vertices are fed back after vertex color clamping, but before
        [flatshading and] clipping."

        For reference, the original EXT_transform_feedback extension has
        this same language ("The vertices are fed back after vertex color
        clamping, but before clipping.") but follows that sentence with:
        "If a geometry shader is active, the vertices recorded are those
        emitted from the geometry shader."  Technically geometry shading
        takes place prior to even vertex color clamping.

        Clearly flat shading needs to happen prior to clipping so that
        clipped vertices can share the flat shaded attributes of the
        primitive prior to any potential clipping.

        This resolution is consistent with DirectX 10's behavior.
        Technically, DirectX 10 says that vertices output through
        transform feedback (which DirectX 10 calls "stream output")
        only have to be defined for constant attributes of the primitive's
        leading vertex (constant attributes are those that OpenGL would
        call flatshaded).  Other constant attributes for non-leading
        vertices may be undefined.  Leaving such constant attributes
        undefined is undesirable, particularly given how OpenGL operates.
        It is well-defined and more useful to simply output the value
        of the vertex's attribute prior to any flatshading.  This is
        particularly desirable for OpenGL because with this extension
        (and even prior to supporting this extension), the provoking
        vertex is not always the leading vertex.

        To clarify further, while this resolution is consistent with
        DirectX 10, an OpenGL implementation that supports transform
        feedback has no undefined behavior specified.  The simplest way
        to describe what happens is that attribute components written
        out by transform feedback are the attribute component values
        of vertices AFTER (optional) geometry shading and vertex color
        clamping but PRIOR to flatshading and primitive clipping.

    20. Why don't the new tokens and entry points in this extension have
        "ARB" suffixes like other ARB extensions?

        RESOLVED: Unlike most ARB extensions, this is a strict subset of
        functionality already approved in OpenGL 3.2. This extension
        exists only to support that functionality on older hardware that
        cannot implement a full OpenGL 3.x driver. Since there are no
        possible behavior changes between the ARB extension and core
        features, source code compatibility is improved by not using
        suffixes on the extension.

Revision History

    Rev.    Date    Author     Changes
    ----  -------- ---------  ---------------------------------------------
      1   5/19/09  dgkoch     copy & rename from EXT_provoking_vertex v12
      2   6/26/09  dgkoch     resync with GL 3.2. reorder edits so they
                              fit better with ARB_compatibility. update
                              dependencies with GL 3.1, 3.2 and
                              ARB_compatibility.
      3   8/02/09  Jon Leech  Reformat to 80 columns and assign ARB
                              extension number.