Name

    AMD_vertex_shader_tessellator

Name Strings

    GL_AMD_vertex_shader_tessellator

Contact

    Bill Licea-Kane, AMD ( Bill.Licea-Kane 'at' amd.com )

Status

    Complete

Version

    Last Modified Date: 2009-03-06
    Author Revision: 8

Number

    363

Dependencies

 
    OpenGL 2.0 is required.

    EXT_gpu_shader4 affects the definition of this extension.

    EXT_geometry_shader4 affects the definition of this extension.

    This extension interracts with AMDX_vertex_shader_tesselator.

    This extension is written against the OpenGL Shading Language
    1.20 Specification.

    The extension is written against the OpenGL 2.1 Specification.

Overview

    The vertex shader tessellator gives new flexibility to the shader
    author to shade at a tessellated vertex, rather than just at a
    provided vertex.

    In unextended vertex shading, the built-in attributes such as
    gl_Vertex, gl_Normal, and gl_MultiTexcoord0, together with the
    user defined attributes, are system provided values which are
    initialized prior to vertex shader invocation.

    With vertex shading tessellation, additional vertex shader special 
    values are available:

        ivec3 gl_VertexTriangleIndex; // indices of the three control
                                      // points for the vertex
        vec3 gl_BarycentricCoord;     // barycentric coordinates
                                      // of the vertex

    i o
      |\
      | \
      *--*
      |\ |\
      | \| \
      *--*--*
      |\ |\ |\
      | \| \| \
    j o--*--*--o k

    Figure 1  A Tessellated Triangle
    o = control point (and tessellated vertex)
    * = tessellated vertex

        ivec4 gl_VertexQuadIndex;   // indices for the four control
                                    // points for the vertex
        vec2 gl_UVCoord;            // UV coordinates of the vertex

    i o--*--*--o k
      |\ |\ |\ | 
      | \| \| \|
      *--*--*--*
      |\ |\ |\ | 
      | \| \| \|
      *--*--*--*
      |\ |\ |\ | 
      | \| \| \|
    j o--*--*--o l

    Figure 2  A Tessellated Quad
    o = control point (and tessellated vertex)
    * = tessellated vertex

    When this extension is enabled, conventional built-in attributes
    and user defined attributes are uninitialized.  The shader writer
    is responsible for explicitly fetching all other vertex data either
    from textures, uniform buffers, or vertex buffers.

    The shader writer is further responsible for interpolating 
    the vertex data at the given barycentric coordinates or uv
    coordinates of the vertex.

IP Status

    No known claims.

New Procedures and Functions

    void TessellationFactorAMD( float factor );
    void TessellationModeAMD( enum mode );

New Types

    (None.)

New Tokens

    Returned by the <type> parameter of GetActiveUniform:

      SAMPLER_BUFFER_AMD                               0x9001
      INT_SAMPLER_BUFFER_AMD                           0x9002
      UNSIGNED_INT_SAMPLER_BUFFER_AMD                  0x9003

    Accepted by TessellationModeAMD

      DISCRETE_AMD                                     0x9006
      CONTINUOUS_AMD                                   0x9007

    Accepted by GetIntegerv

      TESSELLATION_MODE_AMD                            0x9004

    Accepted by GetFloatv

      TESSELLATION_FACTOR_AMD                          0x9005

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

   Modify section 2.15.3, "Shader Variables", page 75

   Add the following new return types to the description of
   GetActiveUniform on p. 81.

      SAMPLER_BUFFER_AMD,
      INT_SAMPLER_BUFFER_AMD,
      UNSIGNED_INT_SAMPLER_BUFFER_AMD.

   Replace section "Samplers" p. 83 with:

    Samplers

    Samplers are special uniforms used in the OpenGL Shading Language
    to identify the texture object or vertex buffer object used for
    each lookup.

    Samplers and Texture objects

    If the sampler is one of the texture types, the value of a sampler
    indicates the texture image unit being accessed.  Setting a
    sampler's value to i selects texture image unit number i. The values
    of i range from zero to the implementation dependent maximum
    supported number of texture image units.

    The type of the sampler identifies the target on the texture image
    unit. The texture object bound to that texture image unit's target
    is then used for the texture lookup. For example, a variable of type
    sampler2D selects target TEXTURE 2D on its texture image unit.
    Binding of texture objects to targets is done as usual with
    BindTexture. Selecting the texture image unit to bind to is done as
    usual with ActiveTexture.

    It is not allowed to have variables of different sampler types
    pointing to the same texture image unit within a program object.
    This situation can only be detected at the next rendering command
    issued, and an INVALID OPERATION error will then be generated.

    Samplers and vertex buffer objects

    If the sampler is one of the vertex types, the value of a sampler
    indicates the vertex array being accessed.  Setting a sampler's
    value to i selects vertex array i.  The values of i range from zero
    to the implementation dependent maximum supported max vertex
    attributes.  Binding of vertex buffer objects to vertex arrays is
    done as usual with BindBuffer.

    It is not allowed to have multiple variables of samplers to the same
    vertex array within a program object.  This situation can only be
    detected at the next rendering command issued, and an INVALID
    OPERATION error will then be generated.

    All samplers

    The location of a sampler needs to be queried with
    GetUniformLocation, just like any uniform variable. Sampler values
    need to be set by calling Uniform1i{v}.  Loading samplers with any
    of the other Uniform* entry points is not allowed and will result
    in an INVALID OPERATION error.

    Active samplers are samplers actually being used in a program
    object. The LinkProgram command determines if a sampler is active or
    not. The LinkProgram command will attempt to determine if the active
    samplers in the shader(s) contained in the program object exceed the
    maximum allowable limits. If it determines that the count of active
    samplers exceeds the allowable limits, then the link fails (these
    limits can be different for different types of shaders). Each active
    sampler variable counts against the limit, even if multiple samplers
    refer to the same texture image unit. If this cannot be determined
    at link time, for example if the program object only contains a
    vertex shader, then it will be determined at the next rendering
    command issued, and an INVALID OPERATION error will then be
    generated.

   Insert section prior to "Validation" on p. 87

    Tessellation

    If a vertex shader enables GL_AMD_vertex_shader_tessellation, then
    the shader writer is responsible for fetching and evaluating the
    vertex attributes at the barycentric coordinates of the vertex.
    (See the shading language specification.)

    Only indexed triangles or indexed quads may be drawn with such a
    shader. Each triangle or quad will introduce generated vertices
    (including the original vertices of the triangle or quad)
    controlled by:

        void TessellationFactorAMD( float factor );

    where the factor is a value between 1.0 and 15.0 inclusive

    The introduction of generated vertices is further controlled by:

       void TessellationModeAMD( enum mode );

    where mode is either DISCRETE_AMD or CONTINUOUS_AMD.

   Add to the list of "begin errors":

     * any two samplers of vertex type refer to the same vertex array.

     * Any sampler bound to a vertex array has vertex buffer object 0
       bound.

     * A vertex shader enables GL_AMD_vertex_shader_tessellation,
       statically reads gl_VertexTriangleIndex or gl_BarycentricCoord
       and the Implicit Begin mode is NOT GL_TRIANGLES

     * A vertex shader enables GL_AMD_vertex_shader_tessellation,
       statically reads gl_VertexQuadIndex or gl_UVCoord and
       the Implicit Begin mode is NOT GL_QUADS

     * A vertex shader enables GL_AMD_vertex_shader_tessellation and
       the command is RasterPos.

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

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

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


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


Additions to Appendix A of the OpenGL 2.1 Specification (Invariance)

Modifications to The OpenGL Shading Language 1.20 Specification

    Including the following line in a shader can be used to control the
    language features described in this extension:

      #extension GL_AMD_vertex_shader_tessellator : <behavior>

    where <behavior> is as specified in section 3.3.

    A new preprocessor #define is added to the OpenGL Shading Language:

      #define GL_AMD_vertex_shader_tessellator 1

Additions to Chapter 1 of the OpenGL Shading Language 1.20 Specification
(Introduction)

Additions to Chapter 2 of the OpenGL Shading Language 1.20 Specification
(Overview of OpenGL Shading)

    2.1  Vertex Processor

    Change 2nd paragraph to:

    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.  While a tessellated vertex however has LIMITED
    knowledge of the immediately adjacent control points (three for a
    triangle, four for a quad), the vertex processor is still operating
    on one tessellated vertex at a time.  The vertex shaders running on
    the vertex processor must compute the homogeneous position
    of the incoming vertex.


Additions to Chapter 3 of the OpenGL Shading Language 1.20 Specification
(Basics)

    3.6  Keywords

    Add the keywords

    __samplerVertexAMD
    __isamplerVertexAMD
    __usamplerVertexAMD

Additions to Chapter 4 of the OpenGL Shading Language 1.20 Specification
(Variables and Types)

4.3.4 Attribute, Change third sentence:

    "Attribute values are read-only as far as the vertex shader is
    concerned, unless GL_AMD_vertex_shader_tessellator is enabled.  If
    GL_AMD_vertex_shader is enabled, they are read-write with undefined
    initial values."

Additions to Chapter 5 of the OpenGL Shading Language 1.20 Specification
(Operators and Expressions)

Additions to Chapter 6 of the OpenGL Shading Language 1.20 Specification
(Statements and Structure)

Additions to Chapter 7 of the OpenGL Shading Language 1.20 Specification
(Built-in Variables)

7.1 Vertex Shader Special Variables

Add the list of intrinsically declared with the following types:

        // if GL_AMD_vertex_shader_tessellator enabled

        ivec3 gl_VertexTriangleIndex; // may be read
                                      // indices of the three control
                                      // points for the vertex
        vec3 gl_BarycentricCoord;     // may be read
                                      // barycentric coordinates of the
                                      // vertex

        ivec4 gl_VertexQuadIndex;     // may be read
        vec2 gl_UVCoord;              // may be read


    If gl_VertexTriangleIndex and/or gl_BarycentricCoord is statically
    read by the shader, the shader is a Triangle Tessellator shader.

    If gl_VertexQuadIndex and/or gl_UVCoord is statically read by the
    shader, the shader is a Quad Tessellator shader.

    It is a link error if both a Triangle Tessellator shader and a Quad
    Tessellator shader are attached to a program.

7.3 Vertex Shader Built-In Attributes

Add the following paragraph.

    If GL_AMD_vertex_shader_tessellator is enabled, the values of the
    built-in Attributes are undefined.

Additions to Chapter 8 of the OpenGL Shading Language 1.20 Specification
(Built-in Functions)

8.7 Texture Lookup Functions

Rename section to "Lookup Functions"

Add in front of first sentence:

    Vertex lookup functions are available to the vertex shader.

Add to the front of the table of functions:

    Syntax:
    vec4  vertexFetchAMD( __samplerVertexAMD sampler, int i );
    ivec4 vertexFetchAMD( __isamplerVertexAMD sampler, int i );
    uvec4 vertexFetchAMD( __usamplerVertexAMD sampler, int i );

    Description:

    If GL_AMD_vertex_shader_tessellator is enabled, fetch the "ith"
    element from  the vertex buffer bound to the vertex array bound to
    the sampler.

Additions to Chapter 9 of the OpenGL Shading Language 1.20 Specification
(Shading Language Grammar)

Additions to Chapter 10 of the OpenGL Shading Language 1.20
Specification (Issues)

Additions to the AGL/EGL/GLX/WGL Specifications

    None


Dependencies on ARB_vertex_shader

    ARB_vertex_shader is required.

Interactions with EXT_gpu_shader4

    If EXT_gpu_shader4 is not supported, remove all references to:

    __isamplerVertexAMD
    __usamplerVertexAMD
    ivec4 vertexFetchAMD
    uvec4 vertexFetchAMD

Interactions with EXT_geometry_shader4

    If EXT_geometry_shader4 is supported, change the last
    paragraph of Section 2.16, Geometry Shaders to:

    A program object that includes a geometry shader must also include
    a vertex shader; otherwise a link error will occur.  If a program
    object that includes a geometry shader also includes a vertex shader
    with that has enabled GL_AMD_vertex_shader_tessellator, a link error
    will occur.

Interactions with AMDX_vertex_shader_tessellator

    This extension is symantically identical to the experimental
    AMDX_vertex_shader_tessellator.  (It has been "promoted" to
    non-experimental status.)

    Only the prefix AMDX has been changed to AMD.
    Only the suffix AMDX has been changed to AMD.

    We encourage applications and shader writers to migrate from
    AMDX to AMD.  However, the AMDX entry points, enums, keywords
    and function names are not yet deprecated.

Errors

New State

Add to Table 6.5 Vertex Array Data

    Get Value                  Type    Get Command     Value         Description          Sec.    Attribute
    ---------                  ----    --------------- -------       -------------------- ----    ---------
    TESSELLATION_FACTOR_AMD    R       GetFloatv       1.0           tessellation factor  2.8     vertex-array
    TESSELLATION_MODE_AMD      Z_2     GetIntegerv     DISCRETE_AMD  tessellation mode    2.8     vertex-array
 
New Implementation Dependent State

    None.

Sample Code

#extension GL_AMD_vertex_shader_tessellator : require

__samplerVertexAMD Vertex;
__samplerVertexAMD Normal;
__samplerVertexAMD Texcoord0;
__samplerVertexAMD Temperature;
__samplerVertexAMD Pressure;

attribute float myTemperature;

void main ( void )
{
    gl_Vertex = vec4( 0.0 );
    gl_Normal = vec4( 0.0 );
    gl_MultiTexCoord0 = vec4( 0.0 );
    myTemperature = 0.0;
    float myPressure = 0.0;  // Don't have to interpolate to attribute

    for ( int i=0; i<3; i++ )
    {
       float weight = gl_BarycentricCoord[i];

       gl_Vertex         += weight*vertexFetchAMD( Vertex,      gl_VertexTriangleIndex[i] );
       gl_Normal         += weight*vertexFetchAMD( Normal,      gl_VertexTriangleIndex[i] );
       gl_MultiTexCoord0 += weight*vertexFetchAMD( Texcoord0,   gl_VertexTriangleIndex[i] );
       myTemperature     += weight*vertexFetchAMD( Temperature, gl_VertexTriangleIndex[i] ).x;
       myPressure        += weight*vertexFetchAMD( Pressure,    gl_VertexTriangleIndex[i] ).x;

    }
    // Rest of vertex shader goes here....
}

Issues

1) Does this belong conceptually in the pipe as subsuming geometry
   shader (after primitive combine) or vertex unpack.

   Vertex unpack.  Even though there is "primitive information" it
   is limited to the immediate neighborhood.

2) Do we need a new stage?

   If we add a "tessellation" stage:

   Input to the tessellator is the unpacked vertex attributes, but
   each attribute is now an array of size 3, the "superprim" attributes,
   plus a barycentric coordinate.

   The output of the tessellator is the varying.

   The varying output of the tessellator then becomes the attributes
   input to the vertex shader.

   Alternatively, we can make the "unpack" part of the vertex shader
   responsibility.

   No.  We'll just make the attributes undefined, and the
   "vertex unpack" stage naturally collapses into the vertex shader.

3) Why make attributes undefined but writable?

   This is the easiest way to have an unpack shader merged into
   existing shaders.

4) What variants of vertexFetch do we need.

   1D is probably all we need, and probably all we will ever need.
   The return types should be vec4, ivec4 and uvec4.
   So, we need:

   vec4  vertexFetchAMD( __samplerVertexAMD  sampler, int i );
   ivec4 vertexFetchAMD( __isamplerVertexAMD sampler, int i );
   uvec4 vertexFetchAMD( __usamplerVertexAMD sampler, int i );

5) How does __samplerVertexAMD and vertexFetchAMD interact with vertex arrays?

   The __samplerVertexAMD becomes an active uniform.  As existing
   samplers are bound to texture units, the samplerVertex is bound to a
   VertexAttrib array, and similarly, the "enable" of the
   VertexAttribArray is ignored.  vertexFetchAMD will use the size,
   type, normalized and stride to fetch the "ith" element from the array
   as the following pseudocode:

   if (generic vertex attribute j array normalization flag is set, and
       type is not FLOAT or DOUBLE)
     VertexAttrib[size]N[type]v
       (j, generic vertex attribute j array element i);
   else
     VertexAttrib[size][type]v
       (j, generic vertex attribute j array element i);

6) What happens if a buffer object is not bound to an array?

    There is no reason why it shouldn't work, but there's also no good
    reason to make it work.  Undefined.

7) What about "conventional" OpenGL array state (Vertex, Color, Normal,
   TexCoord, etc....)?

   By binding the buffer objects to the appropriate vertexAttrib array,
   and setting appropriate size, type, normalized and stride, the
   application programmer can access all "conventional" OpenGL array
   state?

8) Are attributes declared or used in the shader "active?"

    For the purposes of GetActiveAttrib, GetAttribLocation and
    BindAttribLocation, no.

9) What about geometry shaders and tessellation?

    Future hardware may relax this restriction, but you can not
    successfully link a program that includes a vertex shader that has
    enabled GL_AMD_vertex_shader_tessellator and a geometry shader.

10) What draw calls do we support?

    To the shader writer, everything looks like indexed triangles or
    indexed quads, with discrete and continuous tessellation.   These
    indexed triangles result from a  polygon Begin/End object, a
    triangle resulting from a triangle strip, triangle fan,  or series
    of separate triangles, or a  quadrilateral arising from a
    quadrilateral strip,  series of separate quadrilaterals, or a Rect
    command.

    Points, Lines and pixel rectangles and bitmaps are unsupported by
    a tessellation shader.

11) Do we need additional enables?

    Lets first see how "implicit" enable of vertex arrays and
    tessellation draw calls works. The first follows precedent (samplers
    override texture enable hierarchy.)  The second seems to follow.

11) What about begin errors?

    They are evil, but I don't see how they can be avoided.
    Clearly sampler validation needs to follow precedent.

12) What about quads?

    Quads are necessary for subdivision surfaces such as Catmull-Clark.
    We have received several significant requests to support subdivision
    surfaces.

Revision History

    Revision 1, 2007-06-26 wwlk
    Preliminary review document

    Revision 2, 2007-08-16 wwlk
    Review document

    Correct spelling of "tessellate" throughout.  Blush.
    Rename special variables.
    Add additional sampler types.
    Remove "1D" from sampler types and vertex fetches.
    Add core OpenGL api spec changes.
    Add interactions with EXT_gpu_shader4.
    Add many issues.
    Expanded example shader.

    Revision 3, 2007-08-17 wwlk
    Correct edit headers
    (OpenGL 1.5 -> OpenGL 2.0)
    (Shading Language 1.10 -> Shading Language 1.20)

    Revision 4, 2007-09-21 wwlk
    Fix typo in reserved keywords (remove "1D")
    Added support for all polygon calls, explicitly disallowing points
    lines and RasterPos,
    List additional BEGIN errors - yes they are evil.

    Revision 5, 2008-05-22 wwlk
    Add quad support

    Revision 6, 2009-03-05 wwlk
    General cleanup to ready for posting to repository

    Revision 7, 2009-03-05 wwlk
    Promote from AMDX to AMD.

    Revision 8, 2009-03-06 wwlk
    Minor update to enums section.
    Cleaned up typos and <cr><lf>.