xref: /third_party/openGLES/extensions/EXT/EXT_draw_elements_base_vertex.txt

Name

    EXT_draw_elements_base_vertex

Name Strings

    GL_EXT_draw_elements_base_vertex

Contact

    Daniel Koch, NVIDIA (dkoch 'at' nvidia.com)

Contributors

    Shannon Woods, Google
    Dominik Witczak, Mobica
    Contributors to ARB_draw_elements_base_vertex

Notice

    Copyright (c) 2009-2014 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

    Portions Copyright (c) 2014 NVIDIA Corporation.

Status

    Complete.

Version

    Last Modified Date:  August 11, 2020
    Version:             4

Number

    OpenGL ES Extension #204

Dependencies

    This specification is written against the OpenGL ES 3.1 (June 4, 2014)
    Specifications, but can apply to prior specifications.

    Requires OpenGL ES 2.0.

    This extension interacts with OpenGL ES 3.1.

    This extension interacts with OpenGL ES 3.0.

    This extension interacts with EXT_draw_instanced.

    This extension interacts with NV_draw_instanced.

    This extension interacts with EXT_instanced_arrays.

    This extension interacts with ANGLE_instanced_arrays.

    This extension interacts with NV_instanced_arrays.

    This extension interacts with EXT_multi_draw_arrays.

Overview

    This extension provides a method to specify a "base vertex offset"
    value which is effectively added to every vertex index that is
    transferred through DrawElements.

    This mechanism can be used to decouple a set of indices from the
    actual vertex array that it is referencing. This is useful if an
    application stores multiple indexed models in a single vertex array.
    The same index array can be used to draw the model no matter where
    it ends up in a larger vertex array simply by changing the base
    vertex value. Without this functionality, it would be necessary to
    rebind all the vertex attributes every time geometry is switched and
    this can have larger performance penalty.

    For example consider the (very contrived and simple) example of
    drawing two triangles to form a quad. In the typical example you
    have the following setup:

          vertices                indices
         ----------                -----
      0 | (-1,  1) |            0 |  0  |
      1 | (-1, -1) |            1 |  1  |
      2 | ( 1, -1) |            2 |  2  |
      3 | ( 1,  1) |            3 |  3  |
         ----------             4 |  0  |
                                5 |  2  |
                                   -----
    which is normally rendered with the call

       DrawElements(TRIANGLES, 6, UNSIGNED_BYTE, &indices).

    Now consider the case where the vertices you want to draw are not at
    the start of a vertex array but are instead located at offset 100
    into a larger array:

           vertices2             indices2
           ----------             -----
              ....             0 | 100 |
      100 | (-1,  1) |         1 | 101 |
      101 | (-1, -1) |         2 | 102 |
      102 | ( 1, -1) |         3 | 103 |
      103 | ( 1,  1) |         4 | 100 |
              ....             5 | 102 |
           ----------             -----

    The typical choices for rendering this are to rebind your vertex
    attributes with an additional offset of 100*stride, or to create an
    new array of indices (as indices2 in the example). However both
    rebinding vertex attributes and rebuilding index arrays can be quite
    costly activities.

    With the new drawing commands introduced by this extension you can
    instead draw using vertices2 and the new draw call:

       DrawElementsBaseVertexEXT(TRIANGLES, 6, UNSIGNED_BYTE, &indices, 100)

New Procedures and Functions

    void DrawElementsBaseVertexEXT(enum mode, sizei count, enum type,
                                   const void *indices, int basevertex);

    [[ If OpenGL ES 3.0 is supported: ]]

    void DrawRangeElementsBaseVertexEXT(enum mode, uint start, uint end,
                                        sizei count, enum type,
                                        const void *indices, int basevertex);

    void DrawElementsInstancedBaseVertexEXT(enum mode, sizei count,
                                            enum type, const void *indices,
                                            sizei instancecount,
                                            int basevertex);

    [[ If EXT_multi_draw_arrays is supported: ]]

    void MultiDrawElementsBaseVertexEXT(enum mode,
                                        const sizei *count,
                                        enum type,
                                        const void * const *indices,
                                        sizei drawcount,
                                        const int *basevertex);

New Tokens

    None

Additions to Chapter 10 of the OpenGL ES 3.1 Specification (Vertex
Specification and Drawing Commands)

    Modify section 10.3.7 "Array Indices in Buffer Objects" p. 244,
    adding the following to the end of the third paragraph (beginning
    with "While a non-zero buffer object name..."

    "DrawElementsBaseVertexEXT, DrawRangeElementsBaseVertexEXT, and
    DrawElementsInstancedBaseVertexEXT also source their indices from that
    buffer object, adding the <basevertex> offset to the appropriate vertex
    index as a final step before indexing into the vertex buffer; this does
    not affect the calculation of the base pointer for the index array."

    [[ If EXT_multi_draw_arrays is supported: ]]

    "Finally, MultiDrawElementsEXT and MultiDrawElementsBaseVertexEXT also
    source their indices from that buffer object, using its <indices>
    parameter as a pointer to an array of pointers that represent
    offsets into the buffer object."


    Modify section 10.5 "Drawing Commands Using Vertex Arrays" as follows:

    Replace the definition and paragraph describing
    DrawElementsInstancedBaseVertex, bottom of p. 252 with the following:

    "The commands
      void DrawElementsBaseVertexEXT(enum mode, sizei count, enum type,
                                    const void *indices, int basevertex);

      void DrawRangeElementsBaseVertexEXT(enum mode, uint start, uint end,
                                          sizei count, enum type,
                                          const void *indices, int basevertex);

      void DrawElementsInstancedBaseVertexEXT(enum mode, sizei count,
                                              enum type, const void *indices,
                                              sizei instancecount,
                                              int basevertex);

    are equivalent to the commands with the same base name (without the
    "BaseVertexEXT" suffix) except that the <i>th element transferred by
    the corresponding draw call will be taken from element
       <indices>[<i>] + <basevertex>
    of each enabled array. If the resulting value is larger than the maximum
    value representable by <type> it should behave as if the calculation were
    upconverted to 32-bit unsigned integers (with wrapping on overflow
    conditions). The operation is undefined if the sum would be negative and
    should be handled as described in Section 6.4. For
    DrawRangeElementsBaseVertexEXT, the index values must lie between <start>
    and <end> inclusive, prior to adding the <basevertex> offset. Index values
    lying outside the range [<start>,<end>] are treated in the same way as
    DrawRangeElements."

    [[ If EXT_multi_draw_arrays is supported: ]]

    "The command

      void MultiDrawElementsBaseVertexEXT(enum mode,
                                          const sizei *count,
                                          enum type,
                                          const void *const *indices,
                                          sizei drawcount,
                                          const int *basevertex);

    behaves identically to DrawElementsBaseVertexEXT except that
    <drawcount> separate lists of elements are specified instead. It has
    the same effect as:

      if (<mode> or <drawcount> is invalid)
          generate appropriate error
      else {
          for (i = 0; i < <drawcount>; i++)
              if (<count>[i] > 0)
                  DrawElementsBaseVertexEXT(<mode>, <count>[i], <type>,
                                            <indices>[i], <basevertex>[i]);
      }"

Additions to the EGL/AGL/GLX/WGL Specifications

    None

Dependencies on OpenGL ES 3.1

    If OpenGL ES 3.1 is not supported apply the following modifications to the
    OpenGL ES 3.0.3 specification:

    Add the following to the end of Section 2.8.1 "Transferring Array Elements"

    "When one of the *BaseVertex drawing commands specified in section
    2.8.3 is used, the primitive restart comparison occurs before the
    <basevertex> offset is added to the array index."

    Replace the following references relative to the ES 3.1 specification
    with the following references to the ES 3.0.3 specification:

    Edits to section 10.3.7 "Array Indices in Buffer Objects" in ES 3.1 become
    edits to section 2.9.7 "Array Indices in Buffer Objects" in ES 3.0.3.

    Edits to section 10.5 "Draw Command using Vertex Arrays" in ES 3.1 become
    edits to section 2.8.3 "Drawing Commands" in ES 3.0.3.

    Replace references to section 6.4 in ES 3.1 with references to section
    2.9.4 in ES 3.0.3.

Dependencies on OpenGL ES 3.0

    If OpenGL ES 3.0 is not supported, ignore all references to
    DrawElementsInstanced and DrawElementsInstancedBaseVertexEXT (unless one
    of the instanced_array or draw_instanced extensions is present).

    If OpenGL ES 3.0 is not supported, ignore all references to
    DrawRangeElements and DrawRangeElementsBaseVertexEXT.

    If OpenGL ES 3.0 is not supported, ignore all references to primitive
    restart index.

Dependencies on the EXT_draw_instanced extension

    If EXT_draw_instanced is supported, the functionality provided by
    DrawElementsInstancedBaseVertexEXT can also be described in terms of
    DrawElementsInstancedEXT instead of DrawElementsInstanced.

Dependencies on the NV_draw_instanced extension

    If NV_draw_instanced is supported, the functionality provided by
    DrawElementsInstancedBaseVertexEXT can also be described in terms of
    DrawElementsInstancedNV instead of DrawElementsInstanced.

Dependencies on the EXT_instanced_arrays extension

    If EXT_instanced_arrays is supported, the functionality provided by
    DrawElementsInstancedBaseVertexEXT can also be described in
    terms of DrawElementsInstancedEXT instead of DrawElementsInstanced.

Dependencies on the ANGLE_instanced_arrays extension

    If ANGLE_instanced_arrays is supported, the functionality provided by
    DrawElementsInstancedBaseVertexEXT can also be described in
    terms of DrawElementsInstancedANGLE instead of DrawElementsInstanced.

Dependencies on the NV_instanced_arrays extension

    If ARB_instanced_arrays is supported, the functionality provided by
    DrawElementsInstancedBaseVertexEXT can also be described in
    terms of DrawElementsInstancedNV instead of DrawElementsInstanced.

Errors

    The *BaseVertexEXT commands have identical error conditions to the
    non-*BaseVertexEXT functions, and all values of <basevertex> are legal
    (with the exception of ones which cause accesses outside of vertex
    arrays or bound buffers as described in Section 6.4).

New State

    None

New Implementation Dependent State

    None

Issues

    Note: These issues apply specifically to the definition of the
    EXT_draw_elements_base_vertex specification, which is based on the OpenGL
    extension ARB_draw_elements_base_vertex as updated in OpenGL 4.4.
    ARB_draw_elements_base_vertex can be found in the OpenGL Registry.

    (0) This extension is based on ARB_draw_elements_base_vertex.  What are
    the major differences?

        - rebased on OpenGL ES 3.1
        - renamed the "primcount" parameter to "instancecount" per GL4
        - MultiDrawElementsBaseVertexEXT is only available if
          GL_EXT_multi_draw_arrays is supported.

    (1) Should we include MultiDrawElementsBaseVertexEXT in this extension?

    RESOLVED: Yes, but only if EXT_multi_draw_arrays is supported, since
    multi draw calls are not available in unextended OpenGL ES 2 or 3.

    (2) Should we allow client memory to be used for vertex attributes and
    for the <indices> in the *BaseVertexEXT commands?

    RESOLVED: Yes. Since these are defined in terms of the non-BaseVertex
    commands which already supports client memory for vertex attributes and
    indices it makes sense to include support for these new commands as well.

    (3) Which commands are supported on OpenGL ES 2.0 implementations that
    support this extension?

    RESOLVED: Only DrawElementsBaseVertexEXT, unless one of the instancing
    extensions is supported in which case DrawElementsInstancedBaseVertexEXT
    is also supported. DrawRangeElementsBaseVertexEXT is not supported because
    OpenGL ES 2.0 doesn't include DrawRangeElements.

    (4) Which commands are supported on OpenGL ES 3.0 implementations?

    RESOLVED: All of the new drawing commands are applicable to OpenGL ES 3.0
    implementations.

    (5) Does the value of gl_VertexID in the shading language include the
    the value of the <basevertex> offset?

    RESOLVED: Yes. This is as clarified by Khronos bugs 12202 and 12756
    and is consistent with the overview of ARB_shader_draw_parameters.
    Essentially, the value of gl_VertexID should be the index of the
    vertex that is being passed to the shader.
     - DrawArrays: first + i
     - DrawElements: indices[i]
     - DrawElementsBaseVertex: indices[i] + basevertex


Revision History

    Rev.    Date    Author     Changes
    ----  -------- ---------  ----------------------------------------------
     4    08/11/20  pdaniell  Renamed the 'primcount' parameter of
                              MultiDrawElementsBaseVertexEXT in the
                              "New Procedures and Functions" section to
                              'drawcount' to match the extension spec body.
                              
     3    09/30/14  dkoch     Resolved issues 1, 2 as proposed.
                              Added issue 5. Mark complete.

     2    06/24/14  dkoch     Add EXT_multi_draw_arrays interactions
                              and typographical fixes from Dominik.

     1    04/05/14  dkoch     Intial version for ES based on
                              ARB_draw_elements_base_vertex v3.