third_party/angle/extensions/ANGLE_base_vertex_base_instance.txt

Name

    ANGLE_base_vertex_base_instance

Name Strings

    GL_ANGLE_base_vertex_base_instance

Contributors

    Shrek Shao, Google Inc.
    Contributors to the OES_draw_elements_base_vertex specification
    Contributors to the EXT_draw_elements_base_vertex specification
    Contributors to the EXT_multi_draw_arrays specification
    Contributors to the ARB_shader_draw_parameters specification

Contact

    Shrek Shao (shrekshao 'at' google.com)

Status

    Incomplete

Version

    Last Modified Date: August 13, 2019
    Author Revision: 1

Number

    OpenGL ES Extension XX

Dependencies

    OpenGL ES 3.1 is required.

    This extension is written against the OpenGL ES 3.1 specification, the
    OpenGL ES 3.2 specification, and the OpenGL ES Shading Language 3.0
    specification.

    GL_ANGLE_multi_draw is required.

Overview

    This extension exposes the *BaseVertex* draw call in
    OES_draw_elements_base_vertex/EXT_draw_elements_base_vertex together with
    their newly added *BaseInstance and MultiDraw* variants in addition to the
    vertex shader builtins <gl_BaseVertex> and <gl_BaseInstance> exposed by
    ARB_shader_draw_parameters for OpenGL.

    *BaseInstance behaves identically to its counterpart draw calls except that
    <instanceCount> instances of the range of elements are executed and the
    value of <instance> advances for each iteration. Those attributes that have
    non-zero values for <divisor>, as specified by VertexAttribDivisor, advance
    once per <divisor> instances of the set(s) of vertices being rendered.
    Additionally, <baseInstance> specifies the first element within the
    instanced vertex attributes.

    *BaseVertex* is equivalent to its counterpart draw calls except that
    the value of the base vertex passed into the driver is <baseVertex> instead
    of zero, and that <instances> of the set of elements are executed and the
    value of <instance> advances between each set.

    Additionally, <gl_BaseVertex> and <gl_BaseInstance> builtins are added
    to the shading language.
    For any *BaseVertex* draw call variant, the <baseVertex> of the draw may be
    read by the vertex shader as <gl_BaseVertex>. For non *BaseVertex* calls,
    the value of <gl_BaseVertex> is 0.
    For any *BaseInstance draw call variant, the baseInstance of the draw may
    be read by the vertex shader as <gl_BaseInstance>. For non *BaseInstance
    calls, the value of <gl_BaseInstance> is 0.

IP Status

    No known IP claims.

New Procedures and Functions

    void DrawArraysInstancedBaseInstanceANGLE(enum mode,
                                              GLint first,
                                              GLsizei count,
                                              GLsizei instanceCount,
                                              GLuint baseInstance);

    void DrawElementsInstancedBaseVertexBaseInstanceANGLE(GLenum mode,
                                                        GLsizei count,
                                                        GLenum type,
                                                        const GLvoid *indices,
                                                        GLsizei instanceCount,
                                                        GLint baseVertex,
                                                        GLuint baseInstance);

    void MultiDrawArraysInstancedBaseInstanceANGLE(enum mode,
                                                const GLint* firsts,
                                                const GLsizei* counts,
                                                const GLsizei* instanceCounts,
                                                const GLint* baseInstances,
                                                const GLsizei drawcount);

    void MultiDrawElementsInstancedBaseVertexBaseInstanceANGLE(enum mode,
                                                const GLint* counts,
                                                GLenum type,
                                                const GLvoid* const* indices,
                                                const GLsizei* instanceCounts,
                                                const GLint* baseVertices,
                                                const GLint* baseInstances,
                                                const GLsizei drawcount);

New Tokens

    None.

Additions to Chapter 10 of the OpenGL ES 3.2 Specification

    Section 10.5 Drawing Commands Using Vertex Arrays:

    The command

    void DrawArraysInstancedBaseInstanceANGLE (GLenum mode,
        GLint first,
        GLsizei count,
        GLsizei instanceCount,
        GLuint baseInstance);

    Behaves identically to DrawArraysInstanced except that <baseInstance> is
    passed down to DrawArraysOneInstance instead of zero.

    The base index of the instance may be read by a vertex shader as
    <gl_BaseInstance>.

    The command

    void MultiDrawArraysInstancedBaseInstanceANGLE(GLenum mode,
        const GLint* firsts,
        const GLsizei* counts,
        const GLsizei* instanceCounts,
        const GLuint* baseInstances,
        GLsizei drawcount);

    Behaves identically to DrawArraysInstancedBaseInstanceANGLE except that a
    list of arrays is specified instead. The number of lists is specified in
    the <drawcount> parameter. It has the same effect as:

      for(i=0; i<drawcount; i++) {
        if (*(counts+i)>0) DrawArraysInstancedBaseInstance(
                mode,
                *(counts+i),
                *(instanceCounts+i),
                *(firsts+i),
                *(baseInstance+i)
            );
      }

    The index of the draw (<i> in the above pseudo-code) may be read by
    a vertex shader as <gl_DrawID> which is defined in GL_ANGLE_multi_draw.

    The command

    void DrawElementsInstancedBaseVertexBaseInstanceANGLE (GLenum mode,
        GLsizei count,
        GLenum type,
        const GLvoid* indices,
        GLsizei instanceCount,
        GLint baseVertex,
        GLuint baseInstance);

    Behaves identically to DrawElementsInstanced except that <baseVertex> and
    <baseInstance> are passed down to DrawElementsOneInstance instead of zero.

    The base index of the vertex may be read by a vertex shader as
    <gl_BaseVertex>. This value is also added to the <gl_VertexID>.

    The base index of the instance may be read by a vertex shader as
    <gl_BaseInstance>. Note that this value is not added to the
    <gl_InstanceID>.

    The command

    void MultiDrawElementsInstancedBaseVertexBaseInstanceANGLE(GLenum mode,
        const GLsizei* counts,
        GLenum type,
        const GLvoid* const* indices,
        const GLsizei* instanceCounts,
        const GLint* baseVertices,
        const GLuint* baseInstances,
        GLsizei drawcount);

    Behaves identically to DrawElementsInstancedBaseVertexBaseInstanceANGLE
    except that a list of arrays is specified instead. The number of lists is
    specified in the <drawcount> parameter. It has the same effect as:

      for(i=0; i<drawcount; i++) {
        if (*(counts+i)>0) DrawElementsInstancedBaseVertexBaseInstanceANGLE(
                mode,
                *(counts+i),
                type,
                *(instanceCounts+i),
                *(indices+i),
                *(baseVertices+i),
                *(baseInstances+i)
            );
      }

    The index of the draw (<i> in the above pseudo-code) may be read by
    a vertex shader as <gl_DrawID> which is defined in GL_ANGLE_multi_draw.

Errors

    DrawArraysInstancedBaseInstanceANGLE and
    DrawElementsInstancedBaseVertexBaseInstanceANGLE
    generate the same errors as DrawArraysInstanced and DrawElementsInstanced.

    MultiDrawArraysInstancedBaseInstanceANGLE and
    MultiDrawElementsInstancedBaseVertexBaseInstanceANGL
    generate the same errors as DrawArraysInstanced and DrawElementsInstanced,
    respectively, for any draw <i> where an error is generated. If any call
    would produce an error, no drawing is performed.

    The error INVALID_VALUE is generated by the MultiDraw* functions if
    <drawcount> is less than zero.

Modifications to the OpenGL ES Shading Language Specification, Version 3.00

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

      #extension GL_ANGLE_base_vertex_base_instance : <behavior>

    where <behavior> is as specified in section 3.5.

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

      #define GL_ANGLE_base_vertex_base_instance 1

Dependencies on GL_OES_draw_elements_base_vertex and
GL_EXT_draw_elements_base_vertex

    If the OpenGL ES context version is less than 3.2, this extension is only
    available when GL_OES_draw_elements_base_vertex or
    GL_EXT_draw_elements_base_vertex are available.

Issues

    None

Revision History

    Rev.    Date    Author       Changes
    ----  --------  ----------   --------------------------------------------
    1     08/13/19   Shrek Shao   First revision.