extensions/ARB/ARB_multi_draw_indirect.txt

Name

    ARB_multi_draw_indirect

Name Strings

    GL_ARB_multi_draw_indirect

Contact

    Graham Sellers, AMD (graham.sellers 'at' amd.com)

Contributors

    Graham Sellers

Notice

    Copyright (c) 2012-2013 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 2012/06/12.

Version

    Last Modified Date: June 9, 2014
    Revision: 4

Number

    ARB Extension #133

Dependencies

    OpenGL 4.0 or ARB_draw_indirect is required.

    The extension is written against the OpenGL 4.1 Specification, Core Profile,
    July 25, 2010

Overview

    The ARB_draw_indirect extension (included in OpenGL 4.0) introduced
    mechanisms whereby the parameters for a draw function may be provided in
    a structure contained in a buffer object rather than as parameters to the
    drawing procedure. This is known as an indirect draw and is exposed as two
    new functions, glDrawArraysIndirect and glDrawElementsIndirect. Each of
    these functions generates a single batch of primitives.

    This extension builds on this functionality by providing procedures to
    invoke multiple draws from a single procedure call. This allows large
    batches of drawing commands to be assembled in server memory (via a buffer
    object) which may then be dispatched through a single function call.

New Procedures and Functions

        void MultiDrawArraysIndirect(enum mode,
                                        const void *indirect,
                                        sizei primcount,
                                        sizei stride);

        void MultiDrawElementsIndirect(enum mode,
                                          enum type,
                                          const void *indirect,
                                          sizei primcount,
                                          sizei stride);

New Tokens

    None.

Additions to Chapter 2 of the OpenGL 4.1 (Core) Specification (OpenGL Operation)

    Additions to Section 2.8.3, "Drawing Commands"

    After the description of MultiDrawArrays and before the introduction of
    DrawElementsInstanced, insert the following on p.36:

        The command

        void MultiDrawArraysIndirect(enum mode,
                                        const void *indirect,
                                        sizei primcount,
                                        sizei stride);

    behaves identically to DrawArraysIndirect, except that <indirect> is
    treated as an array of <primcount> DrawArraysIndirectCommand structures.
    <indirect> contains the offset of the first element of the array within the
    buffer currently bound to the DRAW_INDIRECT buffer binding.<stride>
    specifies the distance, in basic machine units, between the elements of the
    array. If <stride> is zero, the array elements are treated as tightly
    packed. <stride> must be a multiple of four, otherwise an INVALID_VALUE
    error is generated.

    It has the same effect as (assuming no errors are generated):

        if (<mode> is invalid)
            generate appropriate error
        else {
            const ubyte * ptr = (const ubyte *)indirect;
            for (i = 0; i < primcount; i++) {
                DrawArraysIndirect(mode,
                                   (DrawArraysIndirectCommand*)ptr);
                if (stride == 0)
                {
                    ptr += sizeof(DrawArraysIndirectCommand);
                } else
                {
                    ptr += stride;
                }
            }
        }

    <primcount> must be positive, otherwise an INVALID_VALUE error will be
    generated.

    After the description of DrawElementsIndirect and before the introduction
    of MultiDrawElementsBaseVertex, insert the following on p.39:

        The command

        void MultiDrawElementsIndirect(enum mode,
                                       enum type,
                                       const void *indirect,
                                       sizei primcount,
                                       sizei stride);

    behaves identically to DrawElementsIndirect, except that <indirect> is
    treated as an array of <primcount> DrawElementsIndirectCommand structures.
    <indirect> contains the offset of the first element of the array within the
    buffer currently bound to the DRAW_INDIRECT buffer binding. <stride>
    specifies the distance, in basic machine units, between the elements of the
    array. If <stride> is zero, the array elements are treated as tightly
    packed. <stride> must be a multiple of four, otherwise an INVALID_VALUE
    error is generated.

    It has the same effect as (assuming no errors are generated):

        if (<mode> or <type> is invalid)
            generate appropriate error
        else {
            const ubyte * ptr = (const ubyte *)indirect;
            for (i = 0; i < primcount; i++) {
                DrawElementsIndirect(mode,
                                     type,
                                     (DrawElementsIndirectCommand*)ptr);
                if (stride == 0)
                {
                    ptr += sizeof(DrawElementsIndirectCommand);
                } else
                {
                    ptr += stride;
                }
            }
        }

    Modifications to Section 2.9.8 "Indirect Commands in Buffer Objects"

    Modify both instances of "DrawArraysIndirect and DrawElementsIndirect" on
    p.51 to read "DrawArraysIndirect, DrawElementsIndirect,
    MultiDrawArraysIndirect and MultiDrawElementsIndirect".

Additions to Chapter 3 of the OpenGL 4.1 (Core) Specification (Rasterization)

    None.

Additions to Chapter 4 of the OpenGL 4.1 (Core) Specification (Per-Fragment Operations
and the Framebuffer)

    None.

Additions to Chapter 5 of the OpenGL 4.1 (Core) Specification (Special
Functions)

    None.

Additions to Chapter 6 of the OpenGL 4.1 (Core) Specification (State and
State Requests)

    None.

Additions to the AGL/GLX/WGL Specifications

    None.

GLX Protocol

    None.

Errors

    MultiDrawArraysIndirect and MultiDrawElementsIndirect may generate the
    same errors as the corresponding DrawArraysIndirect and
    DrawElementsIndirect commands in the equivalent pseudocode. In addition,

    INVALID_VALUE is generated by MultiDrawArraysIndirect or
    MultiDrawElementsIndirect if <primcount> is negative.

    INVALID_VALUE is generated by MultiDrawArraysIndirect or
    MultiDrawElementsIndirect if <stride> is not a multipe of four.

New State

    None.

New Implementation Dependent State

    None.

Issues

    None, so far.

Revision History

    Rev.    Date      Author    Changes
    ----  --------    --------  -----------------------------------------
     4    06/09/2014  Jon Leech Note that errors can be generated from   
                                the non-Multi Draw*Indirect commands
                                in the equivalent pseudocode (Bug 11946).
     3    06/14/2012  Jon Leech Change to ARB extension & remove suffixes
     2    02/14/2011  gsellers  Add stride parameters
     1    01/06/2011  gsellers  Initial draft