man glDrawElementsIndirect (3): render indexed primitives from array data, taking parameters from memory

C SPECIFICATION

void glDrawElementsIndirect(GLenum mode, GLenum type, const void *indirect);

PARAMETERS

mode

: Specifies what kind of primitives to render. Symbolic constants GL_POINTS, GL_LINE_STRIP, GL_LINE_LOOP, GL_LINES, GL_LINE_STRIP_ADJACENCY, GL_LINES_ADJACENCY, GL_TRIANGLE_STRIP, GL_TRIANGLE_FAN, GL_TRIANGLES, GL_TRIANGLE_STRIP_ADJACENCY, GL_TRIANGLES_ADJACENCY, and GL_PATCHES are accepted.

type

: Specifies the type of data in the buffer bound to the GL_ELEMENT_ARRAY_BUFFER binding.

indirect

: Specifies the address of a structure containing the draw parameters.

DESCRIPTION

glDrawElementsIndirect specifies multiple indexed geometric primitives with very few subroutine calls. glDrawElementsIndirect behaves similarly to glDrawElementsInstancedBaseVertexBaseInstance(), execpt that the parameters to glDrawElementsInstancedBaseVertexBaseInstance() are stored in memory at the address given by indirect.

The parameters addressed by indirect are packed into a structure that takes the form (in C):

    typedef  struct {
        uint  count;
        uint  primCount;
        uint  firstIndex;
        uint  baseVertex;
        uint  baseInstance;
    } DrawElementsIndirectCommand;

    void glDrawElementsIndirect(GLenum mode, GLenum type, const void * indirect)
    {
        const DrawElementsIndirectCommand *cmd  = (const DrawElementsIndirectCommand *)indirect;
        glDrawElementsInstancedBaseVertexBaseInstance(mode,
                                                      cmd->count,
                                                      type,
                                                      cmd->firstIndex + size-of-type,
                                                      cmd->primCount,
                                                      cmd->baseVertex,
                                                      cmd->baseInstance);
    }

If a buffer is bound to the GL_DRAW_INDIRECT_BUFFER binding at the time of a call to glDrawElementsIndirect, indirect is interpreted as an offset, in basic machine units, into that buffer and the parameter data is read from the buffer rather than from client memory.

Note that indices stored in client memory are not supported. If no buffer is bound to the GL_ELEMENT_ARRAY_BUFFER binding, an error will be generated.

The results of the operation are undefined if the reservedMustBeZero member of the parameter structure is non-zero. However, no error is generated in this case.

Vertex attributes that are modified by glDrawElementsIndirect have an unspecified value after glDrawElementsIndirect returns. Attributes that aren't modified remain well defined.

NOTES

The baseInstance member of the DrawElementsIndirectCommand structure is defined only if the GL version is 4.2 or greater. For versions of the GL less than 4.2, this parameter is present but is reserved and should be set to zero. On earlier versions of the GL, behavior is undefined if it is non-zero.

ERRORS

GL_INVALID_ENUM is generated if mode is not an accepted value.

GL_INVALID_OPERATION is generated if no buffer is bound to the GL_ELEMENT_ARRAY_BUFFER binding, or if such a buffer's data store is currently mapped.

GL_INVALID_OPERATION is generated if a non-zero buffer object name is bound to an enabled array or to the GL_DRAW_INDIRECT_BUFFER binding and the buffer object's data store is currently mapped.

GL_INVALID_OPERATION is generated if a geometry shader is active and mode is incompatible with the input primitive type of the geometry shader in the currently installed program object.

GL_INVALID_OPERATION is generated if mode is GL_PATCHES and no tessellation control shader is active.

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