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glDrawElementsIndirect

Name

glDrawElementsIndirect — 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_TRIANGLE_STRIP, GL_TRIANGLE_FAN, and GL_TRIANGLES, 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 glDrawElementsInstanced, except that the parameters to glDrawElementsInstanced are stored in memory at the address given by indirect and the ith 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 is 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.It also supports the addition of a value baseVertex to each index.

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

    typedef  struct {
        uint  count;
        uint  instanceCount;
        uint  firstIndex;
        int   baseVertex;
        uint  reservedMustBeZero;
    } DrawElementsIndirectCommand;

glDrawElementsIndirect is equivalent to:

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

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

glDrawElementsInstancedBaseVertex isn't actually present in OpenGL ES, but is used to describe this functionality.

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.

API Version Support

OpenGL ES API Version
Function Name 2.0 3.0 3.1
glDrawElementsIndirect - -
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