glMultiDrawElementsIndirect

glMultiDrawElementsIndirect: man4/glMultiDrawElementsIndirect.xml

glMultiDrawElementsIndirect specifies multiple indexed geometric primitives with very few subroutine calls. glMultiDrawElementsIndirect behaves similarly to a multitude of calls to glDrawElementsInstancedBaseVertexBaseInstance, execpt that the parameters to glDrawElementsInstancedBaseVertexBaseInstance are stored in an array in memory at the address given by indirect, separated by the stride, in basic machine units, specified by stride. If stride is zero, then the array is assumed to be tightly packed in memory. The parameters addressed by indirect are packed into a structure that takes the form (in C):

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

A single call to glMultiDrawElementsIndirect is equivalent, assuming no errors are generated to:

GLsizei n;
for (n = 0; n < drawcount; n++) {
    const DrawElementsIndirectCommand *cmd;
    if (stride != 0) {
        cmd = (const DrawElementsIndirectCommand  *)((uintptr)indirect + n * stride);
    } else {
        cmd = (const DrawElementsIndirectCommand  *)indirect + n;
    }

    glDrawElementsInstancedBaseVertexBaseInstance(mode,
                                                  cmd->count,
                                                  type,
                                                  cmd->firstIndex + size-of-type,
                                                  cmd->instanceCount,
                                                  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.

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.