glMultiDrawArraysIndirect

glMultiDrawArraysIndirect: man4/glMultiDrawArraysIndirect.xml

glMultiDrawArraysIndirect specifies multiple geometric primitives with very few subroutine calls. glMultiDrawArraysIndirect behaves similarly to a multitude of calls to glDrawArraysInstancedBaseInstance, execept that the parameters to each call to glDrawArraysInstancedBaseInstance 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 an array of structures, each element of which takes the form (in C):

typedef  struct {
    uint  count;
    uint  instanceCount;
    uint  first;
    uint  baseInstance;
} DrawArraysIndirectCommand;

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

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

    glDrawArraysInstancedBaseInstance(mode, cmd->first, cmd->count, cmd->instanceCount, cmd->baseInstance);
}

If a buffer is bound to the GL_DRAW_INDIRECT_BUFFER binding at the time of a call to glMultiDrawArraysIndirect, 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. In contrast to glDrawArraysInstancedBaseInstance, the first member of the parameter structure is unsigned, and out-of-range indices do not generate an error. Vertex attributes that are modified by glMultiDrawArraysIndirect have an unspecified value after glMultiDrawArraysIndirect returns. Attributes that aren't modified remain well defined.

The baseInstance member of the DrawArraysIndirectCommand 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. glMultiDrawArraysIndirect is available only if the GL version is 4.3 or greater.