glMultiDraw functions and gl_InstanceID

Question

When I look at the documentation of glMultiDrawElementsIndirect (or in the Wiki) it says that a single call to glMultiDrawElementsIndirect is equivalent to repeatedly calling glDrawElementsIndirect (just with different parameters).

Does that mean that gl_InstanceID will reset for each of these "internal" calls? And if so, how am I able to tell all these calls apart in my vertex shader?

Background: I'm trying to draw all my different meshes all at once. But I need some way to know to which mesh the vertex, I'm processing in my vertex shader, belongs.

Answer 1

The documentation says "similarly to". "Equivalent" isn't the same thing. It also points to glDrawElementsInstancedBaseVertexBaseInstance, not glDrawElementsInstanced.

But yes, gl_InstanceId for any draw will start at zero, no matter what base instance you provide. That's how gl_InstanceId works, unfortunately.

Besides, that's not the question you want answered. You're not looking to ask which instance you're rendering, since each draw in the multi-draw can be rendering multiple instances. You're asking which draw in the multi-draw you are in. An instance ID isn't going to help.

And if so, how am I able to tell all these calls apart in my vertex shader?

Unless you have OpenGL 4.6 or ARB_shader_draw_parameters, you can't. Well, not directly.

That is, multidraw operations are expected to produce different results based on rendering from different parts of the current buffer objects, not based on computations in the shader. You're rendering with a different base vertex that selects different vertices from the arrays, or you're using different ranges of indices or whatever.

The typical pre-shader_draw_parameters solution would have been to use a unique base instance on each of the individual draws. Of course, since gl_InstanceId doesn't track the base instance (as previously stated), you would need to employ instanced arrays instead. So you'd get the mesh index from that.

Of course, 4.6/shader_draw_parameters gives you gl_DrawId, which just tells you what the index is within the multidraw command. It's also dynamically uniform, so you can use it to access arrays of opaque types in shaders.