Does OpenGL guarantee that primitives in a vertex buffer will be drawn in order?

Question

I've been reading through the openGL specification trying to find an answer to this question, without luck. I'm trying to figure out if OpenGL guarantees that draw calls such as GLDrawElements or GLDrawArrays will draw elements in precisely the order they appear in the VBO, or if it is free to process the fragments of those primitives in any order.

For example, if I have a vertex buffer with 30 vertices representing 10 triangles, each with the same coordinates. Will it always be the case that the triangle corresponding to vertices 0, 1 and 2 will be rendered first (and therefore on the bottom); and the triangle corresponding to vertices 28, 29, 30 always be rendered last (and therefore on top)?

Answer 1

The specification is very careful to define an order for the rendering of everything. Arrays of vertex data are processed in order, which results in the generation of primitives in a specific order. Each primitive is said to be rasterized in order, and later primitives cannot be rasterized until prior ones have finished.

Of course, this is all how OpenGL says it should behave. Implementations can (and do) cheat by rasterizing and processing multiple primitives at once. However, they will still obey the "as if" rule. So they cheat internally, but will still write the results as if it had all executed sequentially.

So yes, there's a specific order you can rely upon. Unless you're using shaders that perform incoherent memory accesses; then all bets are off for shader writes.