Render queue

Issuing draw calls in whatever order objects happen to sit in the scene is almost always wrong. A render queue collects the calls and sorts them by a packed key: render pass and layer first, then opaque geometry front-to-back so early depth testing discards hidden pixels, transparent geometry back-to-front so alpha blending composites correctly, and finally by material, shader and texture to minimise expensive state changes. Christer Ericson's 2008 write-up on sort keys is the canonical description. The result is less overdraw and fewer pipeline switches — the same image with far fewer calls into the driver.