Is drawing front-to-back necessary for optimizing renders?

Question

I've seen the occasional article suggest ordering your vertices from nearest to furthest from the camera when sending them to OpenGL (for any of the OpenGL variants). The reason suggested by this is that OpenGL will not fully process/render a vertex if it is behind another vertex already rendered.

Since ordering vertices by depth is a costly component of any project, as typically this ordering frequently changes, how common or necessary is such design?

I had previously thought that OpenGL would "look" at all the vertices submitted and process its own depth buffering on them, regardless of their order, before rendering the entire batch. But if in fact a vertex gets rendered to the screen before another, then I can see how ordering might benefit performance.

Is drawing front-to-back necessary for optimizing renders?

Answer 1

Once a primitive is rasterized, its z value can be used to do an "early z kill", which skips running the fragment shader. That's the main reason to render front-to-back. Tip: When you have transparent (alpha textured) polygons, you must render back-to-front.

The OpenGL spec defines a state machine and does not specify in what order the rendering actually happens, only that the results should be correct (within certain tolerances).

Edit for clarity: What I'm trying to say above is that the hardware can do whatever it wants, as long as the primitives appear to have been processed in order

However, most GPUs are streaming processors and their OpenGL drivers do not "batch up" geometry, except perhaps for performance reasons (minimum DMA size, etc). If you feed in polygon A followed by polygon B, then they are fed into the pipeline one after the other and are processed independently (for the most part) of each other. If there are a sufficient number of polys between A and B, then there's a good chance A completes before B, and if B was behind A, its fragments will be discarded via "early z kill".

Edit for clarity: What I'm trying to say above is that since hw does not "batch up" geometry, it cannot do the front-to-back ordering automatically.