Draw front-to-back

[kkaefer]

Draw layers front to back so that we can use the depth buffer to cull completely opaque fragments.

[ansis]

Its probably worth also trying to sort by state instead of just front-to-back as described in these slides. Which is faster depends on where the bottleneck is.

I'm seeing potential conflicts with out of order rendering and current approaches for anti-aliasing and degenerate triangles:

For example, two lines interesect. If the top line is drawn first, its antialiased edges will also be drawn and written to the depth buffer. The underlying line won't be drawn to these pixels, causing a crack. It would be fairly easy to fix. Just draw anti-aliasing separately in a later call. This could have performance implications for line rendering.

Right now degenerate triangles work by increasing the z value of the degenerate vertex to something > 1.0, so that all fragments will have interpolated z values > 1.0 (drawn features always have a z of 1.0). If drawn features have a z < 1.0, then parts of degenerate triangles could still be drawn. This could be worked around by keeping z values as close to 1.0 as possible (determined by depth buffer precision) and increasing the value for degenerate vertices.