Created attachment 388 [details]
Sample map to test

Odamex uses ZDoom 1.22's code for texture-mapping non-sloped visplanes (floors and ceilings).  While being very fast, it suffers from accuracy problems.  This can be noticed when two adjoining sectors share the same floor texture and one of the sectors has a flickering light effect (see attached test map).  The floor texture will appear to move as the light flickers.

As a quick test, I had Odamex use the sloped visplane texture-mapping functions for all visplanes.  These functions are much more precise and fixed the issued with the test map.  The sloped visplane functions are too slow for general use but can probably serve as an example for making accuracy improvements to our non-sloped visplane functions.