softgpu: Correct accuracy of fog calculation #16005
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Its UV checks already should have generally, but let's be safe. It doesn't validate state like fog, etc.
This matches values from a PSP exactly, with the help of immediate mode vertex values (since this directly allows specifying the fog factor without any floating point math.)
Better this than static. May be easier to handle imm prims correctly.
hrydgard
approved these changes
Sep 11, 2022
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Having figured out how to directly specify the 8-bit fog factor in #15978, I was able to directly verify the fog blending equation on hardware (without Z interpolation and floating point handling questions getting in the way.)
It's the same as the BLEND tex func, i.e. divide by 256 but round up. No halves, no dividing by 255, simple.
I had to do some ugly hacking to get the fog values from imm to the actual verts (not included here, and only working for some prims.) This matches a test of many values/factors exactly,. bit-for-bit. The test included every possible source color + every possible factor + a couple fog colors chosen to catch rounding problems.
-[Unknown]