Tetrahedral is Best

Brendan Bolles <bre...@...>

I was experimenting with the ACES configuration the other day. I
applied the RRT to a grey ramp and saw some subtle color artifacts,
which could be verified by boosting the saturation. I tried applying
the rrt_ut33_sRGB.spi3d LUT directly and saw the same artifacts, until
I switched to tetrahedral interpolation which made them go away. Maybe
there's something to this tetrahedral business after all. ;)

So maybe should we switch ACES and other configurations to use
Tetrahedral when doing a 3D LUT?

Also, one of the interpolation types is "Best", which currently uses
Linear for both 1D and 3D LUTs. Seems to me the best interpolation for
a 3D LUT is Tetrahedral. Ben says it's actually faster than tri-linear
in his tests, although I don't think anyone's ever figured out how to
do it in the GPU.


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