Float->byte rendered image dithering uses triangle noise algorithm. Keep
the algorithm the same, just make some improvements and fix some issues:
1) The hash function for noise was using "trig" hash from "On generating
random numbers" (Rey 1998), but that is not a great quality hash, plus it
can produce very different results between CPUs/GPUs. Replace it with
"iqint3" (recommended by "Hash Functions for GPU Rendering", JCGT 2020),
which is same performance on GPU, faster on CPU, and much better quality.
This is the same hash as Cycles already uses elsewhere. Also it is purely
integer based, so exactly the same results on all platforms.
2) For the above point, replace `dither_random_value` to take integer
pixel coordinates and adjust calling code accordingly. Some previous
callers were (accidentally?) passing integer coordinates already. Other
places actually get a tiny bit simpler, since they now no longer need an
extra multiplication.
3) The CPU dithering path was wrongly introducing bias, i.e. making the
image lighter. The CPU path also needs dither noise to be in [-1..+1]
range (not [-0.5..+1.5]!) just like GPU path does, since the later
float->byte conversion already does rounding.
4) The CPU dithering path was using thread-slice-local Y coordinate,
meaning the dithering pattern was repeating vertically. The more CPU cores
you use, the worse the repetition.
5) Change the way that uniform noise is converted to triangle noise.
Previous implementation was based on one shadertoy from 2015, change it
to another shadertoy from 2020. The new one fixes issues with the old way,
and it just works on the CPU too, so now both CPU and GPU code paths are
exactly the same.
6) Cleanup: remove DitherContext, just a single float is enough
Performance and image comparisons in the PR.
Pull Request: https://projects.blender.org/blender/blender/pulls/135224
02869cc6c8