This uses parallel reduction when doing the octahedral map re-mapping.
The goal is not the speedup but the accuracy of the computation (temporal
stability) and to pave the way for sunlight extraction.
This weight each individual samples using texel solid angle for correct
energy.
After optimization, the cost is not so expensive (1024px² octahedral map):
- new: 263µs remap + 12µs sum
- old: 75µs remap + 180µs irradiance update
We could optimize it more, but that feels unecessary given that the first
two filter pass are 7ms and a more pressing optimization.
The old irradiance update was fast because it was using the mip2 which
was already pre-filtered and using way less pixels (which already yield a
temporally stable output).
This new implementation does consider all pixel in the LOD0 which will
allow for more precise sunlight extraction.
This also comes with a cleanup of the update tagging.
Pull Request: https://projects.blender.org/blender/blender/pulls/119537
f646f4c2b4