Path guiding in volumes do not fully support equiangular sampling #102250
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Reference: blender/blender#102250
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At the moment, path guiding in volumes only initialize a guiding distribution when
the distance sampling decisions for direct and indirect are the same (i.e., no equiangular sampling is performed).
This can lead to a wrong pdf for NEE.
A way to fix this is to have two
VolumeSamplingDistriubtionswhich can be initialized:Added subscriber: @sherholz
Added subscriber: @brecht
Yes, I think you need two distributions. Either stored in the same pointer but re-initialized between direct and indirect light sampling, or separately, whatever is most convenient.
Changed status from 'Needs Triage' to: 'Confirmed'
I generated a DIFF to fix this issue.
One shortcoming of the solution is that we now have to query two
VolumeSamplingDistriubtions, which can cause an unnecessary overhead if the user enablesequiangularormisdistance sampling for direct light and the volume has a boundary that blocks the shadow rays (e.g., rough dielectric).That's a general problem, in such cases we ideally shouldn't even be doing direct light sampling either. But there may be a light inside the volume and it's difficult/expensive in general to determine that in advance. Not something I expect to be solved here.
This issue was referenced by
008cc625aaChanged status from 'Confirmed' to: 'Resolved'