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SPPM implementation is not symmetric #209
I noticed that PBRT's current implementation of SPPM does not account for the symmetric issue caused by the use of shading normals. According to Veach's paper "Non-symmetric Scattering in Light Transport Algorithms", a weighting factor needs to be used in order to correct this. Is the lack of such weighting factor intentional?
indeed, this issue is neglected in the current implementation. The problem with the weighting factor is that it leads to extremely high variance in the photon weights (with a low probability, you end up dividing by a very small number). Depending on the scene, this can make the method unusable, hence the omission.