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SPPM implementation is not symmetric #209

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TzuChieh opened this Issue Nov 11, 2018 · 2 comments

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TzuChieh commented Nov 11, 2018

Hi everyone,

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?

@TzuChieh TzuChieh changed the title SPPM implementation is not symmetric? SPPM implementation is not symmetric Nov 11, 2018

@wjakob

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wjakob commented Nov 11, 2018

Hi @TzuChieh,

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.

Best,
Wenzel

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TzuChieh commented Nov 13, 2018

@wjakob Thanks for clarifying the problem! I just found out that Mitsuba source code has some relevant comment on this issue, and the weighting factor is not applied too.

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