Branched Path Tracing converges to different result than plain Path Tracing
Closed, ResolvedPublic


System Information
OS X 10.11, Nvidia GT 750M

Blender Version
Broken: 2.76b, hash f337fea

Short description of error
In shaders that combine sharp specular with diffuse, when lit with an area light, the diffuse result in branched path tracing differs from the result with path tracing

**Exact steps for others to report

Open the attached file.
Make two renders with Cycles, one set to path tracing and one set to branched path tracing. Save the diffuse pass of each rendered image - the blend file should be set up to save combined, diffuse and glossy passes. Open the diffuse in an image editor and compare.

Expected result: With the exception of noise (variance), path tracing and branched path tracing should converge to the same result.

Actual result: In the diffuse pass of the branched path tracing render, there is a dark spot on Suzanne's forehead. It is in the location of the highlight in the glossy pass. This reproduces in both CUDA and CPU renders, as well as on Windows 10. Mesh lights or large spherical/point lights give similar results.

Regression: This happens only with multiple importance sampling. When multiple importance sampling is turned off on the shaders, branched path tracing converges to the same result as path tracing.

I believe there must be something wrong in how the branched path tracer uses MIS to combine samples. I have tried figuring out in the code, where exactly this happens, but so far I have not succeeded.

Thomas Dinges (dingto) triaged this task as "Confirmed" priority.EditedNov 13 2015, 6:00 PM

Confirmed, the difference only happens with MIS ensabled for the Mesh Light.

Edit: Seems it's not related to Sample All Lights as far as I can tell.

When sampling a BSDF for indirect rays in path tracing, then we all take into account how much other BSDFs contribute. That's in _shader_bsdf_multi_eval. The ray will be marked as having the ray type of the BSDF, but the throughput can contain contributions from other types of BSDFs too. Indirect light passes are affected by this, but also lamp MIS since that is computed as part of indirect lighting.

A solution would be to pass along separate throughputs for the different ray types. This adds quite some code complexity and has an impact on performance. Another would be to not use the contribution from other ray types, but that will increase noise in some cases. It might not be so bad in practice though, perhaps it's worth testing what happens when you modify _shader_bsdf_multi_eval to use only BSDFs with the same ray type.

I never made fixing this a priority because I don't think it's that bad in practice. With more advanced BSDF models the distinction between diffuse and glossy is difficult anyway, and the best you can get is diffuse-like and glossy-like results in these passes, which I think the current code satisfies.

Skipping some BSDF evaluations in _shader_bsdf_multi_eval will also be faster, if you sample e.g. a diffuse BSDF you then skip evaluating glossy BSDFs which can be quite costly. So perhaps it ends up being a net benefit, depends how much it increases noise.

Brecht, I'm not sure I'm following you. In my opinion, the path tracing integrator gives the correct result, where diffuse is uniform, the branched path tracer is incorrect and darkens diffuse in the presence of specular.

PT calls _shader_bsdf_multi_eval() twice, once when it takes a light sample and once when it takes a BSDF sample. The shading pdf used in both cases is derived from all BSDFs on a surface.

In the BPT case, _shader_bsdf_multi_eval() is called for light samples, but not for BSDF samples. I'm not sure if MIS gives us the correct result when we work with different pdf derivations for light sampling and BSDF sampling?

Ah, I assumed too quickly that it was this related to this, if it's the branched path tracing that's wrong then it must be something else.

You may be right that using _shader_bsdf_multi_eval() for light samples but not BSDF samples could be an issue, I haven't worked out the math. If the MIS weight to balance the BSDF and light samples was computed for each BSDF individually then I think it could work, but with all those different BSDF pdfs are accumulated into a single pdf which is the used to compute the MIS weight it's not so clear.

@Brecht Van Lommel (brecht), samping only BSDFs of the ray type in _shader_bsdf_multi_eval() didn't give any measurable difference actually. Also, if you re-combine light passes back into combined then you'll have exact the same result with both regular and branched path tracing. So overall light contribution is preserved, just separation doesn't match between two integrators.

Now, what do we consider a ground-truth here? It's kinda both approaches has own logic here..

The ground truth would be the path traced result without MIS, which matches PT with MIS and BPT without MIS.

I think found a fix for the BPT/MIS case, it appears to work correctly when you don't use the one-sample model for light samples, but instead call power_heuristic individually per closure. That way, both light samples and BSDF samples use the same method for calculating MIS.

I'll do some more testing to make sure, then I can hopefully submit a patch.

Here we go. This would be my proposed fix. I hope my code style isn't too far from your standards.

@Stefan Werner (swerner), Code would need some cleanup (some whitespace and naming) but that'd be easier to do from our side.

@Brecht Van Lommel (brecht), does it seems a correct thing to do for you as well?