OS X 10.11, Nvidia GT 750M
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.