Multiscatter GGX passed through Mix Shader introduces rendering artifacts

Glossy, Anisotropic & Glass BSDFs… all show same bug using Multiscatter GGX.

Path Tracing:

  • shows up occasionally within Interactive Preview only (after switching GPU <> CPU), more often with OSL

Branched PT:

  • rendering artifacts are always visible, even in final render
  • observed also with Material consisting of nothing more but Glossy mGGX + OSL script
  • if only a single shader (no OSL) is passed through Mix the artifacts are often less obvious than mixing with another shader using mGGX.

I wonder… Looks as if buckets which exceed some limit (?) get darkened/black… similar to what’s happening with denoise branch in testing on Windows.

Would be helpful if anyone can confirm…

Tests were performed on win7 with latest build (Hash: f23fecf) from buildbot, altho i already experienced such failures with previous builds.

HDRI is Overcast Breezeway, Soda Hall from
Scene / Test file: mGGX_bug.blend (
OSL script: source post

Hi, and thanks for the test file!
It seems you had perfect timing with posting this, I just committed two fixes for another issue before seeing this post, and it seems that they solve this problem as well - at least the black tiles with Branched PT and Glass are gone now (I could reproduce them with yesterday’s master, but not with the current master).
So, please try again with tomorrow’s buildbot version - if there still are errors, please tell me (or just open a bug report on

Will do, thanks.

Tested with today’s windows builds Hash: eececb0 (Official & Experimental VS 2015)
Problem solved!
Thanks again.

Result with Branched PT: