Shadow Catcher bug with transparency and Branched Path Tracing
Closed, ResolvedPublic

Description

System Information

Ubuntu Linux 16.04, CPU rendering

Blender Version
Broken: (example: 2.78.5 ba256b32ee5)

Short description of error

When a transparent or semi-transparent material is used over a Shadow Catcher plane, the plane gets rendered in front of the transparent object when using Branched Path Tracing sampling.

Exact steps for others to reproduce the error

Just add a transparent object in front of a plane with Shadow Catcher enabled and use Branched Path Tracing sampling.

For example, this is the expected render obtained with Path Tracing sampling:

This is the broken one obtained with Branched Path Tracing:

This is the material and sampling settings used with the broken plane:

Setup for the example above:

Sergey Sharybin (sergey) triaged this task as "Confirmed" priority.Tue, Jul 11, 9:36 AM
Sergey Sharybin (sergey) claimed this task.

Thanks for confirming the issue.

Is the fix for this likely to be in 2.79? The branched tracing greatly improves the rendering time in cases where a significant portion of the image is empty, which turns out to be a common case for the Shadow Catcher since it's integrating into existing footage.

Thanks again for looking into this.

The fix will be committed soon. Currently working on some unit tests to help testing shadow catcher in the future.

For the record, PT result was not correct either: if you render on a transparent film and alpha-over on an actual background you'll have artifacts. So both PT and BPT will be changed in render results.