Shadow Catcher bug with transparency and Branched Path Tracing #52021
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Reference: blender/blender#52021
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System Information
Ubuntu Linux 16.04, CPU rendering
Blender Version
Broken: (example: 2.78.5
ba256b32ee
)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:
Changed status to: 'Open'
Added subscriber: @niemeyer
Setup for the example above:
shadow-catcher-bug.blend
Added subscriber: @SteffenD
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.
This issue was referenced by blender/cycles@a3f6911e07
This issue was referenced by
5f35682f3a
Changed status from 'Open' to: 'Resolved'
Thank you!
Is this supposed to be fixed in the latest builds? I'm still seeing artifacts in the nightly of 2.79 (more specifically, 2.78.5
1bd9531fda
).I've reproduced by just putting a fully transparent object in front of a shadow catcher plane, and traces of the shadow remain on it.
I can try a new reproducer if that's useful.
Added subscriber: @brecht
The .blend file attached to this report seems to render correctly for me. If there's a different case that renders wrong, please do attach a .blend for that.
With that same example .blend file, just turn the factor all the way down to zero. The shadow (or AO, more likely) remain:
Here is a more clear example showing the issue and including the more noisy artifacts, in white background so it's easier to spot.
Rendered with 0.3 opacity it looks plausible (2.78.5
1bd9531fda
):Going all the way down to zero shows the issue:
And this composites the phantom shadow and the extra noise on top of the prior scene:
Here is the blend file for the scene:
shadow-catcher-bug-2.blend
@niemeyer, thanks for the detailed files. I am currently at siggraph, so really limited on time for bugfixing. But definitely looking into figuring this out.
What you might want to do is to copy the details to a new report dedicated to this particular problem. This way it'll make it easier to reference the issue from code and track changes back in the future when needed.
Here is a potential fix:
P518: Potential fix for transparency and shadow catcher
There are still some artifacts with less trivial topology than a box and AO-only lit scenes. Will need to have a closer look to see if that's something easily to solve.
P.S. If you don't mind, it's still handy to have separate report for this particular issue.
@Sergey Thanks again for your attention on this.
I've create a more appropriate task as requested: #52229