When using the Random Walk method for subsurface scattering, I get dark areas, where faces are touching. In the screenshot below you can see two cubes stacked on top of each other. The left upper cube sits straight on top of the lower one. In the right stack, I moved the upper cube upwards by 0.000001cm and the issue is gone. All the cubes are separate objects.
The problem can somewhat be solved in this simple example, but I often import STEP models and when I use plastic shaders that rely on subsurface scattering, I always get dark areas where faces touch. And I can’t really go through the whole model and separate faces. So I often end up using the Chistensen-Burley algorithm or try to come up with some Volume shader solution. It would be great though if Random Walk would be able to handle those cases better.
Has anybody got an idea if this issue can be addressed with the algorithms that are used now or is there some workaround for that? I would appreciate any feedback.
I used Blender 5.2 here using the new Random Walk algorithm. But the problem also appear with the Legacy and the Skin variants of the algorithm.
Thanks for the feedback. I’m aware that this is because of the coplanar faces. That’s why I moved the right top cube to show that a miniscule movement gets rid of the issue. However, when importing complex STEP models, adjusting faces is a nightmare and pretty much impossible. That’s why I was wondering if there was some solution for that - maybe some render setting that I have overlooked.
I found sort of a workaround. Adding a Smooth modifier and setting the Factor to a very low value like 0.00001 (which pretty much does nothing) seems to solve the issue. But of course it would be great if the shader could deal with that by itself.
I’m cautious about filing bug report for things that might not be considered bugs. I was rejected in the past when I filed something as a bug reports that might rather be seen as a feature request. Unfortunately there’s no place where feature requests can be brought before the developers anymore, like it has been in the past.
I also found the same issue right now. When I switch the device to None or CUDA, the issue is gone. When I use Optix, the problem appears. I’ll submit a bug report.
Oh, yes, that looks like one huge bug that definitely needs to be submitted in that case. I was very wrong to dismiss this like that before apparently.
Well.. when OptiX produce a problem but Cuda does not.. and maybe “simple” CPU render also does not.. then the problem lies somewhere in OptiX and is not “a bug” which can be solved by the blender foundation/studio/developers. ( Maybe it’s about the precision used in OptiX ? IDK..)
Also overlapping faces are always a problem when rendereing. I do not use CAD software and it may be no problem in there and maybe there is some possibility for detecting overlapping surfaces and produce “better” STEP outputs ?? (Or are there some “cleaning”/ optimize tools for this ?)
There are often (in general) problems when transfereing data from one domain to another. Here from CAD to polygonal modeler and renderer and one can hear, read, watch a lot about those specific problem (and i will not start to talk about color management between different apps.. or the “correct” workflow for this used in which domain well.. part of ).
You could report a bug as well.
well.. part of 
).