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Cycles Glass Black artifacts
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Kubuntu x64 14.10

62b13c6 (From Git)
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Hi Folks!
I'm working on a project for medical instruments.
I found a problem with refraction materials. I don't know exactly if this worked fine in some version.

The problem:
The refraction shaders (glass and refraction+glossy) have black artifacts in some faces. This problem becomes worse when roughness appear, because this is expanded.
The black artifacts give less credibility to the realism, it's a huge problem.
I tried with more light paths bounces... but without success.
I solve this with image edition (krita), it's a patch, but is not the best solution.
Other alternative solution is set the subsurf to high values!.

In the image example you can see model, a simple cube with 2 extrude and bevel.
Environment white, for reduce the error percentage.
High values to light paths. Integrator in Branched.
The model haven't subsurface.
I leave 2 settings ready, glass and refraction + glossy.


Thanks a bunch!



Event Timeline

Eugenio Pignataro (oscurart) created this task.
Eugenio Pignataro (oscurart) raised the priority of this task from to Needs Triage by Developer.
Thomas Dinges (dingto) triaged this task as Normal priority.

This is a typical issue with smooth normals when they don't match the actual geometry sufficiently. If you use flat normals, or add a subsurf modifier to get a smaller difference between the actual geometry and the normals then the problem goes away. It's another variation of the terminator problem with no easy solution.

In the past we haven't considered this a bug, as all raytracers suffer from this one way or another, and it's more something to research once, not something that can just be fixed.

I'm familiar with this, and don't much like the particular algorithm since it doesn't handle bump mapping, diffuse or any roughness > 0. Not that there are ideal solutions, but this is quite complex to only solve a special case.

Sergey Sharybin (sergey) closed this task as Archived.May 11 2015, 1:08 PM
Sergey Sharybin (sergey) claimed this task.

There are number of possible ways improving behavior in this case, but wouldn't really consider this a bug still. Improvements are happening outside of the bug tracker.

So thanks for the report, but archiving it now.