Incorrect rendering in cycles with skin modifier #81298
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Reference: blender/blender#81298
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System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: GeForce RTX 2080 SUPER/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 456.38
Blender Version
Broken: version: 2.91.0 Alpha, branch: master, commit date: 2020-09-29 00:11, hash:
b150071ece
Worked: The last one I tried (2.90.1) also had this issue
Short description of error
Object with skin modifier isn't getting rendered correctly in cycles, both with CPU and GPU. Eevee OK.
Exact steps for others to reproduce the error
Open blender and apply skin modifier on the default cube. Render in cycles with CPU, you will wrong render. Same problem with GPU (I tried OptiX). Now try with NR (I tried OptiX), you again get a wrong render, except with different errors. See the attached files for my results.
skin.blend
Added subscriber: @Sekhar
Added subscriber: @mano-wii
There’s a difference between the way ngon normals are handled in cycles compared to Eevee or Workbench.
I don't know if this limitation is described anywhere, but it has always been that way.
I don't think it's a bug.
Added subscriber: @jeacom
I suspect adding a triangulate modifier should fix this discrepancy.
Added subscribers: @Blendify, @iss
As @mano-wii said, the issue here is n-gons - the face is non-planar and it is triangulated so it can be rendered "realistically".
See https://blenderartists.org/t/modeling-rendering-issue/604780 for example
I don't know if we can fit this information nicely into documentation. @Blendify what do you think?
Closed as duplicate of #48937
Added subscriber: @brecht
This is a bug in the skin modifier, it randomly gives different outputs unrelated to the specific device or tesselation that happens after.