Index of refraction < 1 renders wrong in cycles #38439
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Reference: blender/blender#38439
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System Information
Windows 7, Nvidia gtx550Ti
Blender Version
Broken: Cycles Render, 2.69
fd0b104
Worked: Internal Render, 2.69
fd0b104
Short description of error
In cycles, the glass shader with IOR < 1 does not render correctly. It behaves like IOR = 1. It should behave according to Snell's law http://en.wikipedia.org/wiki/Snell's_law and bend the at surfaces, no matter from which direction the light is coming.
In Blender Internal renderer, the behaviour is correct.
IOR < 1 is necessary for example to render underwater air bubbles. Then the IOR of the air bubble is 0.75 (the inverse of the water IOR).
Exact steps for others to reproduce the error
See attached file and screenshots.
Note: The drag and drop thing in the new bug tracker does not work for me, it asks me how I want to open it. Opera 12.10. I'm uploading it elsewhere. Would be nice if the old standard attach and browse button would come back (optionally).
http:///www.ania.xibo.at/tmp/ior.jpg
http://www.ania.xibo.at/tmp/cycles_ior.blend
http://www.ania.xibo.at/tmp/internal_ior.blend
Changed status to: 'Open'
Added subscriber: @AnnaCelarek
http://www.ania.xibo.at/tmp/ior.jpg
Added subscribers: @brecht, @ThomasDinges
Ania: You can submit the report without files, and then upload them in the first comment (Choose "Upload File" in the Action menu.
Developer Note:
@brecht: We clamp the IOR to "1.0f + 1e-5f" in svm_closure.h. We could change it to "0.0f + 1e-5f"? This way we would still avoid a division by zero later. The rendered result then looks similar to BI.
Added subscriber: @DriesvanOosten
Would it be an idea to just avoid zero? A negative IOR is interesting to render metamaterials.
Added subscriber: @Sergey
You can just flip the normals on the object to get the same result.
I'm fine with changing the limit to 0.0 + 1e-5f though. Negative IOR I'm not sure about, I don't think our fresnel formulas work with that.
This issue was referenced by blender/blender-addons-contrib@eff3bd4e98
This issue was referenced by
eff3bd4e98
Changed status from 'Open' to: 'Resolved'
Closed by commit
eff3bd4e98
.Flipping the normals could cause other unexpected results (normal maps, backfaces etc.).
Thanks for changing!