Multiscatter GGX produces large difference in refraction brightness compared to GGX #96606
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Reference: blender/blender#96606
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System Information
Operating system: Windows-10-10.0.19043-SP0 64 Bits
Graphics card: GeForce GTX 1080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 431.07
Blender Version
Broken: version: 3.0.0, branch: master, commit date: 2021-12-02 18:35, hash:
f1cca30557
Worked: (newest version of Blender that worked as expected)
Short description of error
When using the Principled BSDF (transparency=1) or the Glass BSDF with Multiscatter GGX the shader converges to a different result than GGX or Beckman. The difference is especially visible at Suzanne's right eye in the images below.
Exact steps for others to reproduce the error
#96606 - Multiscatter brightness difference.blend
Switch between the Multiscatter GGX and GGX or Beckmann to see the difference. GGX and Beckmann converge to the same result. The difference is especially visible at Suzanne's right eye.
Thank you and have a great day!
Added subscriber: @tempdevnova
Added subscriber: @Alaska
Changed status from 'Needs Triage' to: 'Confirmed'
I can confirm this difference.
System Information
Operating system: Linux-5.16.0-3-amd64-x86_64-with-glibc2.33 64 Bits
Graphics card: NVIDIA GeForce RTX 3060/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 510.54
Broken version: 3.2.0 Alpha, branch: master, commit date: 2022-03-17 21:23, hash:
6d97a5f93c
The Multiscatter GGX issues trilogy (Appendix (Glass BSDF/Principled BSDF (transparency=1) ))to Multiscatter GGX produces large difference in refraction brightness compared to GGXAdded subscriber: @brecht
@brecht are you able to look at this and confirm if this is a bug, or a limitation of Multiscatter GGX. If it is a bug, is it related to other reports? (E.G. #82500, #85259, #92520)
Added subscriber: @LukasStockner
I'm not sure what the problem is here - regular microfacet models are too dark at high roughnesses, especially for transmission, the entire point of Multiscattering GGX is to fix that. If it produced the same result, why even have the option at all?
Added subscriber: @moisessalvador
The point of Multiscatter is to preserve more energy and look brighter. Yes, it's supposed to be different
Well the problems are:
First:
In this case Multiscatter GGX is significantly
darker
than GGX.Secondly:
The roughness in this case low (0.148) , meaning that multiple scattering shouldn't have a great effect.
Thirdly:
At peak roughness regular GGX usually has an energy loss of up to 70%. However in this case GGX is up to 400 times (40000%) bighter than Multiscatter GGX.
Now taking #96631 into consideration the problem may also actually lie in regular GGX and Beckman, while Multiscatter GGX is actually correct.
The strong brightening in the case of regular GGX and Beckman does seem rather strange, but could also just be a result of total internal reflection
Taking #97962 into account I isolated the origin of the The strong brightening in eye in the case of regular GGX and Beckman. When ray depth is exactly 3 the brightness in the right eye suddenly skyrockets. Taking the geometry of suzanne into account this means that the ray has had 2 transmission and 1 glossy interactions. Again this could be the result of total internal reflection, which would further support my theory that this problem is lying with regular GGX and not Multiscatter GGX. This would then probably be caused by #96631, however when comparing the fresnel behaviour of Multiscatter GGX and GGX for internal reflection (divide 1 by IOR value) at the roughness value of 0.148 shows that the differnece isn't even that big...
This is solved in 4.0 with the updated Principled BSDF.