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Cycles Principled shader Fresnel glitches
Needs Triage, NormalPublic

Description

System Information
Consistent across 3 diffrent devices and operating systems.

Blender Version
Broken: v2.8 stable
still Broken: v2.83 Alpha, 9d90cad3ed39, 2020-01-17 00:13

Short description of error
Specific combinations of roughness and specular cause weird visual artefacts in cylces, that also look diffrent in contrast to eevee. It is possible for the edges of an object to look brighter or darker, than it's surroundings in a uniformly lit environment or an object appearing darker overall despite only roughness being changed.

Using all white background to better visiualize artefacts.
Cycles:


Eevee:

Exact steps for others to reproduce the error
Delete cube+light, set environment to be white color, add spheres, assign principled shader, try out diffrent spec+rough+metal values.
(Happens on both smooth and flat shaded objects...)

Here's a 2.80 file to try it out:

Event Timeline

Found by user Atair and discussed on devtalk, but no bug reports were filed due to negative response by community: https://devtalk.blender.org/t/cycles-principled-bsdf-violating-energy-conservation/10828/1

Btw: I've made a chart for better troubleshooting...

SpecularMetallicRougnessCyclesEevee
000dark fresnelevenly lit
000.5evenly litevenly lit
001bright fresnelevenly lit
100bright fresnelbright fresnel
100.5evenly litevenly lit
101bright fresnelevenly lit
010slightly darker, but even lightingslightly darker, but even lighting
00>0.05>0.50nothing out of ordinarywhen changing "metallic" object suddently gains a bright fresnel, that then disappears
011dark lighting with slightly brigher rimdark lighting with slightly brigher rim

There are differences between eevee and cycles in this white room test (and Idk ... I don't see, how both can be compliant with the paper at the same time, if they differ) and there appear to be at least some weird artefacts in my opinion.

Hope that helps ...

There is some expected difference between EEVEE and Cycles, the fresnel term is one of them. I don't know about cycles but for eevee I don't see anything wrong.

for eevee I don't see anything wrong,

Okay then I'd just like to point out some weird obversations, I've made when checking eevee:

  1. When specular is 1 and metallic is 0, changing the roughness will introduce a bright "fresnel-rimlight" at smaller roughness values.
  2. When specular and roughness are at 0, changing the metallic value from 0 to slightly abve that suddently introduces a bright fresnel highlight, that fades away as you increase the metallic value. Once you've reached it's max value at 1, that bright "rimlight" is completely gone and the objeckt is evenly lit.


Image is supposed to visualize the second mention, the sphere has a metal_value of 0 at the top and 1 at the bottom and you can see a weird rimlight appearing...