Incorrect Fresnel layer weight shading on backfacing geometry #81108

New Issue

slowburn · 2020-09-23T23:08:59+02:00

slowburn commented

2020-09-23 23:08:59 +02:00

System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: Radeon (TM) RX 470 Graphics ATI Technologies Inc. 4.5.13587 Core Profile Context 20.4.2 26.20.15029.27017

Blender Version
Broken: version: 2.91.0 Alpha, branch: master, commit date: 2020-09-20 18:23, hash: e14894aad2

Short description of error
Fresnel layer weight creates a sharp boundary instead of a smooth falloff when viewed on backfacing geometry.
In this example there are 2 hemispheres with the same material, but the left one has inverted face normals.

fresnel bug.blend

**System Information** Operating system: Windows-10-10.0.18362-SP0 64 Bits Graphics card: Radeon (TM) RX 470 Graphics ATI Technologies Inc. 4.5.13587 Core Profile Context 20.4.2 26.20.15029.27017 **Blender Version** Broken: version: 2.91.0 Alpha, branch: master, commit date: 2020-09-20 18:23, hash: `e14894aad2` **Short description of error** Fresnel layer weight creates a sharp boundary instead of a smooth falloff when viewed on backfacing geometry. In this example there are 2 hemispheres with the same material, but the left one has inverted face normals. ![fresnel bug.jpg](https://archive.blender.org/developer/F8912952/fresnel_bug.jpg) [fresnel bug.blend](https://archive.blender.org/developer/F8912959/fresnel_bug.blend)

slowburn commented

2020-09-23 23:08:59 +02:00

Added subscriber: @slowburn

Alaska commented

2020-09-24 08:24:40 +02:00

Added subscriber: @Alaska

Alaska commented

2020-09-24 08:24:40 +02:00

I believe this is a known issue or intended behavior. @brecht acknowledged it here: https://devtalk.blender.org/t/diffuse-translucency-way-too-difficult-to-set-up-in-cycles/807/8

From what I can tell the Fresnel node is taking into consideration effects like Total internal reflection. Whether or not the Fresnel node is is just Fresnel or takes into account total internal reflections is based on the surface normals.

I will leave the marking of this task to someone else as I'm not sure what the approach to this one is.

I believe this is a known issue or intended behavior. @brecht acknowledged it here: https://devtalk.blender.org/t/diffuse-translucency-way-too-difficult-to-set-up-in-cycles/807/8 From what I can tell the Fresnel node is taking into consideration effects like `Total internal reflection`. Whether or not the Fresnel node is is just Fresnel or takes into account `total internal reflections` is based on the surface normals. I will leave the marking of this task to someone else as I'm not sure what the approach to this one is.

Alaska commented

2020-09-24 08:24:50 +02:00

Added subscriber: @brecht

slowburn commented

2020-09-24 12:15:39 +02:00

So it's for simulating natural effects, like Snell's window . Pretty cool, I didn't think of that.
And the "proper" workaround for back faces is to use Fresnel node and plug 1/[ior] into the IOR slot? I guess it could work, but there's almost no chance that new users could figure out how to use Fresnel falloff correctly without a full explanation. I've already seen it used wrong in a couple tutorials.

So it's for simulating natural effects, like [Snell's window ](https://en.wikipedia.org/wiki/Snell%27s_window). Pretty cool, I didn't think of that. And the "proper" workaround for back faces is to use Fresnel node and plug 1/[ior] into the IOR slot? I guess it could work, but there's almost no chance that new users could figure out how to use Fresnel falloff correctly without a full explanation. I've already seen it used wrong in a couple tutorials.

Philipp Oeser commented

2020-12-22 14:43:17 +01:00

Added subscriber: @lichtwerk

Philipp Oeser commented

2020-12-22 14:43:17 +01:00

Changed status from 'Needs Triage' to: 'Confirmed'

Philipp Oeser commented

2020-12-22 14:43:17 +01:00

From what I can tell, there should be some way to make this behave better / easier to set up.
It does not sound like a bug though, more like a TODO.
Not sure if this is covered already by https://wiki.blender.org/wiki/Source/Render/Cycles/Roadmap / D2354, will confirm though and let the Cycles module handle this.

From what I can tell, there should be some way to make this behave better / easier to set up. It does not sound like a bug though, more like a TODO. Not sure if this is covered already by https://wiki.blender.org/wiki/Source/Render/Cycles/Roadmap / [D2354](https://archive.blender.org/developer/D2354), will confirm though and let the Cycles module handle this.

Philipp Oeser removed the

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Incorrect Fresnel layer weight shading on backfacing geometry #81108