Principled shader clearcoat roughness inconsistent change from 0.0177 to 0.0178. #53038

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James Russell · 2017-10-09T20:03:50+02:00

James Russell commented

2017-10-09 20:03:50 +02:00

CPU: AMD 860K
RAM: 8GB
GFX: nVidia 960 GTX, 4 GB
OS: Xubuntu 16.04
Drivers: 375.82 from nvidia-375 (open source), installed from "Additional Drivers" section of settings window.
Blender: 2.79 2017-09-11 10:43, Hash = 5bd8ac9abf

On the left, the plane has a principled shader. It only has a clear coat on it. No SSS, diffuse is RGB 0,0,0, no sheen, and no normal/bump mapping. The "Clearcoat Roughness" setting is at 0.0177 (and of course displays it at 0.018 unless you click on it).

However, the right plane is exactly the same, except the "Clearcoat Roughness" setting is at 0.0178, 0.0001 more. As you can see in the above picture, it's completely different, not just slightly different.

Here is the blend file:
cleacoat_rough_bug.blend

Any questions, I'll respond as quickly as I can.

CPU: AMD 860K RAM: 8GB GFX: nVidia 960 GTX, 4 GB OS: Xubuntu 16.04 Drivers: 375.82 from nvidia-375 (open source), installed from "Additional Drivers" section of settings window. Blender: 2.79 2017-09-11 10:43, Hash = 5bd8ac9abfa On the left, the plane has a principled shader. It only has a clear coat on it. No SSS, diffuse is RGB 0,0,0, no sheen, and no normal/bump mapping. The "Clearcoat Roughness" setting is at 0.0177 (and of course displays it at 0.018 unless you click on it). ![clearcoat_rough_bug.png](https://archive.blender.org/developer/F993096/clearcoat_rough_bug.png) However, the right plane is exactly the same, except the "Clearcoat Roughness" setting is at 0.0178, 0.0001 more. As you can see in the above picture, it's completely different, not just slightly different. Here is the blend file: [cleacoat_rough_bug.blend](https://archive.blender.org/developer/F993108/cleacoat_rough_bug.blend) Any questions, I'll respond as quickly as I can.

James Russell commented

2017-10-09 20:03:50 +02:00

Changed status to: 'Open'

James Russell commented

2017-10-09 20:03:50 +02:00

Added subscriber: @JamesRussell

Brecht Van Lommel self-assigned this 2017-10-09 21:15:37 +02:00

Brecht Van Lommel commented

2017-10-18 23:32:45 +02:00

Added subscriber: @PascalSchon

Brecht Van Lommel commented

2017-10-18 23:32:45 +02:00

@PascalSchon, the root of the issue here seems tricky to solve.

The clearcoat BSDF importance samples a microfacet normal for the GGX distribution, but computes the pdf as if it sampled from the GTR1 distribution. This then breaks multiple importance sampling, and just generally means our clearcoat results are quite broken.

There's a few possible solutions:

Implement importance sampling of the GTR1 distribution. I don't think there's a known method that is nearly as good as GGX visible normals sampling however.
Keep doing GGX importance sampling, and correct the pdf to match. This mismatch will result in too much noise.
Switch to GGX for the clearcoat.

I think both 1) and 2) will increase noise too much. I'm tempted to go with 3), it's not actually clear to me that Unity / Unreal / Substance are even using GTR1 or that the subtle difference is worth it.

@PascalSchon, the root of the issue here seems tricky to solve. The clearcoat BSDF importance samples a microfacet normal for the GGX distribution, but computes the pdf as if it sampled from the GTR1 distribution. This then breaks multiple importance sampling, and just generally means our clearcoat results are quite broken. There's a few possible solutions: 1) Implement importance sampling of the GTR1 distribution. I don't think there's a known method that is nearly as good as GGX visible normals sampling however. 2) Keep doing GGX importance sampling, and correct the pdf to match. This mismatch will result in too much noise. 3) Switch to GGX for the clearcoat. I think both 1) and 2) will increase noise too much. I'm tempted to go with 3), it's not actually clear to me that Unity / Unreal / Substance are even using GTR1 or that the subtle difference is worth it.

Pascal Schön commented

2017-10-20 07:07:26 +02:00

@brecht, yes, I see this like you do. I don't think that we get a real benefit out of the GTR1 distribution. The difference (especially because clearcoat is only applied with a pre-factor of 0.25) should be pretty low and can thus be neglected. For me it seems ok to switch to GGX, but I will at first try to compare both versions today before adding a patch.

Pascal Schön commented

2017-10-20 08:31:27 +02:00

Maybe also the fixed roughness of 0.25 for the G term can cause some issues. Disney didn't really say in their paper from 2012 what the purpose of this hack was, but only that it's "found to be plausible and artistically pleasing". I will try at first to change only the D term to GGX without changing the G term (which means keeping the 0.25 roughness here) and look if this causes more problems and if so, I think we should go with having a consistent roughness throughout the whole clearcoat calculation. By doing so, we wouldn't need to have a distinction between clearcoat and non-clearcoat in the Microfacet BSDF evaluation (except for the sample weight) anymore.

Brecht Van Lommel commented

2017-10-20 14:05:54 +02:00

Right, the fixed 0.25 roughness is definitely a hack, hopefully we can get rid of that too.

Brecht Van Lommel was unassigned by Dalai Felinto

2019-12-23 16:36:27 +01:00

Dalai Felinto commented

2019-12-23 16:36:27 +01:00

Added subscriber: @brecht

Tomás Cejas commented

2022-10-27 00:34:04 +02:00

Added subscriber: @TomasCJ

Tomás Cejas commented

2022-10-27 00:34:04 +02:00

The issue still exists in version 3.3. It's a shame because it impedes glossy clearcoats, as the 0.0178 rougness is quite high. The issue can be "hacked" by reducing the clearcoat value itself, getting less roughness, but then it messes up the clearcoat normal.

Philipp Oeser removed the

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Principled shader clearcoat roughness inconsistent change from 0.0177 to 0.0178. #53038