Principled BSDF: Sheen uses wrong normals #112948

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Alaska · 2023-09-27T11:17:13+02:00

Alaska commented

2023-09-27 11:17:13 +02:00

System Information
Operating system: Windows-10-10.0.22621-SP0 64 Bits
Graphics card: NVIDIA GeForce RTX 4090/PCIe/SSE2 NVIDIA Corporation 4.6.0 NVIDIA 537.42

Blender Version
Broken: version: 4.0.0 Alpha, branch: main, commit date: 2023-09-26 06:58, hash: 3514d6ed52a3
Worked: NA

Short description of error
In the Principled BSDF, materials are layered in this order: Diffuse -> Coat -> Sheen (obviously ignoring many configurations).
The diffuse component has it's own normals. Coat has it's own normals. And Sheen inherits the normals of the diffuse layer. This is incorrect.

For example, you have a diffuse material. You use bump mapping to give it a bumpy look. Then a smooth clearcoat is applied on top. In real life, this clearcoat would fill in the bumps to create the smooth clearcoat surface. Lastly, the sheen layer will be applied on top of the coat. The normals of the sheen layer should be smooth since it is sitting on top of the smooth coat, but instead the normals are bumpy because the sheen layer inherits the normals of the diffuse layer.

Ideally the Sheen layer should switch between Diffuse or Coat normals based on the presence of the coat layer, or the Sheen layer should have it's own normal input.

Exact steps for others to reproduce the error

Create an object with a Principled BSDF material.
Turn up the coat weight.
Turn up the sheen weight.
Adjust the normals of the coat and base Principled BSDF and notice that the base normals adjust the normals of the sheen, not the coat normals.

I have attached a .blend file with things setup.

**System Information** Operating system: Windows-10-10.0.22621-SP0 64 Bits Graphics card: NVIDIA GeForce RTX 4090/PCIe/SSE2 NVIDIA Corporation 4.6.0 NVIDIA 537.42 **Blender Version** Broken: version: 4.0.0 Alpha, branch: main, commit date: 2023-09-26 06:58, hash: `3514d6ed52a3` Worked: NA **Short description of error** In the Principled BSDF, materials are layered in this order: `Diffuse -> Coat -> Sheen` (obviously ignoring many configurations). The diffuse component has it's own normals. Coat has it's own normals. And Sheen inherits the normals of the diffuse layer. This is incorrect. For example, you have a diffuse material. You use bump mapping to give it a bumpy look. Then a smooth clearcoat is applied on top. In real life, this clearcoat would fill in the bumps to create the smooth clearcoat surface. Lastly, the sheen layer will be applied on top of the coat. The normals of the sheen layer should be smooth since it is sitting on top of the smooth coat, but instead the normals are bumpy because the sheen layer inherits the normals of the diffuse layer. Ideally the Sheen layer should switch between Diffuse or Coat normals based on the presence of the coat layer, or the Sheen layer should have it's own normal input. **Exact steps for others to reproduce the error** 1. Create an object with a Principled BSDF material. 2. Turn up the coat weight. 3. Turn up the sheen weight. 4. Adjust the normals of the coat and base Principled BSDF and notice that the base normals adjust the normals of the sheen, not the coat normals. I have attached a .blend file with things setup. ![Sheen normal issue.png](/attachments/9d69180c-4096-42af-910b-6581a725fdba)

Sheen normals test file.blend

931 KiB

Sheen normal issue.png

722 KiB

Alaska added the

labels 2023-09-27 11:17:13 +02:00

Alaska changed title from ~~Principled BSDF: Sheen uses incorrect normals~~ to Principled BSDF: Sheen uses wrong normals

2023-09-27 11:17:24 +02:00

Alaska commented

2023-09-27 11:19:49 +02:00

This is more a design question than a bug report. In the code, the Sheen layer uses the base normals of the Principled BSDF, so everything is working as expected. But it's unexpected that the Sheen would inherit the normals of the layers below the coat since the coat is technically "resetting" the normals of the material.

Germano Cavalcante commented

2023-09-28 00:07:09 +02:00

Think this could be something for:
#99447: Principled v2 BSDF

Not sure if we can consider this a bug.

For user feedback and feature requests we have:
https://wiki.blender.org/wiki/Communication/Contact#User_Feedback_and_Requests

See also: https://wiki.blender.org/wiki/Reference/Not_a_bug

Needs input from the developers.

Think this could be something for: #99447: Principled v2 BSDF Not sure if we can consider this a bug. For user feedback and feature requests we have: https://wiki.blender.org/wiki/Communication/Contact#User_Feedback_and_Requests See also: https://wiki.blender.org/wiki/Reference/Not_a_bug Needs input from the developers.

Germano Cavalcante added

Module

Render & Cycles

Status

Needs Info from Developers

and removed

Status

Needs Triage

labels 2023-09-28 00:07:28 +02:00

Brecht Van Lommel referenced this issue

2023-10-05 16:10:13 +02:00

Principled v2 BSDF #99447

Brecht Van Lommel commented

2023-10-05 16:51:22 +02:00

CC @LukasStockner

I think we could blend the base and coat normals with the coat weight?

I brought this up for OpenPBR as well.

CC @LukasStockner I think we could blend the base and coat normals with the coat weight? I brought this up for OpenPBR as well.

Lukas Stockner commented

2023-10-06 01:49:42 +02:00

Yeah, using mix(Normal, CoatNormal, saturate(CoatWeight)) as the Sheen normal seems reasonable to me. I'll add it for now, we can align with OpenPBR later if they go a different route.

A separate normal input just for Sheen seems overkill to me - unlike specular layers, it's typically not sensitive enough to the exact normal to warrant e.g. a separate bump map.

Yeah, using `mix(Normal, CoatNormal, saturate(CoatWeight))` as the Sheen normal seems reasonable to me. I'll add it for now, we can align with OpenPBR later if they go a different route. A separate normal input just for Sheen seems overkill to me - unlike specular layers, it's typically not sensitive enough to the exact normal to warrant e.g. a separate bump map.

Lukas Stockner self-assigned this 2023-10-06 01:49:48 +02:00

Lukas Stockner referenced this issue from a commit

2023-10-06 02:26:22 +02:00

Fix #112948: Cycles: Principled Sheen over Coat should use Coat normal

Lukas Stockner closed this issue

2023-10-06 02:32:05 +02:00

Blender Bot added

Status

Resolved

and removed

Status

Needs Info from Developers