Regression: Cycles fresnel emission in Principled BSDF produces triangle artifacts #119813

New Issue

Brad Smith · 2024-03-23T03:51:33+01:00

Brad Smith commented

2024-03-23 03:51:33 +01:00

System Information
Operating system: Linux
Graphics card: NVIDIA RTX 3060Ti

Blender Version
Broken: 4.0.2 and 4.1.0 Release Candidate (v41.b9ef66693adf)
Worked: 3.6.2

Short description of error
When using a Fresnel node's output as an emission strength with any BSDF node, cycles renders may produce strange triangle artifacts.

I noticed this issue when working in a scene with dim lighting where I was using a Fresnel node as input to the emission strength of a Principled BSDF to make an object glow from its outer edges. This material was working fine in 3.6.2, but 4.0.2 introduced odd triangle rendering artifacts. I also confirmed this issue is still present in the current 4.1.0 Release Candidate. After some trial and error, I identified the cause of the weird fresnel triangles and created a minimal blend file with the following steps.

Exact steps for others to reproduce the error

Start with default new project with factory settings.
In Scene settings, switch Render Engine to Cycles. Device can be either CPU or GPU (I've observed this issue with both).
In Scene settings, to increase visibility of this issue, consider setting Color Management: View Transform to Standard with Look Very High Contrast. (optional but might help you see the effect depending on your display)
Delete default cube and default light.
Add a new plane object.
With the plane selected, switch to edit mode, and:
1. Subdivide with 5 cuts.
2. Mesh: Transform: Randomize with amount set to 0.2.
3. Select all vertices, and scale on Z axis to 0. This should result in a flat plane that just has some vertices jittered in the X and Y directions.
4. Note that those last two steps seem to be the most important to trigger this rendering issue. If the vertices are perfectly uniform on the subdivided plane, the output looks fine. However, if the vertices form slightly irregular quads, even if they are perfectly flatly aligned at z=0, the weird triangles appear in the Cycles rendered output.
Switch back to object mode, go to the shading tab, and:
1. Add a new material.
2. In the new Principled BSDF, set emission color to white (FFFFFF).
3. Add a Fresnel node, and connect its Fac output to the Principled BSDF's Emission Strength input.
Switch the viewport shading to Rendered and rotate/pan until you can see the fresnel effect on the object. Since this is a flat plane, the effect will be very subtle and only visible as you approach it edge-on.
Compare the difference between 3.6.2 and 4.0.2. Looks great and smooth in 3.6.2, but weird triangles appear in 4.0.2.

This problem isn't limited to just the Principled BSDF. I also tried feeding the Fresnel into a simple Emission shader and using an Add Shader node to combine that with another BSDF, and the same buggy triangles appeared.

If I connect the Fresnel directly to the Material Output node without a BSDF node between them, the material looks smooth and normal as I would expect. So, the problem appears to be in combining the fresnel with other material properties somehow.

Attached files are:

fresnel-shader-issue.principled-bsdf.blend - minimal demonstration file created using the steps I described above
fresnel-shader-issue.add-shader-node.blend - basically the same, but trying to use the Add Shader node (which didn't help)
3.6.2-blender.png - Blender 3.6.2 viewport in Cycles rendered mode. Looks good! ✅
3.6.2-principled-bsdf-render.png - rendered output from Blender 3.6.2 using Cycles. Looks good! ✅
4.0.2-blender.png - Blender 4.0.2 viewport in Cycles rendered mode. Looks bad. ❌
4.0.2-principled-bsdf-render.png - rendered output from Blender 4.0.2 using Cycles. Looks bad. ❌
4.0.2-add-shader-node.png - Blender 4.0.2 viewport in Cycles rendered mode, trying the Add Shader instead of one Principled BSDF node. Still looks bad. ❌
original-real-world-example.png - 4.0.2 render from original real-world project where I discovered this issue. Boosted levels and sketched on (purple) after noticing glitches in the render. In this case, the mesh with the offending fresnel artifacts is also perfectly flat (on the ZY plane) but is made up of irregular triangles and quads, which is what I found to be the key issue in the reproduction steps above.

**System Information** Operating system: Linux Graphics card: NVIDIA RTX 3060Ti **Blender Version** Broken: 4.0.2 and 4.1.0 Release Candidate (v41.b9ef66693adf) Worked: 3.6.2 **Short description of error** When using a Fresnel node's output as an emission strength with any BSDF node, cycles renders may produce strange triangle artifacts. I noticed this issue when working in a scene with dim lighting where I was using a Fresnel node as input to the emission strength of a Principled BSDF to make an object glow from its outer edges. This material was working fine in 3.6.2, but 4.0.2 introduced odd triangle rendering artifacts. I also confirmed this issue is still present in the current 4.1.0 Release Candidate. After some trial and error, I identified the cause of the weird fresnel triangles and created a minimal blend file with the following steps. **Exact steps for others to reproduce the error** 1. Start with default new project with factory settings. 1. In Scene settings, switch Render Engine to Cycles. Device can be either CPU or GPU (I've observed this issue with both). 1. In Scene settings, to increase visibility of this issue, consider setting Color Management: View Transform to `Standard` with Look `Very High Contrast`. (optional but might help you see the effect depending on your display) 1. Delete default cube and default light. 1. Add a new plane object. 1. With the plane selected, switch to edit mode, and: 1. Subdivide with 5 cuts. 1. Mesh: Transform: Randomize with amount set to `0.2`. 1. Select all vertices, and scale on Z axis to `0`. This should result in a **flat** plane that just has some vertices jittered in the X and Y directions. 1. **Note that those last two steps seem to be the most important to trigger this rendering issue.** If the vertices are _perfectly uniform_ on the subdivided plane, the output looks fine. However, if the vertices form _slightly irregular quads, even if they are perfectly flatly aligned at z=0_, the weird triangles appear in the Cycles rendered output. 1. Switch back to object mode, go to the shading tab, and: 1. Add a new material. 1. In the new Principled BSDF, set emission color to white (`FFFFFF`). 1. Add a Fresnel node, and connect its `Fac` output to the Principled BSDF's `Emission Strength` input. 1. Switch the viewport shading to Rendered and rotate/pan until you can see the fresnel effect on the object. Since this is a flat plane, the effect will be very subtle and only visible as you approach it edge-on. 1. Compare the difference between 3.6.2 and 4.0.2. Looks great and smooth in 3.6.2, but weird triangles appear in 4.0.2. This problem isn't limited to just the Principled BSDF. I also tried feeding the Fresnel into a simple Emission shader and using an Add Shader node to combine that with another BSDF, and the same buggy triangles appeared. If I connect the Fresnel *directly* to the Material Output node without a BSDF node between them, the material looks smooth and normal as I would expect. So, the problem appears to be in **combining** the fresnel with other material properties somehow. Attached files are: * `fresnel-shader-issue.principled-bsdf.blend` - minimal demonstration file created using the steps I described above * `fresnel-shader-issue.add-shader-node.blend` - basically the same, but trying to use the Add Shader node (which didn't help) * `3.6.2-blender.png` - Blender 3.6.2 viewport in Cycles rendered mode. Looks good! ✅ * `3.6.2-principled-bsdf-render.png` - rendered output from Blender 3.6.2 using Cycles. Looks good! ✅ * `4.0.2-blender.png` - Blender 4.0.2 viewport in Cycles rendered mode. Looks bad. ❌ * `4.0.2-principled-bsdf-render.png` - rendered output from Blender 4.0.2 using Cycles. Looks bad. ❌ * `4.0.2-add-shader-node.png` - Blender 4.0.2 viewport in Cycles rendered mode, trying the Add Shader instead of one Principled BSDF node. Still looks bad. ❌ * `original-real-world-example.png` - 4.0.2 render from original real-world project where I discovered this issue. Boosted levels and sketched on (purple) after noticing glitches in the render. In this case, the mesh with the offending fresnel artifacts is also perfectly flat (on the ZY plane) but is made up of irregular triangles and quads, which is what I found to be the key issue in the reproduction steps above.

fresnel-shader-issue.principled-bsdf.blend

906 KiB

fresnel-shader-issue.add-shader-node.blend

894 KiB

4.0.2-add-shader-node.png

395 KiB

3.6.2-principled-bsdf-render.png

699 KiB

4.0.2-principled-bsdf-render.png

725 KiB

3.6.2-blender.png

366 KiB

4.0.2-blender.png

390 KiB

original-real-world-example.png

524 KiB

Brad Smith added the

labels 2024-03-23 03:51:34 +01:00

Alaska commented

2024-03-23 06:13:51 +01:00

I can confirm this issue in 4.1, but it's no longer present in Blender 4.2.

I assume this was fixed with 8cbc386152 which is only avaliable in Blender 4.2. @weizhen or @brecht , Do you want to add it to the list of changes to be back ported to 4.1.1 (See #119631)? Or just leave it as a fix for 4.2?

I can confirm this issue in 4.1, but it's no longer present in Blender 4.2. I assume this was fixed with 8cbc386152f3652f197f43af13402918d946c19e which is only avaliable in Blender 4.2. @weizhen or @brecht , Do you want to add it to the list of changes to be back ported to 4.1.1 (See #119631)? Or just leave it as a fix for 4.2?

Alaska added

Status

Needs Info from Developers

and removed

labels 2024-03-23 06:14:08 +01:00

Alaska added this to the 4.1 milestone 2024-03-23 06:14:12 +01:00

Brad Smith commented

2024-03-23 19:36:56 +01:00

Thank you for checking and confirming so quickly! I did not realize this was already fixed in 4.2, but yes a backport to 4.1 would certainly be appreciated.

I just downloaded the latest 4.2 Alpha build, and I confirmed that this issue appears to be fixed for both my minimal reproduction file and the original blend file where I first noticed it. For now, I'll try using these 4.2 Alpha builds when I need this kind of emission in my renders.

Thank you for checking and confirming so quickly! I did not realize this was already fixed in 4.2, but yes a backport to 4.1 would certainly be appreciated. I just downloaded the latest 4.2 Alpha build, and I confirmed that this issue appears to be fixed for both my minimal reproduction file and the original blend file where I first noticed it. For now, I'll try using these 4.2 Alpha builds when I need this kind of emission in my renders.

Brecht Van Lommel referenced this issue

2024-03-25 10:29:05 +01:00

4.1: Candidates for corrective releases #119631

Brecht Van Lommel modified the milestone from 4.1 to 4.2 LTS

2024-03-25 10:29:54 +01:00

Brecht Van Lommel commented

2024-03-25 10:30:25 +01:00

Kernel changes are risky one day before the release, so I'm postponing this to 4.1.1 and 4.2.