Weird volumetric artifacts in Cycles (X) rendering #91797

New Issue

Pavel Duong · 2021-09-29T03:12:02+02:00

Pavel Duong commented

2021-09-29 03:12:02 +02:00

System Information
Operating system: Windows 10
Graphics card: NVIDIA GeForce GTX 1650 Super

Blender Version
Broken: 2.93.4LTS (CUDA) and c53ffda8a4 (blender-3.0.0-alpha+master.c53ffda8a496-windows.amd64-release)
Worked: 2.93.4LTS (CPU)

Short description of error
Rendering volumetric under conditions provided in the example blend file below produces different results compared depending on whether Cycles/Cycles X and/or CPU/CUDA is used.

Here is a render from Blender 2.93.4 LTS using the CPU, which I consider correct:
(Note: I am not completely sure if the issue occurs a bit here as well on the left side of the render)

Here are the renders that I consider broken:

Blender 2.93.4 LTS using CUDA:

Blender 3.0.0 Alpha using CPU:

Blender 3.0.0 Alpha using CUDA:

Exact steps for others to reproduce the error

Open the attached scene
Render the scene (If LTS, then use CUDA. If CyclesX builds then either will do)
Notice the artifacts

volumetrics.blend

**System Information** Operating system: Windows 10 Graphics card: NVIDIA GeForce GTX 1650 Super **Blender Version** Broken: `2.93.4LTS` (CUDA) and c53ffda8a496 (`blender-3.0.0-alpha+master.c53ffda8a496-windows.amd64-release`) Worked: `2.93.4LTS` (CPU) **Short description of error** Rendering volumetric under conditions provided in the example blend file below produces different results compared depending on whether Cycles/Cycles X and/or CPU/CUDA is used. Here is a render from *Blender 2.93.4 LTS* using the CPU, which I consider correct: (*Note: I am not completely sure if the issue occurs a bit here as well on the left side of the render*) ![volumetrics_lts_cpu.png](https://archive.blender.org/developer/F10660502/volumetrics_lts_cpu.png) --- Here are the renders that I consider broken: *Blender 2.93.4 LTS* using CUDA: ![volumetrics_lts_cuda.png](https://archive.blender.org/developer/F10660560/volumetrics_lts_cuda.png) *Blender 3.0.0 Alpha* using CPU: ![volumetrics_alpha_cpu.png](https://archive.blender.org/developer/F10660562/volumetrics_alpha_cpu.png) *Blender 3.0.0 Alpha* using CUDA: ![volumetrics_alpha_cuda.png](https://archive.blender.org/developer/F10660564/volumetrics_alpha_cuda.png) **Exact steps for others to reproduce the error** 1. Open the attached scene 2. Render the scene (If LTS, then use CUDA. If CyclesX builds then either will do) 3. Notice the artifacts [volumetrics.blend](https://archive.blender.org/developer/F10660466/volumetrics.blend)

Pavel Duong commented

2021-09-29 03:12:02 +02:00

Added subscriber: @vignette

Richard Antalik commented

2021-09-29 17:01:37 +02:00

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

Steffen Dünner commented

2021-09-29 17:02:37 +02:00

Added subscriber: @SteffenD

Yegor commented

2021-09-29 19:07:13 +02:00

Added subscriber: @YegorSmirnov

Yegor commented

2021-09-29 19:07:13 +02:00

By the way, Branched Path Tracing in 2.93.4 doesn't have this problem.

William Leeson commented

2021-10-14 22:26:08 +02:00

Added subscriber: @leesonw

William Leeson commented

2021-10-14 22:26:08 +02:00

I did some investigating here and it seems you can remove the fog cube and just focus on the mist it seems like there are some differences when rendering the meshes. For 2.93 it looks like this

and for 3.0 is looks like this

to get this I just rendered the surfaces with a diffuse BSDF. It seems the geometry is behaving differently. Zooming in on the bottom right corner for 2.93 I get

and for 3.0 I get

I think maybe something about the geometry is causing the issues.

I did some investigating here and it seems you can remove the fog cube and just focus on the mist it seems like there are some differences when rendering the meshes. For 2.93 it looks like this ![volumetrics_2.94.png](https://archive.blender.org/developer/F11110553/volumetrics_2.94.png) and for 3.0 is looks like this ![volumetrics_3.0.png](https://archive.blender.org/developer/F11110610/volumetrics_3.0.png) to get this I just rendered the surfaces with a diffuse BSDF. It seems the geometry is behaving differently. Zooming in on the bottom right corner for 2.93 I get ![vol_close_up_293.png](https://archive.blender.org/developer/F11111016/vol_close_up_293.png) and for 3.0 I get ![vol_close_up_30.png](https://archive.blender.org/developer/F11111033/vol_close_up_30.png) I think maybe something about the geometry is causing the issues.

Yegor commented

2021-10-14 22:35:08 +02:00

I bet it's due to the shadow terminator solution (in object properties)

I bet it's due to the shadow terminator solution (in object properties) ![image.png](https://archive.blender.org/developer/F11111326/image.png)

William Leeson commented

2021-10-14 22:52:15 +02:00

Maybe, it kind of seems like the shadow ray are hitting a different geometry.

William Leeson self-assigned this 2021-10-25 14:54:16 +02:00

Pavel Duong commented

2021-11-15 12:59:11 +01:00

I have done some experimenting and found out that the artifacts of overlapping volumetrics happens only (or at least in this case) if the mesh is combined.

On the left the cubes are stacked into each other and are separate objects, while on the right all the cubes are combined into a single mesh. Also from what I've seen, there is a subtle difference between CPU and CUDA render on the left cube (the CUDA render has a dark spot in the middle).

Test File
Using Blender version: 3.0.0 Beta, branch: master, commit date: 2021-11-11 18:49, hash: bd734cc4419a, type: release
volumetrics_2.blend

CPU Render

CUDA Render

I have done some experimenting and found out that the artifacts of overlapping volumetrics happens only (or at least in this case) if the mesh is combined. On the left the cubes are stacked into each other and are separate objects, while on the right all the cubes are combined into a single mesh. Also from what I've seen, there is a subtle difference between CPU and CUDA render on the left cube (the CUDA render has a dark spot in the middle). **Test File** Using Blender version: `3.0.0 Beta, branch: master, commit date: 2021-11-11 18:49, hash: bd734cc4419a, type: release` [volumetrics_2.blend](https://archive.blender.org/developer/F11808812/volumetrics_2.blend) **CPU Render** ![cpu_300.png](https://archive.blender.org/developer/F11808804/cpu_300.png) **CUDA Render** ![gpu_compute_300.png](https://archive.blender.org/developer/F11808800/gpu_compute_300.png)

Brecht Van Lommel commented

2021-11-17 21:21:31 +01:00

Added subscriber: @brecht