Cycles: Magic Texture in Volumetric World Shaders render differently with the CPU and GPU #92036

New Issue

Alaska · 2021-10-08T08:54:09+02:00

Alaska commented

2021-10-08 08:54:09 +02:00

System Information
Operating system: Linux-5.14.0-1-amd64-x86_64-with-glibc2.32 64 Bits
Graphics card: NVIDIA GeForce RTX 3090/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 470.63.01

Blender Version
Broken: version: 3.0.0 Alpha, branch: master, commit date: 2021-10-08 02:23, hash: de07bf2b13
Worked: Unsure as prior to Cycles-X the cause was a lack of equiangular sampling support on the GPU.

Short description of error:
When rendered with a certain volumetric world shader, it can appear drastically different between the CPU and GPU which lead to issues with rendering with CPU+GPU


CPU Rendering	GPU Rendering (CUDA or OptiX)	CPU+GPU (The portion of the image affected depends on GPU and CPU speed)

Exact steps for others to reproduce the error:
Download the attached file below and render it with the CPU, GPU, and test with CPU+GPU and observe the different results.
TT92036 - Volumetric rendering differences.blend

Change the render engine to Cycles
Create a World Shader with Magic Texture with the output connected to a Color Ramp, with the output of the Color Ramp connected to the input of the Color of Volume Scatter node which is connected the World Output node.
Shift the handles on the Color Ramp (primarily shifting the black handle to the left) until the shader disappears when rendering in Cycles with the GPU.
Now test the scene with CPU and GPU rendering and compare them.

While setting up this material you may notice that the Magic Texture node produces a different result for the GPU and CPU when in the World Shader. This can be tested by connecting the Color output from the Magic Texture directly to the Color input of the Volume Scatter node. This is probably what's causing the issue.

**System Information** Operating system: Linux-5.14.0-1-amd64-x86_64-with-glibc2.32 64 Bits Graphics card: NVIDIA GeForce RTX 3090/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 470.63.01 **Blender Version** Broken: version: 3.0.0 Alpha, branch: master, commit date: 2021-10-08 02:23, hash: `de07bf2b13` Worked: Unsure as prior to Cycles-X the cause was a lack of equiangular sampling support on the GPU. **Short description of error:** When rendered with a certain volumetric world shader, it can appear drastically different between the CPU and GPU which lead to issues with rendering with CPU+GPU |![CPU.png](https://archive.blender.org/developer/F10868236/CPU.png)|![GPU.png](https://archive.blender.org/developer/F10868243/GPU.png)|![GPU + CPU.png](https://archive.blender.org/developer/F10868240/GPU___CPU.png)| | -- | -- | -- | |CPU Rendering|GPU Rendering (CUDA or OptiX)|CPU+GPU (The portion of the image affected depends on GPU and CPU speed)| **Exact steps for others to reproduce the error:** Download the attached file below and render it with the CPU, GPU, and test with CPU+GPU and observe the different results. [TT92036 - Volumetric rendering differences.blend](https://archive.blender.org/developer/F10869419/TT92036_-_Volumetric_rendering_differences.blend) 1. Change the render engine to Cycles 2. Create a `World Shader` with `Magic Texture` with the output connected to a `Color Ramp`, with the output of the `Color Ramp` connected to the input of the `Color` of ` Volume Scatter` node which is connected the `World Output` node. 3. Shift the handles on the `Color Ramp` (primarily shifting the black handle to the left) until the shader disappears when rendering in Cycles with the GPU. 4. Now test the scene with CPU and GPU rendering and compare them. While setting up this material you may notice that the `Magic Texture` node produces a different result for the GPU and CPU when in the `World Shader`. This can be tested by connecting the `Color` output from the `Magic Texture` directly to the `Color` input of the ` Volume Scatter` node. This is probably what's causing the issue.

Alaska commented

2021-10-08 08:54:09 +02:00

Added subscriber: @Alaska

Alaska changed title from ~~Cycles: Certain Volumetric World Shaders render differently with the CPU and GPU~~ to Cycles: `Magic Texture` in Volumetric World Shaders render differently with the CPU and GPU

2021-10-08 09:41:58 +02:00

William Leeson commented

2021-10-08 13:35:36 +02:00

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

William Leeson commented

2021-10-08 13:52:52 +02:00

Added subscriber: @leesonw

William Leeson commented

2021-10-08 13:52:52 +02:00

I fiddled a bit and found that it disappears around 0.466 and 0.467 where as on the CPU this happens almost at 1. There appears to be a few differences between the node_ramp_util.h and ramp_util.h that I am investigating.

William Leeson self-assigned this 2021-10-08 14:26:51 +02:00

William Leeson commented

2021-10-08 14:27:32 +02:00

It seems there is a NaN introduced somewhere in the calculations on the GPU :-/

Charlie Jolly commented

2021-10-08 14:34:20 +02:00

Added subscriber: @CharlieJolly

William Leeson commented

2021-10-08 15:38:39 +02:00

It seems the magic node gives huge values for the random input vector i.e. (-9481895287870032301452285067430723584.000000 53184858981249068612401265478211731456.000000 12989808383429901368005745244375089152.000000) which don't play nice with the GPU giving NaNs as results which are causing the backness I believe. These values are coming from the ShaderData P value which is set to a very large value. If you add an object for the rays to hit then the volume shader works :-)

Everywhere the ray hits a surface it has sensible values everywhere else it is NaN. You can also assign a value to the vector input of the Magic Texture and that will work also. The question is what should the value be in this case the Magic Texture says

Vector

  Texture coordinate to sample texture at; defaults to Generated texture coordinates if the socket is left unconnected.

but what does that mean in this case?

It seems the magic node gives huge values for the random input vector i.e. (-9481895287870032301452285067430723584.000000 53184858981249068612401265478211731456.000000 12989808383429901368005745244375089152.000000) which don't play nice with the GPU giving NaNs as results which are causing the backness I believe. These values are coming from the ShaderData P value which is set to a very large value. If you add an object for the rays to hit then the volume shader works :-) ![bg_object.png](https://archive.blender.org/developer/F10879390/bg_object.png) Everywhere the ray hits a surface it has sensible values everywhere else it is NaN. You can also assign a value to the vector input of the Magic Texture and that will work also. The question is what should the value be in this case the Magic Texture says Vector ``` Texture coordinate to sample texture at; defaults to Generated texture coordinates if the socket is left unconnected. ``` but what does that mean in this case?

Brecht Van Lommel commented

2021-10-08 17:19:34 +02:00

Added subscriber: @brecht

Brecht Van Lommel commented

2021-10-08 17:19:34 +02:00

It's just the shading point position in this case. Probably we need to do something in the volume integration code to keep the volume step size reasonable, I guess it's taking very large steps now since such world volumes have unbounded size..

William Leeson commented

2021-10-08 17:41:20 +02:00

Yes that would be my guess :-)

William Leeson commented

2021-10-11 11:06:20 +02:00

I found a nice fix for this with out having to adjust the volume step.

YAFU commented

2021-10-12 11:48:53 +02:00

Added subscriber: @YAFU

YAFU commented

2021-10-12 11:48:54 +02:00

Hi.

I have found that the problem is also reproducible with Wave Texture, do I need to create a different report?
TT92036 - Volumetric rendering differences_wave.blend

By the way, the whole problem does not only occur with 3.0 (just in case fixes need to be applied in LTS versions as well)

Hi. I have found that the problem is also reproducible with Wave Texture, do I need to create a different report? [TT92036 - Volumetric rendering differences_wave.blend](https://archive.blender.org/developer/F11002657/TT92036_-_Volumetric_rendering_differences_wave.blend) By the way, the whole problem does not only occur with 3.0 (just in case fixes need to be applied in LTS versions as well)

William Leeson commented

2021-10-12 14:03:05 +02:00

Thanks for the information. Do you know any other shaders that are affected? This may mean we need to make a change somewhere else? It should be pretty easy to patch the LTS with this.

YAFU commented

2021-10-12 17:57:10 +02:00

@leesonw , With that node setup I find that it renders differently with CPU or GPU also with Checker texture:
TT92036 - Volumetric rendering differences_checker.blend

I don't think it happens with other node textures that I have tried, but I don't know much about nodes setup. It would be good if someone with more knowledge than mine does a better test.

@leesonw , With that node setup I find that it renders differently with CPU or GPU also with Checker texture: [TT92036 - Volumetric rendering differences_checker.blend](https://archive.blender.org/developer/F11011035/TT92036_-_Volumetric_rendering_differences_checker.blend) I don't think it happens with other node textures that I have tried, but I don't know much about nodes setup. It would be good if someone with more knowledge than mine does a better test.

blender-admin commented

2021-12-10 09:02:55 +01:00

This issue was referenced by blender/cycles@3cdf714f1a

This issue was referenced by blender/cycles@3cdf714f1ac792f381318e99b9d2f9a8fd10a366

blender-admin commented

2021-12-10 09:02:55 +01:00

This issue was referenced by 57f46b9d5f

This issue was referenced by 57f46b9d5f370dfacc92786dab596060400a21a8

William Leeson commented

2021-12-10 09:12:26 +01:00

Changed status from 'Confirmed' to: 'Resolved'

William Leeson closed this issue

2021-12-10 09:12:26 +01:00

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Cycles: Magic Texture in Volumetric World Shaders render differently with the CPU and GPU #92036

Cycles: `Magic Texture` in Volumetric World Shaders render differently with the CPU and GPU #92036