CPU + GPU render precision differences for small, far away area light #69535

New Issue

Paul Szajner · 2019-09-05T13:03:34+02:00

Paul Szajner commented

2019-09-05 13:03:34 +02:00

System Information
Operating system: Linux-4.19.0-5-amd64-x86_64-with-debian-10.0 64 Bits
Graphics card: GeForce GTX 780/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 418.74

Blender Version
Broken: version: 2.81 (sub 8), branch: master, commit date: 2019-09-04 21:17, hash: baaa89a0bc
Worked: (optional)

Short description of error
This was recently reported in this thread :
https://blenderartists.org/t/blender-2-8-beta-cpu-render-vs-gpu-render-problem/1136771/2

I can at least say, that when using small area lights below a size of 1 (e.g. meter), and it’s getting darker (the light source is far away from the object) you can clearly see this problem: CPU renders the tiles darker than the CUDA GPU.

Exact steps for others to reproduce the error
Create a new scene with the cube, remove the default light, create an area light, resize it to lets say 10% (or 1%… the smaller the bigger the effect) and rotate it that it lights the default cube.
Here is a .blend that reproduce the issue :
TileProblem.blend.blend

**System Information** Operating system: Linux-4.19.0-5-amd64-x86_64-with-debian-10.0 64 Bits Graphics card: GeForce GTX 780/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 418.74 **Blender Version** Broken: version: 2.81 (sub 8), branch: master, commit date: 2019-09-04 21:17, hash: `baaa89a0bc` Worked: (optional) **Short description of error** This was recently reported in this thread : https://blenderartists.org/t/blender-2-8-beta-cpu-render-vs-gpu-render-problem/1136771/2 I can at least say, that when using small area lights below a size of 1 (e.g. meter), and it’s getting darker (the light source is far away from the object) you can clearly see this problem: CPU renders the tiles darker than the CUDA GPU. ![CPU_GPU_tilies_issue.png](https://archive.blender.org/developer/F7718114/CPU_GPU_tilies_issue.png) **Exact steps for others to reproduce the error** Create a new scene with the cube, remove the default light, create an area light, resize it to lets say 10% (or 1%… the smaller the bigger the effect) and rotate it that it lights the default cube. Here is a .blend that reproduce the issue : [TileProblem.blend.blend](https://archive.blender.org/developer/F7718117/TileProblem.blend.blend)

Paul Szajner commented

2019-09-05 13:03:34 +02:00

Added subscriber: @sozap

blender-admin commented

2019-09-05 13:03:34 +02:00

#82896 was marked as duplicate of this issue

Philipp Oeser commented

2019-09-05 15:13:26 +02:00

Added subscriber: @lichtwerk

Philipp Oeser commented

2019-09-05 15:13:26 +02:00

Hm, can actually confirm.
Need to check if this has been reported before.

Hm, can actually confirm. Need to check if this has been reported before.

Philipp Oeser commented

2019-09-05 15:30:39 +02:00

Here are some reports that mention differences between CPU vs. GPU (though these are more about volumes, DOF, ...)
#57109 (Flickering artifacts in (animated) DOF areas)
#67448 (Shader node float precision issue cause different results on CPU and GPU)
#50193 (Cycles renders volumetrics very differently on CPU vs. GPU)

Here are some reports that mention differences between CPU vs. GPU (though these are more about volumes, DOF, ...) #57109 (Flickering artifacts in (animated) DOF areas) #67448 (Shader node float precision issue cause different results on CPU and GPU) #50193 (Cycles renders volumetrics very differently on CPU vs. GPU)

Brecht Van Lommel changed title from ~~CPU + GPU render tiles differently~~ to CPU + GPU render precision differences for small, far away area light

2019-09-25 16:17:22 +02:00

Marcus Brock commented

2019-10-10 14:24:30 +02:00

Added subscriber: @atomedia

Marcus Brock commented

2019-10-10 14:24:30 +02:00

I want to add that this is not just an error that appears in very few situations. It appears (for my understanding) for nearly every realistic sized light (lights <1m).
I have a big project where I can render no single frame without this error. ~90% of the light comes from area lights (10x10cm floor lights).
It looks like the contrast or brightness setting is different when rendering with CPU or GPU. GPU frames are always a lot brighter than CPU frames. I currently don't know which device produces the correct output. It's annoying and very time consuming that you cannot use both devices at the same time anymore to speed up rendering.

Here for you two comparison frames rendered separatly.

CPU (Threadripper 2950X):

GPU (RTX 2080 + GTX 1060):

CPU+GPU (way more samples):

Maybe the bug can be investigated with a little higher priority.

I want to add that this is not just an error that appears in very few situations. It appears (for my understanding) for nearly every realistic sized light (lights <1m). I have a big project where I can render no single frame without this error. ~90% of the light comes from area lights (10x10cm floor lights). It looks like the contrast or brightness setting is different when rendering with CPU or GPU. GPU frames are always a lot brighter than CPU frames. I currently don't know which device produces the correct output. It's annoying and very time consuming that you cannot use both devices at the same time anymore to speed up rendering. Here for you two comparison frames rendered separatly. CPU (Threadripper 2950X): ![CPU.png](https://archive.blender.org/developer/F7801506/CPU.png) GPU (RTX 2080 + GTX 1060): ![GPU.png](https://archive.blender.org/developer/F7801507/GPU.png) CPU+GPU (way more samples): ![CPUGPU.jpg](https://archive.blender.org/developer/F7801576/CPUGPU.jpg) Maybe the bug can be investigated with a little higher priority.

Germano Cavalcante commented

2019-12-04 19:28:16 +01:00

Closed as duplicate of #72141

Germano Cavalcante closed this issue

2019-12-04 19:28:16 +01:00

Dalai Felinto commented

2020-10-08 12:07:55 +02:00

Added subscriber: @dfelinto

Dalai Felinto commented

2020-10-08 12:07:55 +02:00

Changed status from 'Duplicate' to: 'Confirmed'

Dalai Felinto reopened this issue

2020-10-08 12:07:55 +02:00

Dalai Felinto commented

2020-10-08 12:07:55 +02:00

This was merged into an issue that is different. Re-opening and let the Cycles team decide whether this is something to be tackled or just a known issue.

YAFU commented

2020-11-21 11:27:45 +01:00

Added subscriber: @YAFU

Robert Guetzkow commented

2020-11-21 11:59:59 +01:00

Added subscriber: @arturmachura

Philipp Oeser removed the

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CPU + GPU render precision differences for small, far away area light #69535