Volume Scatter with GPU worse than with CPU in Cycles

Support Lighting and Rendering
1

Hi,

I noticed that when rendering volume scatter in Cycles with GPU you get worse results than when you render it with CPU. Is that a known bug or limitation? Or can this discrepancy be dealt with somehow?

Here is a split image of a volume scattered background and its node setup comparing CPU on the left to GPU on the right. The only thing I changed was the render engine from CPU to GPU and the tile size from 16 to 128. The samples in both renderings are set to 2048.
Windows 7
GTX 670
Blender 2.75 (I noticed the same in, I think, 2.73 and 2.74 as well as the 2.76 test version)

BlenderVolumeCPU_GPU.png1280×720 884 KB

Attachments


2

hmm - interesting. Can say I have noticed this but I may have a play tonight. I have a scene that would show off this difference nicely.

3

One thing to check. I have just rendered with CPU using the latest version of blender and I noticed that when rendering using CPU - there is a “pattern” option on the sampling section of the render tab.

This was set to “Correlated multi-jittered”. When I rendered using GPU however this option disappeared - which means you’ll be rendering using “Sobol” pattern. You can activate “Correlated Multi-Jittered” for GPU - but it is an experimental feature.

I did this and the results for CPU and GPU appear identical. Here are three renders:

  1. CPU (correllated multi-jittered)
  2. GPU (correllated multi-jittered)
  3. CPU (sobol)




4

Can you try rendering with more samples? The difference seems to be visible only over 1024.

5

I rendered these with 1200 samples (CPU first, GPU second) - identical to the pixel as long as the sampling pattern is the same (Sobol in this case)



This final one was GPU with correlated multi-jittered. IMO it’s considerably worse - especially near the light entry point.


6

It’s possible that one or two of the available sampling techniques for volumes is simply not available for use by the GPU (due to CUDA limitations).

To verify, try doing a CPU render without the use of Multiple Importance and Equi-angular sampling.

7

I did some more comparisons with all kinds of settings. All renderings with 2048 samples.

BlenderVolumeComparison.jpg1280×1440 307 KB