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Alrighty. I've searched all over the place and even put in a couple of requests from every deity I could think of, but nothing on my issue. So, I'm messing around with making up volumetric light cones for spotlights in Cycles for a project. I've got all of the parts set up and in scene. I start rendering and I get some odd results. With GPU Compute and GPU+CPU I get wicked amounts of noise inside the cones. As if the density of the scatter isn't high enough. Then I try CPU only, it comes out smooth as butter. Now I'm running a 9900k 32GB RAM and an RTX 2080 on the current stable release of 2.80. I just can't figure it out. I mean, I can handle just using my CPU and eating the much longer render times, but I'd much rather use the full power of my rig and get the same if not better results. I'll put up some pics for examples of what I've got going on.CPU only render. Nice and clean. ish.

GPU only render. Not so clean.

Martynas Žiemys
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JBJB2495
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    Please edit the question and give it an informative title. It's best to form it as a clear question that tells people looking through questions what your problem is about. It would improve your chances of getting a good answer sooner. So you are using the exact same render settings and switching between CPU and GPU Compute only on the official 2.80 release? – Martynas Žiemys Nov 19 '19 at 06:11
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    It seems I can replicate the issue with a simple scene with GTX970 and i72600K. It seems it is a bug. It should be reported to the bug tracker. If you report it, could you share the link to the report? – Martynas Žiemys Nov 19 '19 at 06:24
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    Looks like a difference in the sampling approach: https://mobile.twitter.com/stefan_3d/status/1123292775850680321 – Robert Gützkow Nov 19 '19 at 08:11
  • @Martynas Ziemys. Yeah, sadly I was working on this until the wee hours of the morning and had a hard enough time remembering where I saved the renders. They are the exact same settings in Official 2.80, yes. Thank you for confirming that it is an issue, and now that I've read what Robert Gützkow it would seem it's a known issue and I'll just have to slog through CPU rendering in any scenes that have volumetrics. Not a huge deal really. The same scene took 5 minutes with CPU which isn't too bad. Thank you. – JBJB2495 Nov 19 '19 at 13:33
  • @Robert Gützkow Thanks for posting that. Looks like CPU it is. I'll mess around with some settings to make it a bit more efficient. Thankfully I think my CPU is up to the task. Looks like I might have to dust off Twitter and look there for this stuff more often. – JBJB2495 Nov 19 '19 at 13:34
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    @RobertGützkow , could you maybe post this nicely formed as a full answer? I think this one is important to have fully answered for others to be able to find it. – Martynas Žiemys Nov 19 '19 at 13:41
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    @JBJB2495, I hope you don't mind me changing the title. It will be easier for others to find this. – Martynas Žiemys Nov 19 '19 at 13:45
  • @Marynas Ziemys Not at all! I appreciate that. I totally could not figure out a way to phrase it. That works perfectly. – JBJB2495 Nov 19 '19 at 14:18

1 Answers1

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The following answer is based on a Twitter thread by Stefan Werner.

Volumetric rendering on the CPU can be less noisy with fewer samples, because Cycles uses equi-angular sampling. This importance sampling technique by C. Kulla and M. Fajardo is currently not implemented for GPU rendering.

The slides from the Eurographics Symposium on Rendering 2012 may help to understand the paper.

Robert Gützkow
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