topic Re: Shocking difference in GPU and CPU rendering while using SSS active in shader in Arnold for Houdini Forum

Shocking difference in GPU and CPU rendering while using SSS active in shader

robert.pashayan — Mon, 14 Jul 2025 16:25:08 GMT

Hello everyone, I'm having shocking difference in how SSS is rendered in GPU versus CPU.
It's done in the IPR of Solaris, and I can't figure out if it's a software limitation or some settings are off.
Not only the redness of the SSS radius not coming out but also the scale seems off.
Does someone had the same issues ?

Re: Shocking difference in GPU and CPU rendering while using SSS active in shader

Stephen.Blair — Mon, 14 Jul 2025 18:27:28 GMT

Can you post a repro ?

What sampling settings? What's the shader setup? What sss type? What version of Arnold?

Re: Shocking difference in GPU and CPU rendering while using SSS active in shader

robert.pashayan — Mon, 14 Jul 2025 18:45:51 GMT

Repro ? I Didn't get what you mean by that.
Render Settings are similar, I only boost the Max Samples for Adaptive Sampling from 10(CPU) to 24 for GPU, it's a simple grey shader with RandomWalk v2 SSS with a displacement.

HtoA 6.4.2.2 | Arnold 7.4.2.0

Re: Shocking difference in GPU and CPU rendering while using SSS active in shader

Stephen.Blair — Mon, 14 Jul 2025 19:18:46 GMT

How do I reproduce the problem?

Re: Shocking difference in GPU and CPU rendering while using SSS active in shader

robert.pashayan — Mon, 14 Jul 2025 20:15:09 GMT

You do a CPU render of an object with SSS, then add another render settings node in the end, where you activate the GPU render in Arnold and add AA Samples(optional). That's all.
See bellow, the rest is exactly the same.

Re: Shocking difference in GPU and CPU rendering while using SSS active in shader

thiago.ize — Tue, 15 Jul 2025 01:19:47 GMT

"repro" is short for "Reproducer". You need to upload a scene that reproduces the problem so that we are testing exactly the same thing. I suspect that what you're seeing could be due to certain SSS settings and if we don't use your settings or you mesh we won't have the problem you cite.

In any case, I suspect your problem is the same as https://forums.autodesk.com/t5/arnold-for-cinema-4d-forum/sss-issue-in-gpu-mode-randomwalk-v2/m-p/13004622#M9650 which is due to numerical precision error. Scaling everything to be larger is the current workaround until this is fixed in Arnold.

Re: Shocking difference in GPU and CPU rendering while using SSS active in shader

robert.pashayan — Tue, 15 Jul 2025 11:44:23 GMT

Hi Thiago, spot on, thank you.
By multiplying the scene scale by 100 and adjusting the SSS value I got a very similar result.