small SSS scale values not working with GPU render

[Brian_Re]

Hi there!

I found an issue with Arnold GPU rendering. The issue is that SSS scaling does not work when using small values in GPU rendering.

I've tried a few tests (see the image below), but the problem still exists. I'm rendering a real-world-scale cookie in this test. The cookie measures about 18 centimeters in length. It appears OK when I render it with the CPU. However, when I switch to the GPU, the SSS scale stops working after a specific value (which varies by scene). I tried all three of Arnold's most recent releases, and the problem persists. I also tested five different scenes, and the problem is the same: when rendering with GPU there is a bug with the SSS scale .

[Stephen.Blair]

I did a quick test and couldn't repro

Is this cookie at or near the world origin, or far away?

Displacement / texture ?

GPU doesn't support diffusion and will switch to randomwalk v2

[Brian_Re]

Did you use a small object like a 3 cm cube? because I'm seeing this issue on multiple machines. anyways. The cookie is in the center of origin, and it has displacement. 

Someone already mentioned this problem before here: SSS Issue in GPU mode (Randomwalk V2)

[Stephen.Blair]

Units in Arnold are arbitrary. So I did an 18 unit cube in Maya where Maya defaults to cm

I'll make it smaller.

[peter_horvath]

While Maya uses cm as the default unit, scenes are exported from Cinema 4D with meter as the default, therefore values are smaller. This is a known GPU numerical precision issue, as mentioned in the linked thread, and tracked internally with ticket ARNOLD-11163. Hasn't been fixed yet, unfortunately.

As a workaround, you can try to change the Export Scale to 1 centimeter in the Arnold Render Settings under the System tab. That will basically scale up your Arnold scene.