SSS Issue in GPU mode (Randomwalk V2)

[jacksonboss]

I noticed this issue quite a while ago, and although it didn’t bother me too much at first, I’d still like to report it.

I do a lot of work using SSS with small objects, and I’ve noticed a bug/problem when using the GPU for this.

In my latest project involving a protein bar, I noticed that the broken part had the SSS Scale set to 0.08, yet it still looked very strong, almost like a wax material. Even when setting the Scale to 0, there was still a minimum SSS effect on the bar.

I've been aware of this for over a year, but I wasn’t too concerned, thinking it would be resolved in a future update. I usually worked around it by scaling up my objects up to 10 times their size to avoid the issue, but in some cases, I can't do that.

 

The SSS in CPU mode is perfect, but obviously much slower. I did a small test with a cube, as you can see below. Please take a look and draw your own conclusions (I’m not sure why the displacement differs when switching from CPU to GPU mode—maybe that’s a bug as well).

I forgot to clarify that this only happens on small surfaces. I usually resolve this issue by excessively scaling up my object, but as I mentioned, there are some cases where I can’t do that.

 

I always use Randomwalk V2 for SSS

[lee_griggs]

Thanks very much for reporting this issue. Would you be so kind as to upload a small scene that replicates the issue and demonstrates it? I would be interested to know if you have the same SSS issue with openpbr_surface.

Incidentally, are you using transmission_scatter for the caramel(?) center? There is a honey example in the openpbr_surface docs here.

[thiago.ize · Accepted solution]

Hi, no need for a scene. This is a known GPU numerical precision bug already being tracked internally with ticket ARNOLD-11163.

[jacksonboss]

@lee_griggs Thank you very much for responding, the caramel is just transmission and is still in the development phase, I was referring to the broken part of the chocolate.

@thiago.ize  I'm glad to know it's a known bug, and I hope it's resolved as soon as possible. I love using Arnold GPU for its speed and wouldn't want to have to use the CPU for this.

Thank you both for the quick response, and have a great weekend!

[jamie.portsmouth]

Hi jacksonboss

Can you clarify what the length units are here? So the scale of 0.02 corresponds to 2cm (i.e. you're working in meters), or 0.02cm (i.e. you're working in cm)?