Stuck buckets with Vray 2Sided materials

[joaquim_montserrat1]

Hi Chaos,

This issue has come back from time to time and is annoying because of its finicky nature. We have a bunch of instances with various Vray 2sided materials with average amounts of translucency. In a seemingly random way, Vray will get stuck on some specific buckets, the dynamic bucket splitting will not take place and this will clog the renderfarm with no gain...
Last project this happened on a few frames in the middle of a long animation and we have had no choice but to spend as much as 16h to render a single bucket (on a threadripper 3970x), to give an idea of how annoying this can be...
After a long time trying to isolate the issue and find a workaround, we're out of ideas and seek help.

What we discovered while investigating:

• This feels super random, happening on some frames and not others, with minute changes on the scene (change from camera to perspective for example) causing the bug to appear somewhere else entierly (making it even harder to debug).
• For some unknown reason, the dynamic bucket splitting will stop working (fixing this would solve the issue imo)
• If rendering with progressive mode, the issue remains, a specific pass will get stuck, and disregard the time limit if any is set, still clogging the renderfarm.
• This seems to be linked with brightness and translucency amounts, dark will cause less issues, bright more. That said we tried clamping every shaders and stick to conservative values, without success to make the bug disappear.
• This seems to also be affected by the use of the VrayMultiSubTex with randomness by Instance ID, but I think succeeding by bypassing it entierly is not a win and also not a sure thing, as the bug seems to come back and my gut feeling is that it is tied to the 2sided material translucency.

We're using VRay 6.10.02 bld 00000
Color management as ACEScg

We tried to isolate this in a scene to share with you.



Please help,

Thanks,

Ju

[^Lele^]

I can confirm the bug as you describe it.
The issue seems to be with the 2Sided material itself, as skipping it renders fine.
Perhaps, while a fix arrives, you could change the shader for those roses to be a standard VRayMtl + Uniform SSS?
Looks WILL change, rendertime not so much at all, but crucially you'll have time to do something about that in post, with your sequence safely rendered.

[joaquim_montserrat1]

Thanks for the tip Lele, your workaround works well and I didn't know this behavior could work on volumeless objects.