Disappearing reflection through refractive object CPU vs GPU

[thebeals]

Hi all,

I am trying to render a plastic sphere inside a solid glass cube in Maya 2017 Vray 3.60.02. When the plastic sphere and the glass cube have a similar low ior (1.6 for example), the reflection on the sphere disappears when I render on the CPU. If I change the sphere to have a different IOR such a 1.4 or 1.8 and leave the glass cube at 1.6, the reflection shows back up. When I render on the GPU, the reflections appear correctly in all cases.

I can't find a depth setting or material to fix this and I'm not quite sure what is going on. I originally thought it was a weird law of physics that I didn't know about, but since it works on the GPU, I'm stumped and am thinking it's more related to how lights are being traced. Attached is an example of the image being renderered on the CPU (nor reflection) and GPU (reflection). I've also attached the scene file in case anyone wants to dig into it.

Does anyone know what is going on or have any suggestions of things to try?

[damaggio]

Thanks for your test scene thebeals, confirming the same results here.
Support should have info soon.

[yolov]

OK, that's very interesting.
Back in V-Ray 3.5 or 3.6 we changed the way the VRayMtl is exported.
The old 'layered' export was causing some troubles and was making it difficult to introduce some new features. So we changed it and is since exported as BRDFVRayMtl.
It seems that the old export is getting the proper reflections, but the new one is not.
While we are checking why the new export is doing this, you can switch to the old one. The export can be changed per material, so only the material on the sphere is done in the old way.
You'll need to set the opacity mode to 'normal', disable 'glossy fresnel' and make sure 'use legacy materials' is enabled in render settings > overrides > materials.

[damaggio]

Interesting , thanks Alex.

[thebeals]

That is very interesting. Thank you for the help!