CPU GPU Difference: Studio Setup with Light underneath the Floor works in CPU but not in GPU?

[olika]

Hi community,
i encountered some strange behavior i can't figure out whether it's intended engine behavior, my stupidness or maybe a buggy thing.

I have a fairly simple dark Studio Setup (sorry for the small render crops but everything is under NDA).

Because of the shape of the product i need to place a light underneath the floor to get the reflection on the product as i want it.
There is some action going on on the floor i cant make the floor refractive as you would do it in real life.

- Therefore i excluded the Floor from the Vray light underneath which works perfectly on CPU (you have to turn on Reflective Caustics in GI to make it work). See Picture 1

- But when i change to GPU with the same settings the bottom light is not showing up anymore in the reflections of the object. See Picture 2

- What irritates me is that you can see the setup behaving properly while calculating the lightcache. But when the renderpass starts its not there? See Picture 3

- No matter what combination i'm trying, ie. turn off cast shadows, visible for reflection etc. i just cannot not make the setup work like in CPU. See Picture 4

Vray Version is the latest: 4.30.02
I hope i explained my problem in an understandable way, please see the attached picture for more information.

I really really would like to get this solved, because RTX really plays out the power so it's rendering like 4 times quicker.
Would be so great to be able to utilize the GPU.
Otherwise it would be another animation i'm stucked to CPU because of little little things
Fingers crossing there's a solution and thanks in advance!



Greetings Olika

[hermit.crab]

I am uncertain if I understood the issue correctly. I did the following: a light underneath a plane and a sphere on top; unlinked the light from the plane and turned off the plane's 'cast shadows'. Is this correct or am I missing something? It would be really helpful to attach a simple scene with the issue recreated.

[olika]

Hi hermit.crab
Thanks for your fast reply and interest in that topic.
As i really would love to investigate and solve with you what's going on here i'll try my best to provide you with information and files!
Please see attachment where i put together another sheet to explain and both files for CPU and GPU.

Looking forward what you think about it, thanks!
Grüße Oli Kentner

[hermit.crab]

Thanks for the provided scene. To get the desired effect you would need to enable the "Affect Diffuse" on the bottom light since there is no Lighting information without it, hence nothing to (indirectly) reflect. To get the specular reflections through the plane, you would need to disable the "Visible in reflections" option in the plane's V-Ray properties. There might be a slight difference in GPU, however, it is due to the engine's functionality. Try the suggestions and report with feedback.