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Hello, long time no see!

I've just gotten back into 3D again and noticed that when rendering this bedroom test, that caustics created by my wardrobe mirror doors are also reflected back into the mirrors when using GPU, but not when using CPU. It doesn't seem like a case of needing to wait longer for the CPU, it seems as though the GPU is just calculating extra bounces? Any ideas or explanation? Thanks!

Check your “console” tab for any kind of error.

GPU engine and CPU engine work a bit differently since some features are not implemented yet inside the GPU engine.

Have you tried CPU + Denoiser to see whether you can get a high enough SL that the caustics on the wall etc. aren't visible?

I'm not sure that you can get caustics from solid objects, only where there is transmission of light through a refractive surface. Mirrors reflect. Therefore, the CPU result is correct and what I would expect to see in an internal visual.

The GPU is rendering fireflies by the looks of things. Using the denoiser and Firefly remover option, it should hopefully get rid of this noise. Also, check in your scene to make sure there are no 100% white materials in the reflection 0 slot. Any bright textures you have can be clamped to a max of 230 if need be.

If I remember, there is an issue with CPU caustics in that they only show up clearly after SL20 or something?

CDRDA wrote: ↑

Mon Sep 05, 2022 11:58 am

I'm not sure that you can get caustics from solid objects, only where there is transmission of light through a refractive surface. Mirrors reflect. Therefore, the CPU result is correct and what I would expect to see in an internal visual.

Yes, you're right - I thought it sounded odd when I sent it, but in a rush and it had already been sent

I think is a consequence of different algorithm. Production uses BPT and GPU (and Draft) uses PT. BPT may have problem conecting paths when both the light and the camera rays have to refract or reflect.

Hi again, sorry for the delay getting back to this.

It actually was just a matter of the different algorithms converging on the result at different speeds. Here after a much longer render the CPU has caught up a lot more with the GPU result:
Caustics is generated from both reflective and refractive surfaces. It's really just the same type of light bouncing as with any surface, but in a more concentrated direction as opposed to a diffuse/blurry interaction.

I'm glad the two engines end up agreeing!