Noise in alpha in V-Ray GPU

[simon_lundberg]

I have issues with noisy alphas through transparency in V-Ray GPU. I set up a simple scene with a sphere and a sheet of glass in front of it (using the "Glass" VrayMtl preset from the dropdown). When rendering with just one sample per pixel, the alpha is a completely solid 1.0000 in all the areas seen directly by the camera, while parts seen through transparency are all over the place. This is not a problem in V-Ray; only in V-Ray GPU.

This noise doesn't really make sense to me. V-Ray CPU and Cycles (branched) both like expected from a branching path tracer with a single sample per pixel. mPath and Cycles in PT mode both look like I would expect from a non-branching path tracer. I don't understand why V-Ray GPU assumes that paths that bounce off the glass have almost-zero alpha. This feels like a bug to me. With one sample per pixel, each pixel alpha should be either exactly one or exactly zero. Values in between shouldn't be possible.

This causes some problems in our render pipeline because we're doing some post-processing that makes assumptions about the alpha. Is there a workaround for this strange behaviour?

[vladimir_krastev]

Hi, thanks for posting. The issue is reproducing on my end. I have reported it under VGPU-6309 in our internal system for review.

[simon_lundberg]

Thanks, Vladimir! No known workarounds I suppose? I'll try to solve it with image filtering in the meanwhile.

[vladimir_krastev]

I tried to clear it out with denoiser but no luck. If you raise the rays per pixel a lot, the noise is almost entirely gone.