GPU Raytracing Multipass Compositing Issue

[christian.woelfel]

Hello all!

I hope I have not overlooked the problem and would like to ask for your advice here:

I'm currently trying (for the first time) via GPU raytracing to linearly reconstruct the beauty pass in Photoshop using the combined passes, however it doesn't work as usual with CPU raytracing: the contrasts look a bit different and the brightness differences don't match the beauty pass.

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The following passes are used:

Combined Channels / Color Channels: Beauty (for comparison), Diffuse IBL/Light/Indirect, Glossy IBL/Light/Indirect, Specular Reflection, Transluency, Transparency Color, Incandescence, Background Color.

Export as Multi Layer .exr OR Single Passes as .exr -> Compositing in Photoshop 2021/2022 via Linear Add

No Tonemapping applied & 32 Bit Output Format.

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With CPU raytracing it works fine - now I wonder if GPU raytracing is not yet supported or if I am missing something? I use a RTX 3090 and VRED 2022.3.

Many greetings and thanks!

[michael_nikelsky]

This sounds like a bug, they are supposed to match. I will try to reproduce it but if you have an example exr or scene to show the differences it would be good.

One thing to make sure: Always disable clamping when doing compositing like this. Especially for the GPU raytracer this is important since it can not shot more than 1 ray for reflections compared to the CPU raytracer.

[michael_nikelsky]

Ok, just tried it in VRED 2022.3 and except for some noise differences, especially on the shadow material if the occlusion intensity is not set to 1, the results are matching between the beauty and the combined image. So I assume it must be maybe a specific material type that causes issues for you or some specific setting or the clamping if that is enabled.

By the way: You can also just write out a multilayer PSD in HDR format, then the layers are already properly setup and you don´t have to do it manually.

[christian.woelfel]

Thanks Michael!

I'll do another test by myself without any lights besides the hdri. It`s an interieur shot with some local lights and there are also some hard edges popping up - the light attenuation is always set to realistic. 

[christian.woelfel]

Tested it again without any lights besides the HDRI - it works so it seems that there are some issues with lights?

Thanks for the hint saving .psd files - works fine but I won't get access to the crypto masks. 

[michael_nikelsky]

I have tested it with some area lights and the results matched. Can you create an example scene that shows the issue?

[michael_nikelsky · Accepted solution]

Turns out there is an issue when enabling a cryptomatte renderpass while not having export alpha channel turned on and set to premultiply alpha. In that case the images will have non-premultiplied alpha which photoshop can´t handle correctly.

Enabling Export Alpha Channel and Premultiply Alpha fixes the issue.