Arnold GPU Rendering with Alpha (Transparency)

[darkmaister]

Hi, i'm trying to render out an image and it's not carrying the alpha channel and has no transparency in GPU. However, when I switch to CPU it renders properly with alpha. Is this not a supported feature yet?

[aaronfross]

Hello... it appears that alpha channel rendering is not supported on the GPU. This is an absolutely critical feature. I was utterly shocked when I discovered that a completely blank 3ds Max scene renders with opaque alpha in Arnold. At first I thought it was just that an environment map was not rendering as transparent alpha. But it does appear that alpha is not supported at all on the GPU.

Please address this issue immediately. It is of absolute top priority. Much higher priority that anything I can think of. GPU rendering is not at all useful for a real production environment if alpha transparency is not supported.

Thank you.

[Stephen.Blair]

That's because there's a default background.

If I change the Background to Custom Map and leave the Custom Map slot empty, then I can get the alpha of the objects in the scene.

GPU rendering isn't production ready, it's still in beta.

[aaronfross]

Thank you, Stephen, that's very helpful. However, this works differently than rendering on the CPU. It should work the same. Users are not going to be able to discover this on their own.

[aaronfross]

Also, this does not help in the use case where the user wants an environment map to be visible in RGB channels, but invisible to alpha. That's a common scenario, it's the default behavior in 3ds Max, and that's how it works when rendering on the CPU.

[Stephen.Blair]

We have a ticket for this issue.

[Anonymous]

Hello. Is this issue fixed now? I do have the same problem and its quite a big deal.

[darren.d.floyd]

I'm just running into this issue now.  I'm in maya 2018 and finding that my old workflow of batch exporting as .png with alpha (that still works outside of arnold) is borked by arnold:  .pngs from arnold don't have alpha channels at all; .tif files have the alphas, but they don't work properly due the the light dome being rendered in the scene as a white background.

This can be fixed by selecting the default light, and in the dome node, under visibility, setting camera to 0.  Doing this and rendering .tif files with the basic settings has produced image sequences with alphas like the old .png workflow does when using MayaVector.

[helisgrafik]

When in glass material Cutout (Opacity) -> OSL: Color Value -> Gray color

[helisgrafik]

When in glass material Cutout (Opacity) -> OSL: Color Value -> Gray color