Alpha channel cut (and request).

[piotrglabski]

In our team, we have had the same workflow applied to hundreds of scenes for about 15 years. It consists of saving the rendering in one file and then using the alpha channel in photoshop.
Recently GPU rendering is prevailing and the lack of a rendered alpha channel is becoming more and more of a problem because it requires additional steps.
Is it somehow possible to get the same effect with GPU rendering while preserving the presented scene setup, i.e. so that VRB receives the alpha channel in one pass.
Is this something that can be added as a feature request? Or is this technically impossible to achieve in the Vray engine?
If this is the wrong channel please correct.

[hermit.crab]

Not sure if I understand. Based on the graphics, the sphere should contribute to the alpha in GPU too, and in fact, it does. There must be something else causing the differences. What host platform are you using? Better recreate the setup in 3d and attach the file here so we can take a look.

[piotrglabski]

Hi,
Basic setup: an object against the background with alpha contribution set to 0.
With Vray 7 alpha is present within VB
Switching to Vray GPU - alpha is NOT present.
Objective is to have alpha cut.
(I'm not allowed to attach *.max files, I'm attaching zipped file).

[hermit.crab]

Okay, just as I thought. Well, the Alpha Contribution parameter is not yet supported on the GPU, however, we do have an improvement request (internal tracker id: VGPU-2828 ). I'll add a bump on your behalf. For now, I suggest you use, f.e, VRayCryptomatte to create masks for whichever object you'd like.

[piotrglabski]

Okay, just as I thought. Well, the Alpha Contribution parameter is not yet supported on the GPU, however, we do have an improvement request (internal tracker id: VGPU-2828 ). I'll add a bump on your behalf. For now, I suggest you use, f.e, VRayCryptomatte to create masks for whichever object you'd like.

Thank you for your answer.
Could you give me some directions how to use VrayCryptomatte to achive such an effect?
It is a Render Element, is that it? So it doesn't solve the problem, because there is separate channel/picture and cannot be saved to one file, like an alpha channel with tiff/tga/ etc...
The goal is to have an image with alpha channel and it can be save as a one file.
With non-GPU it is possible and quicker.

[hermit.crab]

It is a Render Element, is that it? So it doesn't solve the problem, because there is separate channel/picture and cannot be saved to one file, like an alpha channel with tiff/tga/ etc...
The goal is to have an image with alpha channel and it can be save as a one file.
With non-GPU it is possible and quicker.

Yes, it's a render element - you simply add it and render. Afterward, in your post (f.e. Nuke or Photoshop with the EXR-IO plugin), you use it to create masks. Note that this works for multi-channel .exrs only.
If you insist on using formats such as .tiff, .tga, .png, you might want to use another Matte element - MultiMatte. I'm attaching a script I wrote a while back for custom mask creation. You set the objects you want as a mask on a Layer, run the script, choose the layer, and hit the create button - the script will create a MultiMatte for the objects in the layer. Render and use the MultiMatte instead of the Alpha in post.

I don't think there's a way to do this without some adjustments in post.