Denoising apha? (Shadow Catcher)

Is there some technical reason why alpha doesn’t get denoising?

Having shadow catcher is awesome, but one often has to avoid it, resorting to extracting shadows in the old way from a purpose-made scene-duplicate render layer, because otherwise sampling has to be cranked up 10x, to fix alpha that doesn’t get denoising.
While denoising against background works just fine without transparency.

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Have you tried to denoise the alpha with OIDN in the compositor?

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If the alpha is put through the denoiser separately from the image, it does apply denoising indeed and can be “SetAlpha” with the RGB image.
Still makes me wonder why alpha isn’t done at least in this way as an option, but it does seem to work. Thanks!

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Hi kivig, would you mind posting your node setup for this please?

Hi, it’s very simple - just run the alpha through a duplicate denoising node, as if it was a color, and use set alpha node to replace the original alpha. Cumbersome, but it’s something.

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It works! Makes no sense to me, but who cares if it works!:slight_smile: thanks mate

@kivig @martylog

Hey could you show me the node setup to denoise shadows please?
I didnt understand it.

Do you mean like this?

What I have to do with the Set Alpha Node?

Basically yes. But you can plug the image and denoised alpha in Image and Alpha inputs of “Set Alpha” node to combine them back. Then use output Image as you want.

Ah nice, now it works. Thank you :slight_smile: