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I am facing some issues with the shadow-catcher and denoiser in Blender 2.79. Denoising doesn't seem to have any effect on shadow catcher shadow. If I turn off shadow catcher on ground plane the desnoising works well. But if it's turned on - the shadow stays noisy.

Also the shadow become much darker with shadow catcher turned on.

Am I doing something wrong? How do I get noise free contact shadow and lighter shadow like the left render?

Shadow-catcher is noisy even with denoiser also very dark Here is the .blend file of the scene in case someone wants to inspect it: http://pasteall.org/blend/index.php?id=48592

Will really appreciate any help.

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  • $\begingroup$ Please don't use pasteall.org to host files. Links on that site expire after some time and users that might face a similar issue will not be able to learn from your file once it is not available anymore. Please use blend-exchange.giantcowfilms.com to share files used for questions and answers. $\endgroup$
    – user1853
    Dec 26, 2017 at 15:45
  • $\begingroup$ Denoiser is not a substitute for higher number of samples. 128 samples is not a lot. $\endgroup$
    – user1853
    Dec 26, 2017 at 15:53
  • $\begingroup$ Thank you cegaton. Updated the .blend file link. I know 128 sample isn't enough but - the question is not about how to remove noise - it's about how to get denoising work on shadow catcher shadows. Also I the shadow catcher shadow doesn't match the no-shadow catcher ground plane - it's much darker. These seem to be some important issues that anyone can face... $\endgroup$
    – saicode
    Dec 27, 2017 at 6:10
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    $\begingroup$ You can't composite a shadow using a nonlinear application. Further, the proper operation should be a multiply against your background. $\endgroup$
    – troy_s
    Dec 27, 2017 at 6:20
  • $\begingroup$ Thanks Troy. Yes I thought non-linear application could be a reason for the dark shadow as I am using PNG format. It's in multiply mode against a white background. I am not sure how to control the intensity of the shadow as the shadow is in the same layer as the object. Also I follow similar shadow catcher workflow with 8Bit/16Bit PNG format in other software (Modo) but it doesn't look so dark compared to solid background. $\endgroup$
    – saicode
    Dec 27, 2017 at 6:44

2 Answers 2

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It's because denoising works only with RGB-channels, not alpha-channel. You can try to denoise it in compose nodes.

I use Bilateral Blur to denoise alpha channel in render, then set it in result picture by Set Alpha node. Extract ground plane by object ID mask:

enter image description here

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    $\begingroup$ In Blender 2.90, passing the alpha channel through a denoise node cleans the shadow up. $\endgroup$
    – Ajay
    Feb 15, 2021 at 20:08
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If you know what your background will look like later on you can change the sky to that colour. Then the shadow will be denoised. However, this won't have alpha then.
I use that trick very often because I mostly just need a white background but for your application it may vary.

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