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There are two ways that I know I can generate contact shadows in Blender Octane: first - with shadowcatcher, second - with render layers.

Unfortunately, I have issues with both methods.

Shadowcatcher

While the setup is really easy and you get a proper Shadow AOV you can comp in later, there are issues with this approach.

Let's have a simple scene with normal surface to place our object.


As soon as we apply the Shadowcatcher to our surface and comp the image with the pure white background it's instantly loose believability since the object doesn't receive bounce reflections from the surface it's standing on. Standard Surface materials still doesn't have Shadowcatcher option and surfaces with Shadowcatcher on also can't be included/excluded with light linking.



Ok, so I can try to use another method which in theory should work perfect.

Contact Shadows AOV

Well, there is no really such a thing in Octane but let's call it that for simplicity.

I don't like this setup since it's way too many nobs you need to turn on to make it work.

So we need to use Render Layers. So we disable shadow catcher for our surface. We set our surface object Render Layer ID to 2 and enable Global Render Layers. Enable use of Alpha in the render settings. Now, we add Black Layer Shadow AOV to create a proper contact shadow. We won't lose the reflections and proper environment bounces on our main object and get a good Contact Shadow AOV.

BUT, as soon as we comp everything on the pure white background again I see a ton of ugly noise. No matter how much samples I shot it still looks this bad.



Is this the expected behavior? Or am I missing something with both setups?



In general Shadow AOV produces a ton of harsh noise. Including colored noise. Is this on purpose? If so, why?

Why not using the Shadow AOV directly?

That's the output, no need for Shadow Catcher or Layers.

Also, in my test using Render Layers, even in combination with Shadow Catcher is producing a correct result.

linograndiotoy wrote:Why not using the Shadow AOV directly?

I was testing this approach and it wasn't working for me. The shadow pass produced sever colored noise. I am not sure why.

I tried to debug the scene but so far can't figure it out why Shadow AOV does this in my test scene.

The very basic scenes like yours seems to work as expected. I tried it and it works fines but then there are some conditions that makes Shadow AOV produce this strange noise. I haven't figure out why yet.

If it worked for me reliably, it would solve my issue with having the contact shadow in a separate pass.

Is there a way to send my scene to you to have a look as well?

Shadow catcher material is only for shadow. It does not contain any reflection or diffuse data.
At the moment the only solution would be to use render layers as Lino has suggested.

AlexeyAdamitsky wrote:
Is there a way to send my scene to you to have a look as well?

If the scene size is not big, you can attach it to a reply using the Upload Attachment option.

In alternative you can use any upload service out there (Dropbox, Mega...).

I can confirm that it's the same story also in Houdini, version 2022.1.2.0 and 2022.1.1.1. Any kind of scene will do for proving this, even a simple cube and a plane. The shadow pass will contain black and white dots and/or multicolored noise. No way to get rid of it and is not fixable in comp. Reverting to 2022.1.0.6 does not have this bug and fixed the issue for me. It's a major problem in the last two versions and should fixed ASAP.

ghost3d wrote:I can confirm that it's the same story also in Houdini, version 2022.1.2.0 and 2022.1.1.1. Any kind of scene will do for proving this, even a simple cube and a plane. The shadow pass will contain black and white dots and/or multicolored noise. No way to get rid of it and is not fixable in comp. Reverting to 2022.1.0.6 does not have this bug and fixed the issue for me. It's a major problem in the last two versions and should fixed ASAP.

What about 2023 version?

J.C wrote:

ghost3d wrote:I can confirm that it's the same story also in Houdini, version 2022.1.2.0 and 2022.1.1.1. Any kind of scene will do for proving this, even a simple cube and a plane. The shadow pass will contain black and white dots and/or multicolored noise. No way to get rid of it and is not fixable in comp. Reverting to 2022.1.0.6 does not have this bug and fixed the issue for me. It's a major problem in the last two versions and should fixed ASAP.

What about 2023 version?

I am Houdini user and i came from the Houdini DCC subforum where i tried to find a solution to my problem in previous threads. I found the exact same problem here, in the Blender subforum. I couldn't find a 2023 Houdini DCC release there to test , unfortunately.