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?