Giving a Geometry group a Render Layer ID Tweet

[treddie]

Ok, that fixed it. But now I am confused. Turning shadow catcher off, leaves me with an opaque surface with cast shadows. That is ok, since I can composite in PShop with the shadow layer in multiply mode. But it would be nice to have shadows on a transparent shadow-catching surface. Octane does this, but it stops updating after the first pass. At any rate, I thought you HAD to enable shadow catcher to get a shadows-only surface (all other objects invisible). What then is the purpose of the shadow catcher option?

[bepeg4d]

You need to enable the Render Layer option, and also enable Alpha channel in Kernel setting, to make the plane with different layer ID transparent.
The Shadow catcher option was added before the Render Layer option, and should be used only for simple scenes.
ciao beppe