I'm trying to create a shadow catcher on a ground mesh that is also transparent. The ground catches the shadow of items above it but allows the bottom part of a background image (also on a mash) to be seen. See the quick diagram below for a visual explanation.
I have the shadow catcher working but I cannot get it to be transparent ether in the viewport or in a render. I've tried to follow the example here Shadow Catcher makes background objects invisible separating my background and ground (shadow catcher) into separate collections (see image below) but no luck - what am I doing wrong?
I'm using Blender 2.83.