NEW IN R18
Shadow CatcherShadow Catcher Assign Illumination
To be clear upfront: The Shadow Catcher shader is simply a much easier and faster method of achieving the same effect that can also be achieved using other methods, e.g., with the Compositing tag, multi-pass tricks, etc.
In short, the Shadow Catcher shader integrates shadows and, if desired, reflections into the Alpha channel.
Only shadows and/or reflections from objects to which shaders are assigned will land in the Alpha channel (the remaining space in the Alpha channel will be black).
This makes it easy, for example, to insert rendered objects in photos in a compositing program, i.e., insert them into a two-dimensional background (back plates):
The following conditions must be met:
Note that GI effects cannot be taken into account by the Shadow Catcher with regard to shadows since it only works with shadows from light sources.
The Shadow Catcher cannot exclude itself from self-shadowing. To do so, the Self Shadowing option must be disabled in the Compositing tag.