I've posted about this problem before, but wanted to make my post more concise and to upload an image (I'd upload a project, but it appears that I can't?) since i've received no reply.
I have three layers: a light, a 3D shape layer, and a shadow layer--a 3D solid set back in z-space with the "accept shadows" switch on "only."
The solid layer receives shadows, but does not become transparent. When I turn on the transparency grid, I don't see it--I see the bounds of the solid layer, which has turned all white (regardless of its color setting).
When I place something behind my shadow layer, the shadow layer shows it through, with the shadows on top.
My question: is there no way to render out the shadow layer with an alpha channel? I swear I've done this before. Is this a bug? Am I doing something wrong? I really need some help!
You use a white layer for a shadow only layer and then set the blend mode to multiply.
Another option is to use set channels to generate an alpha but this would not composite in a scene as well as using multiply.