I have a background picture showing a room, and a foreground object both in 3D-space. I'm trying to rebuild the room with solids (just roughly) and putting them in between the background and the foreground, so that the shadow coming from the foreground object looks believable. I tried to figure out if and how I can turn the solids invisible while at the same time keeping the shadows. With no luck so far. With lowering the opacity of the solid the shadows disappear also.
I tried the vanishing point feature in Photoshop, but it didn't work for me.
I'm supposing you are using lights in this 3d comp. Then you can turn the solids to cast shadows only in one of their property twirl downs. I don't have AE on at the moment but this is basically what you want to do.
You can also have some solids invisible to the lights if you want.