You can do a shadow catcher using a shadow pass and different render layers to create an alpha channel for the scene:
Move the plane object (shadow catcher) to layer2.
Make a copy of the other objects and move them to layer 2.
The idea is that on layer 1 you only have your objects but not the plane. And on layer 2 a copy of your objects and the plane that will act as shadwow catcher.
- On all of the objects in layer 2, except for the plane, disable Ray visibility for camera. We just want the plane to be visible, the other objects should be transparent but we keep the shadows they project on the plane:
- Make sure your lamps and camera are active in layers 1 and 2.
Set two render layers:
One for the objects in layer one that excludes information from layer 2.
A second render layer for the shadows that includes a shadow pass and excludes layer 1.
- Enable Film. Transparent.
- In the compositor combine the alpha channel of the object's layer and the inverted shadow pass to create a new alpha channel for the scene, this one will include the shadows as part of the alpha.
You can control the transparency of the shadows by changing the color on the empty socket of the mix node. If you make that color white the shadows will be fully opaque, if you make it black you'll have no shadows.
