[findamir]

Hello,
I have set up a scene that consists of a flat building (nothing fancy) a helicopter in front of it and the helicopter is on the ground. The helicopter shadow casts on building and ground. However, I do not need the building and ground for the final render (because I have real footage of ground and building), but for them to ONLY receive shadows. I tried hiding the building and ground and through my render, the helicopter shadows disappear when I look in the Alpha channel. I only see alpha of helicopter and no shadow that is "thrown" on building and ground.
Please help if you can,
Thank you,
Amir