Matte Compositing, How to mask / cut out an object and background in the alpha channel

Hi, I am having a hard time at finding a solution to make a matte compositing on an object in real time.
Basically I want to make a mask on an object so that the matte/mask will be completely cut out of the background in the alpha channel in real time.

Any thoughts on how I can achieve this result with UE4?

This is for an Augmented Reality project, I need to mask the stage (walls and ceiling) of my production shooting and extend the room with so 3d background, everything in realtime.

Thanks a lot,
Romano.

Hi,
I have the same problem… did you find a solution for this?
Ib