Okay: in my comp I have a guy standing up not really moving green-screened, a background, and ants moving (also green-screened). I want the footage of the ants to crawl only on the guy, and not the rest of the video. So, how do I create a matte that moves with the guy's movements and blocks the ants from moving onto the background? Would I even need to create a matte? I'm a newb.. I hope someone can help. Thanks!
Key out the guy (Layer 1).
Place ants above it and key them out (Layer 2).
Duplicate the keyed out guy (Layer 3) and place it above the ants.
Set the "Track Matte" of the ants layer (Layer 2) to Alpha Matte.