Superimpose / Matte Subtraction

[haid]

Hey all,
I've got a video clip of people walking in a main area. In the first frame of the clip, there is no one in this area, but as the clip progresses, people begin to appear and walk through the shot. How can I use the difference matte in a compositing program (specifically After Effects 6.0) to make it so that the people are walking in front of a transparent background? Keep in mind that the first frame has no one in it. Can I use this frame to almost "subtract" the "environment" on which the people walk in subsequent moments within the clip, thus projecting them elsewhere?

Thanks.

 

[Will Vincent]

So, basically you're trying to do a travelling matte (typically done with chroma keying, blue/green screen) from this footage?

Well.. I suppose theoretically assuming the camera is locked off and nothing in the background changes then it could work... Though I'm not sure how straightforward it will be, especially without knowing what the shot looks like.