Need Advice about image extraction before starting

[GlenStewart]

I want to extract a portion of an image, then super impose that extraction over a webcam feed. Allowing the webcam feed to be the background and the extracted portion to be the foreground. Thirdly, I want to have the extracted image to follow a persons movement. I am thinking I will need to use simpleCV for that but i am hoping to do it using image magick. Is this all possible or only partly with this software? Thanks in advance for any constructive advice.

[snibgo]

Given frames of a video, IM can isolate a person moving. The basic idea is to find difference between adjacent frames or (better) to find differences between frames and a known frame that has no people. It can then superimpose the isolated person on the frames of the same or different video.

However, it is too slow to do this in real time. A program like ffmpeg "knows" about movements between frames, because that data is used in video compression, and using that data would be very much faster.