How to make a video follow a man?

    Oct 22 2011 | 11:12 am
    at first i'm a newbie in this software. I'm Thai people and interesting about new medias.
    Now I've an art project for install in gallery. In my project, I want to project the sun images over everybody that walk in front of my wall like this :
    There is a white wall and when people walking pass or stand around there, My projector will be project images of sun (.png or video) over their heads. and wherever that they wanna walk that sun will follow them until they will walk out from my wall.
    so, now I can get realtime video from my camera already. and I change brightness/contrast/saturation for make image from camera to black and white, because it have to make it to easy to track sun video.
    Now how to specify a video position on people head and make it follow them???