dont know the video you are talking about, but maybe the keystone library helps
http://keystonep5.sourceforge.net/

Thanks. Indeed, I am using this excellent library. The point is that I will be projecting an image on an angled surface, then using a laser I will control keystone points, the laser has to be tracked by a web cam. But usually camera view is wider than the projector screen, or it's just never the same size, which causes mismatch of the coordinates. Thus, x,y of the camera has to be x,y of the screen projected, but it also has to be a 4 point calibration since the screen projected is not a perfect rectangle. For that basically I need to crop the camera view to the size of the screen projected and scale the coordinates. The tricky thing is that it's not only about width and height, but all four corners. I hope I am being clear.