first of all, i want to thank you for all your replies 
im going to try to explain myself the best i can. its a kind of what you are saying. i dont know if you could see the quicktime i told you but im going to give you the direct link to it:
http://www.gmunk.com/2001\_NYC\_update/FINN\_movie.mov
the thing is that he/she takes a video and can shift it from any part of it. but if i try to do the same what i get is that the video is moved to the right ( in case i shif for X positive values) and the weith of the video still the same. what i want is the shift to start, for example, from the middle of a video. what i do not want is to get is the image moved to the right ( it still shifts the very first colum of pixels of the left and have not done the sift the middle of it, right? )
i hope you could understand my question this time.
thanks a lot,
sergio