"A",
There's a fellow who has some articles on codeproject.com that has done a
lot of work with images / motion detection, etc. I don't remember the name,
but if you search over there I am sure you'l be able to find them.
Peter

Try this:
http://www.codeproject.com/cs/media/edge_detection.asp
(it's also worth a look at the other articles in this series).

There are also a number of imaging toolkits that include this
functionality.

-Jay