image recogniton?

[Joe Strout]

Li Han wrote:

> Sorry, I oversimplified the question because of my poor english. It is
> an analog compass whose value we need to read  into the computer every
> second. We use a video camera keep shooting it, and the compass and
> camera are  fixed.

If you have any choice about it, it would be greatly simpler, cheaper, 
and more effective to throw out the camera and analog compass, and use 
an electronic compass instead.  These are affordable and work quite well 
provided you keep them level (the same constraint you have with an 
old-fashioned compass too).  You can make some simple interface 
hardware, or spend a bit more for something with a USB or serial 
interface built in, like this: 
<http://www.oceanserver-store.com/osevkitsorus.html>

Best,
- Joe