Programming photoshop difference filter

[defunktlemon]

Hi:
I'm gonna need some steady help over the next 3 weeks, if anyone wishes to be in on this for the long hall. I have to program a version of the Photoshop Difference Filter. To be honest, I'm not sure where to start with it. I guess I need to look at some examples of similar filtering programs, find out how they are working, etc.
Any starting tips appreciated
Thank you