compare images with tolerance at specific regison

[manmanda]

I want to automate compare images at specific region but with a user specified tolerance (ex. 20% diff is accepted).If my comparison result is within that tolerance then i will make my comparison pass otherwise fail. How to achieve that?

[snibgo]

With a script. You can crop two image with "convert", compare these with "compare", and do whatever you want with the result.

You can crop and compare in a single "convert". However, the result can't (as far as I know) be used in an fx: expression, so you still need the script for your 20% pass/fail decision.