[ale] image diff utility?

[christopher at bergeron.com (Christopher Bergeron)]

Does anyone know of any command line utilities that can compare two 
images (jpg, pngs, whatever), and if they visually differ by a set 
threshold they will output a true or false flag?

I've seen this done in big bloated security applications, but I'm 
looking for a simple command line utility or method of doing something 
like this:

if (image1.png) is more than n percent different from (image2.png), exit 
with a true; if not, exit with a false

Heck, I don't even need to have an adjustable threshold; I just need to 
know if 2 images are significantly different.  Does anyone have any 
ideas on how I could accomplish this?

Thanks in advance!
-CB