Regression testing image processing algorithms.

I’ve used them for regression testing web apps:

https://github.com/python-needle/needle

Depending on your needs, detecting duplicates can also be useful – especially if you’re using some kind of CV tool which quickly assesses similarity and you want a second check to confirm that the highly similarity matches aren’t hitting some edge case failure mode.

One challenge is that you sometimes want to allow minor differences - think things like text aliasing or slight differences in the pixel values produced by different compression/color handling/etc. implementations - so there are some neat tools like pdiff which try to emphasize differences which the human visual system is sensitive to:

https://github.com/hectorgon/perceptual-diff

Visual regression testing is a common example, especially with websites.

I've put a few simple examples in Node.js here: https://github.com/umaar/learn-browser-testing/tree/master/3... (just as learning material)

Automated testing of a rendering engine.

Diff testing UIs.

judging the effectiveness of an exercise or supplement program by comparing before and after images of a participant.