pxdoppelganger

https://github.com/otto-de/pxdoppelganger

pXdoppelganger compares two images and can tell you the exact difference (in % of pixels changed). If you compare two screenshots of you app before and after a release, it will help you to automate your design regression tests. It follows the suggestions of image comparison by in Jeff Kreeftmeijers blog: jeffkreeftmeijer.com/2011/comparing-images-and-creating-image-diffs/