Pixel-By-Pixel Image Comparison With IM.js
It scans the given images pixel-by-pixel using HTML5 canvas and outputs their similarity ratios.
The developer has created IM.js for auto-testing the similarity of URL snapshots taken from different browsers which is a great idea for quickly finding out any rendering differences or problems. It can also be used for many other cases like detecting duplicate images.
And, besides the ratio, it can create a canvas where the two different images are joined for easily seeing the differences at one sight.