uprightdiff is a utility that examine differences between two images. It is used as part of the Parsing Team's visual diff testing project.
It is optimised for images which come from browser screenshots. It analyses the image for vertical motion, and annotates connected regions that have the same vertical displacement. Then it highlights any remaining ("residual") differences which are not explained by vertical motion on a pixel-by-pixel basis.
The README has instructions on how to build the binary and then use it.