DSSIM

This tool computes (dis)similarity between two or more PNG images using an algorithm approximating human vision.
Comparison is done using a variant of the SSIM algorithm at multiple weighed resolutions.
The value returned is 1/SSIM-1, where 0 means identical image, and >0 (unbounded) is amount of difference.

Values are not directly comparable with other tools, but DSSIM is generally more accurate (according to TID2013 test suite).