From Brett Slakin's dpxdt + ImageMagick

ImageMagick Diffing

  # Method from http://www.imagemagick.org/Usage/compare/
        return [
            'compare',
            '-verbose',
            '-metric',
            'RMSE',
            '-highlight-color',
            'Red',
            '-compose',
            'Src',
            self.ref_path,
            self.run_path,
            self.output_path,
        ]


http://www.imagemagick.org/Usage/compare/
http://www.imagemagick.org/script/compare.php

compare base.png update.png -verbose -metric RMSE -highlight-color Red -compose Src output.png

http://www.selenic.com/hg/help/revsets