The easy way the Average difference in pixel values. Slightly harder psnr. http://en.wikipedia.org/wiki/Peak_signal-to-noise_ratio http://en.wikipedia.org/wiki

The easy way the Average difference in pixel values. Slightly harder psnr. http://en.wikipedia.org/wiki/Peak_signal-to-noise_ratio http://en.wikipedia.org/wiki/Peak_signal-to-noise_ratio Harder again... ssim http://en.wikipedia.org/wiki/Structural_similarity http://en.wikipedia.org/wiki/Structural_similarity There isn't any real solution to automating similarity as a human sees it. Humans are tricky.