He answers this on the FAQ linked at the top. http://rogeralsing.com/2008/12/09/genetic-programming-mona-lisa-faq/ http://rogeralsing.com/2008/12/09/genetic-pro

He answers this on the FAQ linked at the top. http://rogeralsing.com/2008/12/09/genetic-programming-mona-lisa-faq/ http://rogeralsing.com/2008/12/09/genetic-programming-mona-l... He just loops over the whole image and does a pixel-by-pixel comparison, taking the difference between each of the R, G, and B channels and summing them all.