 <p>I assume you're talking about something like <a href="http://rogeralsing.com/2008/12/07/genetic-programming-evolution-of-mona-lisa/" rel="nofollow noreferrer">Roger Alsing's program</a>.</p> <p>I implemented a version of this, so I'm also interested in alternative fitness functions, though I'm coming at it from the perspective of improving performance rather than aesthetics. I expect there will always be some element of "fade-in" due to the nature of the evolutionary process (though tweaking the evolutionary operators may affect how this looks).</p> <p>A pixel-by-pixel comparison can be expensive for anything but small images. For example, the 200x200 pixel image I use has 40,000 pixels. With three values per pixel (R, G and B), that's 120,000 values that have to be incorporated into the fitness calculation for a single image. In my implementation I scale the image down before doing the comparison so that there are fewer pixels. The trade-off is slightly reduced accuracy of the evolved image.</p> <p>In investigating alternative fitness functions I came across some suggestions to use the <a href="http://en.wikipedia.org/wiki/YUV" rel="nofollow noreferrer">YUV colour space</a> instead of RGB since this is more closely aligned with human perception.</p> <p>Another idea that I had was to compare only a randomly selected sample of pixels. I'm not sure how well this would work without trying it. Since the pixels compared would be different for each evaluation it would have the effect of maintaining diversity within the population.</p> <p>Beyond that, you are in the realms of computer vision. I expect that these techniques, which rely on feature extraction, would be more expensive per image, but they may be faster overall if they result in fewer generations being required to achieve an acceptable result. You might want to investigate the <a href="http://pdiff.sourceforge.net/" rel="nofollow noreferrer">PerceptualDiff</a> library. Also, <a href="http://www.lac.inpe.br/~rafael.santos/JIPCookbook/6050-howto-compareimages.jsp" rel="nofollow noreferrer">this page</a> shows some Java code that can be used to compare images for similarity based on features rather than pixels.</p>