Yet another solver for the MintEye captcha.

captcha.py

import Image
import ImageChops

def diff_images(img_a, img_b):
    # return sumOfAllPixelValues(abs(img_a - img_b))
    return sum(map(sum, ImageChops.difference(img_a, img_b).getdata()))

def solve_captcha(filenames):
    filenames = sorted(filenames)
    images = map(lambda x: Image.open(x), filenames)

    # precompute differences between all neighbouring images
    pre_diffs = []
    for i in range(len(images)-1):
        left, right = images[i:i+2]
        pre_diffs.append(diff_images(left, right))

    # compare each image (except first and last) to its neighbours
    diffs = {}
    for i in range(len(pre_diffs)-1):
        left, right = pre_diffs[i:i+2]
        diffs[i+1] = left-right

    # get extreme values
    max_diff = max(diffs, key=diffs.get) # usually right of solution
    min_diff = min(diffs, key=diffs.get) # usually left of solution

    # solution somewhere in center
    if min_diff == max_diff-2:
        return filenames[min_diff+1]

    # solution in the last two images, no max_diff
    elif min_diff >= len(diffs)-2:
        return filenames[min_diff+1]

    # solution in the first two images, no min_diff
    elif max_diff <= 2:
        return filenames[max_diff-1]

    # no solution
    else:
        return None

def main():
    for captcha_num in range(17):
        filenames = range(30)
        filenames = map(lambda x: "%02d_%02d.jpg" % (captcha_num, x), filenames)

        solution = solve_captcha(filenames)
        if solution is None:
            print "failed to solve %02d" % (captcha_num,)
        else:
            Image.open(solution).save("solved_%02d.jpg" % (captcha_num,))
            print "solved %02d: %s" % (captcha_num, solution)

if __name__ == "__main__":
    main()