docs: def diff_badness(diff):
      """Estimate the "badness" value of a difference map.
  
      Returns 0 if the pictures are identical
  
      Returns a large number if the pictures are completely different
      (e.g. a black field and a white field).  More specifically, returns
      ``255 * width * height`` where ``(width, height) == diff.size``.
  
      Returns something in between for other situations.
      """
      # identical pictures = black image = return 0
      # completely different pictures = white image = return lots
      return sum(i * n for i, n in enumerate(diff.histogram()))