static int compareBWImagesByScanlineOrdering(BWImage a, BWImage b) {
  assertSameSize(a, b);
  int w = a.getWidth(), h = a.getHeight();
  for y to h:
    for x to w: {
      int pix1 = a.getInt(x, y), pix2 = b.getInt(x, y);
      if (pix1 != pix2) ret pix1-pix2;
    }
  ret 0;
}