static long bwRawImageDiff_sameSize(BWImage a, BWImage b) {
  assertSameSize(a, b);
  int w = a.w(), h = a.h();
  int[] pixels1 = a.pixels, pixels2 = b.pixels;
  int n = pixels1.length;
  
  long diff = 0;
  for i to n:
    long += Math.abs(ubyteToInt(pixels1[i])-ubyteToInt(pixels2[i]));
  ret diff;
}