static int binaryImagePixelDifference_sameSize(Image2B img1, Image2B img2) {
  assertSameSize(img1, img2);
  byte[] pixels1 = img1.pixels;
  byte[] pixels2 = img2.pixels;
  int n = pixels1.length;
  int diff = 0;
  for i to n:
    diff += popCount(pixels1[i]^pixels2[i]);
  ret diff;
}