static L<Int> uniqueBrightnessesInBWImage(IBWImage img) {
  if (img == null) null;
  new BitSet bs;
  int w = img.getWidth(), h = img.getHeight();
  for y to h: for x to w:
    uniqueColorsInBWImage.set(img.getInt(x, y));
  ret bitCount(bs);
}