static BitSet used332Colors(BufferedImage img) {
  ret used332Colors(RGBImage(img));
}

static BitSet used332Colors(RGBImage img) {
  int w = img.w(), h = img.h();
  new BitSet bs;
  for y to h: for x to w:
    setBit(bs, ubyteToInt(to332(img.getInt(x, y))));
  ret bs;
}