ifclass RGBImage
static BWImage toBWImage(RGBImage img) {
  ret toBW(img);
}
endif

static BWImage toBWImage(BufferedImage img) {
  ret img == null ?: new BWImage(img);
}

static BWImage toBWImage(MakesBufferedImage img) {
  if (img == null) null;
  if (img cast BWImage) ret img;
  ret new BWImage(toBufferedImage(img));
}