static BWImage saturationChannel(BufferedImage img) {
  if (img == null) null;
  int w = img.getWidth(), h = img.getHeight();
  BWImage img2 = new(w, h);
  for y to h: for x to w:
    img2.setPixel(x, y, (float) saturationFromRGB(img.getRGB(x, y)));
  ret img2;
}

Began life as a copy of #1020182