static L<Pt> pixelsWithNonZeroSaturation(BufferedImage img) {
  if (img == null) null;
  int w = img.getWidth(), h = img.getHeight();
  new L<Pt> out;
  for y to h: for x to w:
    if (saturationFromRGB(img.getRGB(x, y)) != 0)
      out.add(pt(x, y));
  ret out;
}