static RGBImage toGrayScale(RGBImage img, int bits) {
  int steps = 1 << bits;
  int w = img.getWidth(), h = img.getHeight();
  RGBImage bw = new RGBImage(w, h);
  for (int y = 0; y < h; y++)
    for (int x = 0; x < w; x++) {
      float b = img.getPixel(x, y).getBrightness();
      int bx = (int) (b*steps);
      double b2 = Math.min(1f, Math.max(0f, bx/(float) steps));
      bw.setPixel(x, y, new RGB(b2));
    }
  return bw;
}