static IBWImage posterizeIBWImage(int brightnessLevels, IBWImage img) {
  if (brightnessLevels >= 256 || img == null) ret img;
  int w = img.getWidth(), h = img.getHeight();
  
  ret new IBWImage {
    public int getWidth() { ret w; }
    public int getHeight() { ret h; }
    
    public float getFloatPixel(int x, int y) {
      ret (float) quantizeToNLevelsBetweenZeroAndOne_stretch(
        brightnessLevels, img.getFloatPixel(x, y));
    }
  };
}