srecord noeq PosterizeBWImage(int brightnessLevels, BWImage img) is PixelPreservingImageOpResult<BWImage> {
  BWImage result;
  
  run {
    if (brightnessLevels >= 256 || img == null) ret with result = img;
    byte[] pixels = img.getBytes();
    int w = img.getWidth(), h = img.getHeight(), n = pixels.length;
    byte[] pixels2 = new[n];
    var posterizer = cachePosterizer(SinglePixelPosterizer(brightnessLevels));

    for i to n:
      pixels2[i] = (byte) posterizer.get(ubyteToInt(pixels[i]));
  
    result = new BWImage(w, h, pixels2);
  }
  
  public BWImage pixelPreservingSrcImage() { ret img; }
  
  BWImage get() { if (result == null) run(); ret result; }
}