static L<RGBImage> rgbImageToFixedSizeTiles(RGBImage img, int tileW, int tileH) {
  new L<RGBImage> tiles;
  int w = img.getWidth(), h = img.getHeight();
  for (int y = 0; y + tileH <= h; y += tileH)
    for (int x = 0; x + tileW <= w; x += tileW)
      tiles.add(rgbClip(img, Rect(x, y, tileW, tileH)));
  ret tiles;
}