static RGBImage subtractRGBImages(RGBImage image, RGBImage image2) {
  int w = image.getWidth(), h = image.getHeight();
  RGBImage out = new RGBImage(w, h);
  for (int y = 0; y < h; y++)
    for (int x = 0; x < w; x++) {
      RGB a = image.getRGB(x, y), b = image2.getRGB(x, y);
      out.setPixel(x, y, fixRGB(a.r-b.r+0.5f, a.g-b.g+0.5f, a.b-b.b+0.5f));
    }
  ret out;
}