static RGBImage integralImageToRGBImage(IntegralImage img) {
  if (img == null) null;
  int w = img.w, h = img.h;
  RGBImage out = new(w, h);
  for y to h:
    for x to w: {
      int r = integralImage_sumRect(img, x, y, x+1, y+1, 0);
      int g = integralImage_sumRect(img, x, y, x+1, y+1, 1);
      int b = integralImage_sumRect(img, x, y, x+1, y+1, 2);
      out.setPixel(x, y, r, g, b);
    }
  ret out;
}