static RGB rgbAverageColor(RGBImage img) {
  int w = img.w(), h = img.h();
  double r = 0, g = 0, b = 0;
  for y to h: for x to w: {
    RGB rgb = img.getPixel(x, y);
    r += rgb.r;
    g += rgb.g;
    b += rgb.b;
  }
  int n = w*h;
  ret new RGB(r/n, g/n, b/n);
}