// assumes coordinates are in range
static RGB ii_averageColorOfArea(IntegralImage img, int x1, int y1, int x2, int y2) {
  int area = (x2-x1)*(y2-y1);
  int r = integralImage_sumRect(img, x1, y1, x2, y2, 0)/area;
  int g = integralImage_sumRect(img, x1, y1, x2, y2, 1)/area;
  int b = integralImage_sumRect(img, x1, y1, x2, y2, 2)/area;
  ret RGB(rgbInt(r, g, b));
}