// assumes coordinates are in range
static RGB ii_averageColorOfArea(IntegralImage img, int x1, int y1, int x2, int y2) {
  if (img == null) null;
  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));
}

static RGB ii_averageColorOfArea(IntegralImage img, Rect r) {
  ret r == null ? null : ii_averageColorOfArea(img, r.x, r.y, r.x2(), r.y2());
}

// later TODO: sub-pixel accuracy
static RGB ii_averageColorOfArea(IntegralImage img, DoubleRect r) {
  ret ii_averageColorOfArea(img, toRect(r));
}