// assumes coordinates are in range
static int ii_getPixel(IntegralImage img, int x, int y) {
  if (img == null) ret 0;
  int r = integralImage_sumRect(img, x1, y1, x2, y2, 0);
  int g = integralImage_sumRect(img, x1, y1, x2, y2, 1);
  int b = integralImage_sumRect(img, x1, y1, x2, y2, 2);
  ret rgbInt(r, g, b);
}