// assumes coordinates are in range
static int ii_getPixel(IntegralImage img, int x, int y) {
  if (img == null) ret 0;
  int r = img.rectSum(x, y, x+1, y+1, 0);
  int g = img.rectSum(x, y, x+1, y+1, 1);
  int b = img.rectSum(x, y, x+1, y+1, 2);
  ret rgbInt(r, g, b);
}