static int bwIntegralImage_sumRect(BWIntegralImage img, int x1, int y1, int x2, int y2) {
  int bottomLeft  = img.get(x1-1, y2-1);
  int bottomRight = img.get(x2-1, y2-1);
  int topLeft     = img.get(x1-1, y1-1);
  int topRight    = img.get(x2-1, y1-1);
  ret bottomRight+topLeft-topRight-bottomLeft;
}