static double bwii_averageBrightnessOfArea(BWIntegralImage img, int x1, int y1, int x2, int y2) {
  ret doubleRatio(bwIntegralImage_sumRect(img, x1, y1, x2, y2), (x2-x1)*(y2-y1));
}

static double bwii_averageBrightnessOfArea(BWIntegralImage img, Rect r) {
  ret bwii_averageBrightnessOfArea(img, r.x, r.y, r.x2(), r.y2());
}