sinterface IBWIntegralImage {
  public int getWidth();
  public int getHeight();
  public int getIIValue(int x, int y);
  
  // 0.0 to 255.0
  default public double getPixelAverage(int x1, int y1, int x2, int y2) {
    int area = (x2-x1)*(y2-y1);
    ret doubleRatio(bwIntegralImage_sumRect(this, x1, y1, x2, y2), area);
  }
}