final sclass BWIntegralImage_twoParts implements MakesBufferedImage, IBWIntegralImage {
  int th;
  BWIntegralImage top, bottom;

  *() {}
  *(File f) { this(loadImage2(f)); }
  
  *(MakesBufferedImage img) { this(toBufferedImage(img)); }
  
  *(BufferedImage image) ctex {
    int w = image.getWidth(), h = image.getHeight();
    th = (h+1)/2;
    L<BWIntegralImage> l = parallelEval(
      () -> BWIntegralImage(virtualClipBufferedImage(image, 0, 0, w, th)),
      () -> h <= th ? null : BWIntegralImage(virtualClipBufferedImage(image, 0, th, w, h))
    );
    top = first(l);
    bottom = second(l);
  }
    
  public int getIIValue(int x, int y) {
    ret y < th ? top.getIIValue(x, y)
      : bottom.getIIValue(x, y-th);
  }
  
  public int getWidth() { ret w; }
  public int getHeight() { ret h; }
  
  // unoptimized
  public BufferedImage getBufferedImage() {
    ret scaleDownUsingIntegralImageBW(this, w).getBufferedImage();
  }
}