sclass IIVerticalSplit extends Meta implements IIntegralImage {

  // y coordinate of split point

  int splitPoint; 

  // a and b must have same width

  // [0; splitPoint)

  RegisteredReference<IIntegralImage> a = new(this);

  // [splitPoint; height)

  RegisteredReference<IIntegralImage> b = new(this);

  *() {}

  *(IIntegralImage img, int splitPoint) {

    init(img, splitPoint);

  }

  void init(IIntegralImage img, int splitPoint) {

    this.splitPoint = splitPoint;

    int w = img.getWidth(), h = img.getHeight();

    a.set(img.clip(0, 0, w, splitPoint));

    b.set(img.clip(0, splitPoint, w, h-splitPoint));

  }

  *(IIntegralImage img) {

    int h = img.getHeight();

    if (h < 2) fail("Can't split image vertically - too small");

    init(img, h/2);

  }

  *(IIntegralImage img1, IIntegralImage img2) {

    a.set(img1);

    b.set(img2);

    splitPoint = img1.getHeight();

  }

  public int getWidth() { ret a->getWidth(); }

  public simplyCached int getHeight() { ret a->getHeight()+b->getHeight(); }

  public double getIntegralValue(int x, int y, int channel) {

    if (y < splitPoint)

      ret a->getIntegralValue(x, y, channel);

    else ret

      a->getIntegralValue(x, splitPoint, channel)

        + b->getIntegralValue(x, y-splitPoint, channel);

  }

  public double getIntegralValue(int x, int y) {

    if (y < splitPoint)

      ret a->getIntegralValue(x, y);

    else ret

      a->getIntegralValue(x, splitPoint)

        + b->getIntegralValue(x, y-splitPoint);

  }

  public BufferedImage getBufferedImage() {

    BufferedImage img = newBufferedImage(getWidth(), getHeight());

    var g = graphics(img);

    a->drawAt(g, 0, 0);

    b->drawAt(g, 0, splitPoint);

    ret img;

  }

}