// is it a split? is it a join? depends on your view

sclass IIHorizontalSplit extends Meta implements IIntegralImage {

  // x coordinate of split point

  // - isn't this just a.getWidth()?

  int splitPoint;

  // a and b must have same height

  // [0; splitPoint)

  RegisteredReference<IIntegralImage> a = new(this);

  // [splitPoint; width)

  RegisteredReference<IIntegralImage> b = new(this);

  *() {}

  *(IIntegralImage img) {

    int w = img.getWidth();

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

    init(img, w/2);

  }

  *(IIntegralImage img, int splitPoint) {

    init(img, splitPoint);

  }

  *(IIntegralImage img1, IIntegralImage img2) {

    a.set(img1);

    b.set(img2);

    splitPoint = img1.getWidth();

  }

  void init(IIntegralImage img, int splitPoint) {

    this.splitPoint = splitPoint;

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

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

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

  }

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

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

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

    if (x < splitPoint)

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

    else ret

      a->getIntegralValue(splitPoint, y, channel)

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

  }

  public double getIntegralValue(int x, int y) {

    if (x < splitPoint)

      ret a->getIntegralValue(x, y);

    else ret

      a->getIntegralValue(splitPoint, y)

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

  }

  public BufferedImage getBufferedImage() {

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

    var g = graphics(img);

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

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

    ret img;

  }

  IIntegralImage a() { ret a!; }

  IIntegralImage b() { ret b!; }

  void setA(IIntegralImage a) { this.a.set(a); }

  void setB(IIntegralImage b) { this.b.set(b); }

}