// 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); }
}