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;
  }
}

Began life as a copy of #1031958