//static int drawArrowBetweenRects_headSize = 5;
static int drawArrowBetweenRects_width = 4;
static double drawArrowBetweenRects_headScale = 2;

static void drawArrowBetweenRects(Graphics2D g, Rect r1, Rect r2, Color color) {
  // TODO: more cases
  Pt p1, p2;
  if (r2.x > r1.x2()) {
    p1 = pt(r1.x2(), middleOfY(r1));
    p2 = pt(r2.x-1, middleOfY(r2));
  } else {
    p1 = pt(middleOfX(r1), r1.y-1);
    p2 = pt(middleOfX(r2), r2.y+r2.h);
  }
  
  int x1 = p1.x, y1 = p1.y, x2 = p2.x, y2 = p2.y;
  double dist = pointDistance(x1, y1, x2, y2);
  double headSize = drawArrowBetweenRects_width*drawArrowBetweenRects_headScale*drawArrowHead_length-1;
  DoublePt v = blendDoublePts(new DoublePt(x2, y2), new DoublePt(x1, y1), headSize/dist);
  DoublePt p = blendDoublePts(new DoublePt(x2, y2), new DoublePt(x1, y1), 0);
  g.setColor(color);
  g.setStroke(new BasicStroke(drawArrowBetweenRects_width));
  g.draw(new Line2D.Double(x1, y1, v.x, v.y));
  drawArrowHead(g, x1, y1, p.x, p.y, drawArrowBetweenRects_width*drawArrowBetweenRects_headScale);
}