static int drawArrowBetweenRects_headSize = 5;
static int drawArrowBetweenRects_width = 2;

static void drawArrowBetweenRects(Graphics2D g, Rect r1, Rect r2, Color color) {
  // TODO
  Pt p1 = pt(r1.x+r1.w/2, r1.y-1);
  Pt p2 = pt(r2.x+r2.w/2, 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 arrowLen = drawThoughtArrow_size*drawArrowHead_length-1;
  DoublePt v = blendDoublePts(new DoublePt(x2, y2), new DoublePt(x1, y1), (cs+arrowLen)/dist);
  DoublePt p = blendDoublePts(new DoublePt(x2, y2), new DoublePt(x1, y1), cs/dist);
  Graphics2D g = imageGraphics(bg);
  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_headSize);
}