static double rectDistance(Rect r1, Rect r2) {
  double x1, x2, y1, y2;
  double w, h;
  if (r1.x > r2.x) {
     x1 = r2.x; w = r2.w; x2 = r1.x;
  } else {
     x1 = r1.x; w = r1.w; x2 = r2.x;
  }
  if (r1.y > r2.y) {
     y1 = r2.y; h = r2.h; y2 = r1.y;
  } else {
     y1 = r1.y; h = r1.h; y2 = r2.y;
  }
  double a = Math.max(0, x2 - x1 - w);
  double b = Math.max(0, y2 - y1 - h);
  return Math.sqrt(a*a+b*b);
}