/**
   * Computes the unitary normal vector of a segment
   * @param x1 Starting point of the segment
   * @param y1 Starting point of the segment
   * @param x2 Ending point of the segment
   * @param y2 Ending point of the segment
   * @return
   */
  public Point2D.Double normal(int x1,int y1,int x2, int y2) {
    double nx,ny;
    if (x1 == x2) {
      nx = Math.signum(y2-y1);
      ny = 0;
    } else {
      double f = (y2-y1)/(double)(x2-x1);
      nx = f*Math.signum(x2-x1)/Math.sqrt(1+f*f);
      ny = -1*Math.signum(x2-x1)/Math.sqrt(1+f*f);
    }
    return new Point2D.Double(nx,ny);
  }