/**
   * Computes the reflected segment at a point of a curve
   * @param x1 Starting point of the segment
   * @param y1 Starting point of the segment
   * @param xi Intersection of the segment with the curve acting as a mirror
   * @param yi Intersection of the segment with the curve acting as a mirror
   * @param nx Normal of the curve at the point of intersection
   * @param ny Normal of the curve at the point of intersection
   * @return Reflected segment (has the same norm as the falling segment)
   */
  public Point2D.Double reflection(int x1, int y1, int xi, int yi, double nx, double ny ) {
    double rx, ry;
    double dot = (xi-x1)*nx+(yi-y1)*ny;
    rx = (xi-x1)-2*nx*dot;
    ry = (yi-y1)-2*ny*dot;
    return new Point2D.Double(rx,ry);   
  }