/// <summary>
        /// Returns a DistortedOutlineShape based on the current shape and a SpiralDistortion.
        /// A regular polygon with more than six corners is returned unmodified.
        /// </summary>
        /// <param name="r"></param>
        /// <returns></returns>
        private SmoothOutlineShape SpiralDistortedCopy(Random r)
        {
            if (this.windings == 1 && this.corners > 6)
            {
                return(this);
            }

            // d: radial distance between an edge midpoint and a corner
            double d = this.sz - this.xEdge;

            // dr: the same, relative to the shape size
            double dr = d / this.sz;

            // this would wind one corner to the following corner (over the full shape size)
            double maxSpiralWinding = (double)this.windings / (double)this.corners;

            // this would wind a midpoint to a corner (over their radial distance)
            // this is sufficient to make the corner hang over and form a hook
            maxSpiralWinding /= (2.0 * dr);

            // this will produce even stronger overhanging corners
            maxSpiralWinding *= (this.windings == 1 ? 1.5 : 1.2);

            // Choose an actual winding ratio.
            double w = (0.33 + 0.66 * r.NextDouble()) * maxSpiralWinding;

            w = maxSpiralWinding;
            if (r.Next(2) == 0)
            {
                w *= -1;
            }

            DistortedOutlineShape.Distortion distortion = DistortedOutlineShape.SpiralDistortion(this.xc, this.yc, this.sz, w);
            return(this.DistortedCopy(distortion));
        }