/// <summary>
        ///     Gets the centroid of the polyline 2d.
        /// </summary>
        /// <param name="pl">The instance to which the method applies.</param>
        /// <returns>The centroid of the polyline 2d (WCS coordinates).</returns>
        public static Point3d Centroid(this Polyline2d pl)
        {
            var vertices = pl.GetVertices().ToArray();
            var last     = vertices.Length - 1;
            var vertex   = vertices[0];
            var p0       = vertex.Position.Convert2D();

            _ = pl.Elevation;
            var     cen   = new Point2d(0.0, 0.0);
            var     area  = 0.0;
            var     bulge = vertex.Bulge;
            double  tmpArea;
            Point2d tmpPt;
            var     tri = new Triangle2D();
            var     arc = new CircularArc2d();

            if (bulge != 0.0)
            {
                arc     = pl.GetArcSegment2DAt(0);
                tmpArea = arc.AlgebricArea();
                tmpPt   = arc.Centroid();
                area   += tmpArea;
                cen    += (new Point2d(tmpPt.X, tmpPt.Y) * tmpArea).GetAsVector();
            }

            for (var i = 1; i < last; i++)
            {
                var p1 = vertices[i].Position.Convert2D();
                var p2 = vertices[i + 1].Position.Convert2D();
                tri.Set(p0, p1, p2);
                tmpArea = tri.AlgebricArea;
                area   += tmpArea;
                cen    += (tri.Centroid * tmpArea).GetAsVector();
                bulge   = vertices[i].Bulge;
                if (bulge != 0.0)
                {
                    arc     = pl.GetArcSegment2DAt(i);
                    tmpArea = arc.AlgebricArea();
                    tmpPt   = arc.Centroid();
                    area   += tmpArea;
                    cen    += (new Point2d(tmpPt.X, tmpPt.Y) * tmpArea).GetAsVector();
                }
            }

            bulge = vertices[last].Bulge;
            if (bulge != 0.0 && pl.Closed)
            {
                arc     = pl.GetArcSegment2DAt(last);
                tmpArea = arc.AlgebricArea();
                tmpPt   = arc.Centroid();
                area   += tmpArea;
                cen    += (new Point2d(tmpPt.X, tmpPt.Y) * tmpArea).GetAsVector();
            }

            cen = cen.DivideBy(area);
            return(new Point3d(cen.X, cen.Y, pl.Elevation).TransformBy(Matrix3d.PlaneToWorld(pl.Normal)));
        }