The general idea behind the second one is approximation . the control mesh is subdivided until the error between the control vertices and corresponding subdivision surface is less than a given threshold . then the vertices substitute those points on the surface approximately . the last step is to calculate offset point for every vertex and connect them to form an offset surface 第二种算法采用逼近思想，首先通过对控制网格进行细分，当控制顶点和细分曲面的距离满足一定误差要求时，近似用控制顶点代替细分曲面上的点，然后对各个顶点分别求等距点。
