说明：  marchingcubes算法 面绘制算法中的经典算法 (1)体素中由三角面片逼近的等值面计算； (2)三角面片各顶点法向量计算。
(Marchingcubes algorithm -Classical algorithm in surface rendering algorithm (1) the isosurface approximated by triangular patches in voxels. (2) the calculation of the vertex normal vector of the triangulation.)