点集的包围球

http://en.wikipedia.org/wiki/Bounding_sphere

http://blogs.agi.com/insight3d/index.php/2008/02/04/a-bounding/

http://www.flipcode.com/archives/Smallest_Enclosing_Spheres.shtml

http://blog.csdn.net/wangjiannuaa/article/details/6072279

合并包围球

Real-Time Collision Detection, p267, 6.5.2 Merging Two Spheres