现实世界里的物体存在着相互遮挡的关系，当我们用计算机显示三维场景的时候，需要将这种遮挡关系反映出来。确定哪些面和线上不可见的-real world objects exist block of mutual relations, when we used the computer to the display of 3D scenes, the need to block this relationship reflected. To determine which surface and not visible online