显示三维图形的过程中，不仅要将它投影到二维平面上，还要确定一个视见体，在投影之前对物体进行裁剪，只显示我们感兴趣的那部分图形。-show 3D process, it should not only two-dimensional projection of the plane, but also to determine see as a body, the projector right before cutting objects, only shows that we are interested in that part of graphics.