3D图形在平面的投影中如何判断每个面显示的前后关系.

[lin07860]

3D图形坐标(例一个正方体):
{
$0020,$0020,$0020, $0060,$0020,$0020, $0060,$0060,$0020, $0020,$0060,$0020, ;上
$0020,$0020,$0000, $0060,$0020,$0000, $0060,$0060,$0000, $0020,$0060,$0000, ;下
$0020,$0020,$0020, $0020,$0060,$0020, $0020,$0060,$0000, $0020,$0020,$0000, ;左
$0060,$0020,$0020, $0060,$0060,$0020, $0060,$0060,$0000, $0060,$0020,$0000, ;右
$0060,$0060,$0020, $0020,$0060,$0020, $0020,$0060,$0000, $0060,$0060,$0000, ;前
$0020,$0020,$0020, $0060,$0020,$0020, $0060,$0020,$0000, $0020,$0020,$0000, ;后
}
现在显示这正方体在平面上投影,假如我们俯视,那这个正方体的面叠加层次由低到高应为(下/左右前后/上),但我改变视觉角度的时候,这些面的叠加顺序也会有相应的改变,要如何判断在任意角度时3D图形中每个面的图层的叠加顺序呢?

希望各位大侠能给点意见,谢谢!