在DirectX中将屏幕坐标转成空间坐标

Vector3 v1 = new Vector3(p.X, p.Y, 0f);
v1.Unproject(this.GraphicDevice.Viewport, this.GraphicDevice.Transform.Projection,

this.GraphicDevice.Transform.View, this.GraphicDevice.Transform.World);

Vector3 v2 = new Vector3(p.X, p.Y, 1f);
v2.Unproject(this.GraphicDevice.Viewport, this.GraphicDevice.Transform.Projection,

this.GraphicDevice.Transform.View, this.GraphicDevice.Transform.World);

float fParam = (v1.Y - 0f) / (v1.Y - v2.Y);

Vector3 v3 = new Vector3();
v3.X = v1.X - (v1.X - v2.X) * fParam;
v3.Y = 0;
v3.Z = v1.Z - (v1.Z - v2.Z) * fParam;

先算出Z=0,Z=1两种情况的空间点，也就是剪切的最近和最远距离，然后根据Y=f3DY(我这里直接将f3DY设置成0了，参见第10行代码)来进行一次剪切，按比例得到X，Z。

V3便是计算之后的坐标