public Line3D CrossPlane(Plane3D p) //求平面与平面的交线
        {
            Line3D CrossLine = new Line3D();
            float tempA = (p.d - d) - (norm.x - p.norm.x);
            float tempB = (p.d - d) - (norm.y - p.norm.y);
            float tempC = norm.z - p.norm.z;
            if (tempC != 0)
            {
                CrossLine.ptBegin.x = 1;
                CrossLine.ptBegin.y = 0;
                CrossLine.ptBegin.z = tempA /tempC;

                CrossLine.ptEnd.x = 0;
                CrossLine.ptEnd.y = 1;
                CrossLine.ptEnd.z = tempB / tempC;

                return CrossLine;
            }
            else
            {
                return null;
            }
            
        }