public static void InvertZ(this Visual3D v3d)
{
v3d.Transform((Point3D p) => {return new Point3D(p.X, p.Y, -p.Z);},
(System.Windows.Media.Media3D.Vector3D v) => { return new System.Windows.Media.Media3D.Vector3D(v.X, v.Y, -v.Z); });
}