public Vector2 GetMouseGridPosition() {
            Vector3 near = new Vector3(new Vector2(lastpos.X, lastpos.Y), 0);
            Vector3 far = new Vector3(new Vector2(lastpos.X, lastpos.Y), 1);

            Viewport viewport = new Viewport(0, 0, graphics.PreferredBackBufferWidth, graphics.PreferredBackBufferHeight);
            near = viewport.Unproject(near, camera.Projection, camera.View, camera.World);
            far = viewport.Unproject(far, camera.Projection, camera.View, camera.World);

            Vector3 nrm = Vector3.Normalize(far - near);
            Vector3 x = near - nrm * (near.Y / nrm.Y);
            return new Vector2(x.X, x.Z);
        }