[LayaAir 2.0]要让2DUI跟随3D物件，比如说敌人头上有血条。请问要怎么做?

let out = window.app.fcUtil.worldToViewportPoint(this.owner.transform.position, window.GameMag.Camera1);
this.hpNode.x = out.x - this.hpNode.width / 2
this.hpNode.y = out.y + this.hpNode.height *3.5
//this.owner.transform 3d的物体
//this.hpNode 2d的ui
 
//3d坐标转2d屏幕坐标
    /**
     * 
     * @param {*} point 3d世界坐标
     * @param {*} camera 摄像机
     */
    worldToViewportPoint(point, camera) {
        let pos = new Laya.Vector3(point.x, point.y, point.z);;
        let out = new Laya.Vector4(0, 0, 0, 0);
        camera.worldToViewportPoint(pos, out);
        return out;
    }