XR-Frame 实现 始终朝向屏幕（相机）的面片与模型

wxml，xr-frame中plane平面默认是趴在场景中的，需要先绕x轴渲染90度，

// 面片    
    <xr-node id="l" position="-3.0 0 0.0">
      <xr-mesh rotation="90 0 0" geometry="plane" uniforms="u_baseColorFactor:0.2 0.2 0.4 1, u_metallicRoughnessValues: 0 0.6" states="cullOn: false"></xr-mesh>
    </xr-node>

// 模型
    <xr-node id="b" position="0 0 -3.0">
      <xr-gltf rotation="0 -90 0" scale="0.01 0.01 0.01" model="door"></xr-gltf>
    </xr-node>

在场景的ready事件中获取相机，模型，并注册tick请求动画帧事件

   handleReady({detail}) {
      const xrScene = this.scene = detail.value;
      const xrSystem = wx.getXrFrameSystem();

      this.mat = new (xrSystem.Matrix4)();
      const { width, height } = this.scene


      this.cameraTrs = this.scene.getElementById('camera').getComponent(xrSystem.Transform);
      
      this.leftTRS = this.scene.getElementById('l').getComponent(xrSystem.Transform);
      this.backTRS = this.scene.getElementById('b').getComponent(xrSystem.Transform);

      this.FACING = xrSystem.Vector3.createFromNumber(0, 0, 0);
      this.UP = xrSystem.Vector3.createFromNumber(0, 1, 0);


      xrScene.event.add('tick', this.handleTick.bind(this));
    },

 请求动画帧中，计算旋转四元数，更新模型朝向

    handleTick: function () {
      const xrSystem = wx.getXrFrameSystem();

      if (this.leftTRS) {
        const quaternion = this.leftTRS.quaternion;
        // 算出从物体到相机的向量
        this.FACING.set(this.cameraTrs.position).sub(this.leftTRS.position, this.FACING);
        xrSystem.Quaternion.lookRotation(this.FACING, this.UP, quaternion);
      }

      if (this.backTRS) {
        const quaternion = this.backTRS.quaternion;
        // 算出从物体到相机的向量
        this.FACING.set(this.cameraTrs.position).sub(this.backTRS.position, this.FACING);
        xrSystem.Quaternion.lookRotation(this.FACING, this.UP, quaternion);
      }
    }