我是A框架的新手，我试图在标记上显示3D模型。当我放置一个盒子时，它可以工作，但是当我尝试导入自己的3d模型时，它不会显示。
<!-- include A-Frame obviously -->
<script src="https://aframe.io/releases/0.6.0/aframe.min.js"></script>
<!-- include ar.js for A-Frame -->
<script src="https://jeromeetienne.github.io/AR.js/aframe/build/aframe-ar.js"></script>
<body style='margin : 0px; overflow: hidden;'>
<a-scene embedded arjs>
<!-- create your content here. just a box for now -->
<a-box position='0 0.5 0' material='opacity: 0.5;'></a-box>
<!-- define your gltf asset -->
<a-assets>
<a-asset-item id="tree" src="models/scene.gltf"></a-asset-item>
</a-assets>
<!-- use your gltf model -->
<a-entity gltf-model="##tree"></a-entity>
<!-- define a camera which will move according to the marker position -->
<a-marker-camera preset='hiro'></a-marker-camera>
</a-scene>
</body>