我是three.js和一般3d编程的新手，我用three.js绘制了一个扇区，我可以在一个方向上看到对象，但我在相反的方向上看不到它，似乎是三个。这里的js例子有同样的现象，我怎么能看到两个dretions中的对象？
var squareShape = new THREE.Shape();
var arc = 1/6*Math.PI
var len = 20
squareShape.moveTo( 0,0 );
squareShape.absarc( 0, 0, 20, 4/3*Math.PI, 5/3*Math.PI, false );
squareShape.moveTo( 0, 0 );
var geometry = new THREE.ExtrudeGeometry( squareShape, {amount:0.1} );
var mesh = THREE.SceneUtils.createMultiMaterialObject( geometry, [ new THREE.MeshLambertMaterial( { color: 0xff0000 ,opacity: 1.0} ), new THREE.MeshBasicMaterial( { color: 0xff0000, wireframe: true, transparent: true ,opacity: 1.0} ) ] );
mesh.position.set( 10, 10, 10 );
mesh.rotation.set( Math.PI/2, 0, Math.PI/2 );
scene.add( mesh );