Javascript 如何在three.js中查看两侧的自定义二维形状

我不熟悉three.js和3d编程，我用three.js画了一个扇形，我可以在一个方向上看到对象，但我不能在相反的方向上看到它，three.js示例似乎有相同的现象，我怎么能在两个方向上都看到对象

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 );

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在你的情况下，应该是

new THREE.MeshLambertMaterial( { color: 0xff0000, opacity: 1.0, side: THREE.DoubleSide } )