解答：以下程式加入 drawSquare() 函式：

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    var square = new THREE.Geometry();
    square.vertices.push( new THREE.Vector3( x1, y1, 0 ) );
    square.vertices.push( new THREE.Vector3( x2, y1, 0 ) );
    square.vertices.push( new THREE.Vector3( x2, y2, 0 ) );
    square.vertices.push( new THREE.Vector3( x1, y2, 0 ) );
    square.faces.push( new THREE.Face3( 0, 1, 2 ) );
    square.faces.push( new THREE.Face3( 2, 0, 3 ) );
    return square;

更新：Three.js r60 之後 Face4 被移除不應該再繼續使用：https://github.com/mrdoob/three.js/wiki/Migration#r59–r60