如何旋转Z轴的方向？

Question

我最近开始使用Three.js来处理我的游戏项目，但我无法让相机四处走动。

问题是：我可以在X轴上旋转摄像机，但是当它朝向Z轴时，它总是朝着相同的方向，因为旋转视图时Z轴不会改变。到目前为止，这是我的代码：

var circle = Math.PI / 180;

    var scene = new THREE.Scene();
    var camera = new THREE.PerspectiveCamera( 75, window.innerWidth / window.innerHeight, 0.1, 1000 );

    var renderer = new THREE.WebGLRenderer();
    renderer.setSize( window.innerWidth, window.innerHeight );
    document.body.appendChild( renderer.domElement );

    var geometry = new THREE.BoxGeometry( 10, 15, 3 );
    var material = new THREE.MeshBasicMaterial( { color: 0x05ff68 } );
    var cube = new THREE.Mesh( geometry, material );
    scene.add( cube );

    var geometry2 = new THREE.BoxGeometry(10,10,10);
    var sphere = new THREE.Mesh( geometry2, material );
    scene.add(sphere);

    sphere.position.set(-20,0,0);

    camera.position.z = 30;
    camera.position.x = 0;
    camera.position.y = 10;

    camera.rotation.y = 0 * circle;
    camera.rotation.x = 0 * circle;

    cube.position.set(20,0,0)

    function render() {
        requestAnimationFrame( render );
        renderer.render( scene, camera );
    }

    render();

    function checkKey(e) {

        e = e || window.event;

        if (e.keyCode == '87') {
            camera.position.y -= 2;
        }
        else if (e.keyCode == '83') {
            camera.position.y += 2;
        }
        else if (e.keyCode == '65') {
            camera.rotation.y += 2 * circle;
        }
         else if (e.keyCode == '68') {
            camera.rotation.y -= 2 * circle;
        };

    } 

我需要知道如何改变Z的方向。