用CANNON.js大的Raycastvehicle制作一个物理小车，请用代码示例

好的，以下是一个基于CANNON.js的RaycastVehicle物理小车示例代码：

// 初始化物理引擎世界
var world = new CANNON.World();
world.gravity.set(0, -9.82, 0);
var fixedTimeStep = 1.0 / 60.0;
var maxSubSteps = 3;

// 创建车辆底盘刚体
var chassisShape = new CANNON.Box(new CANNON.Vec3(1, 0.5, 2));
var chassisBody = new CANNON.Body({ mass: 150 });
chassisBody.addShape(chassisShape);
chassisBody.position.set(0, 2, 0);
world.addBody(chassisBody);

// 创建车轮刚体
var wheelShape = new CANNON.Sphere(0.5);
var wheelBody1 = new CANNON.Body({ mass: 50 });
wheelBody1.addShape(wheelShape);
wheelBody1.position.set(1, 1, 1);
world.addBody(wheelBody1);

var wheelBody2 = new CANNON.Body({ mass: 50 });
wheelBody2.addShape(wheelShape);
wheelBody2.position.set(-1, 1, 1);
world.addBody(wheelBody2);

var wheelBody3 = new CANNON.Body({ mass: 50 });
wheelBody3.addShape(wheelShape);
wheelBody3.position.set(1, 1, -1);
world.addBody(wheelBody3);

var wheelBody4 = new CANNON.Body({ mass: 50 });
wheelBody4.addShape(wheelShape);
wheelBody4.position.set(-1, 1, -1);
world.addBody(wheelBody4);

// 创建车轮约束
var options = {
    radius: 0.5,
    directionLocal: new CANNON.Vec3(0, -1, 0),
    suspensionStiffness: 30,
    suspensionRestLength: 0.5,
    frictionSlip: 5,
    dampingRelaxation: 2.3,
    dampingCompression: 4.4,
    maxSuspensionForce: 100000,
    rollInfluence: 0.01,
    axleLocal: new CANNON.Vec3(1, 0, 0),
    chassisConnectionPointLocal: new CANNON.Vec3(1, 0, 1),
    useCustomSlidingRotationalSpeed: true,
    customSlidingRotationalSpeed: -30,
};
var vehicle = new CANNON.RaycastVehicle({
    chassisBody: chassisBody,
    indexForwardAxis: 2,
    indexRightAxis: 0,
    indexUpAxis: 1,
});
vehicle.addWheel(options);
vehicle.addWheel(options);
vehicle.addWheel(options);
vehicle.addWheel(options);

// 添加约束到世界
world.addConstraint(vehicle.constraints[0]);
world.addConstraint(vehicle.constraints[1]);
world.addConstraint(vehicle.constraints[2]);
world.addConstraint(vehicle.constraints[3]);

// 更新车辆状态
function updateVehicle() {
    var wheelBodies = [wheelBody1, wheelBody2, wheelBody3, wheelBody4];
    for (var i = 0; i < vehicle.wheelInfos.length; i++) {
        vehicle.updateWheelTransform(i);
        var t = vehicle.wheelInfos[i].worldTransform;
        wheelBodies[i].position.copy(t.position);
        wheelBodies[i].quaternion.copy(t.quaternion);
    }
}

// 更新物理引擎世界
function updatePhysics() {
    world.step(fixedTimeStep, 1, maxSubSteps);
    updateVehicle();
}

// 渲染循环
function render() {
    requestAnimationFrame(render);
    updatePhysics();
    // 在这里进行渲染
}

// 启动渲染循环
render();

这个例子创建了一个车辆底盘刚体和四个车轮刚体，然后使用RaycastVehicle约束将它们连接起来。在每帧渲染循环中，调用 `updatePhysics()` 方法来更新物理引擎世界和车辆状态，然后在渲染函数中进行渲染。你可以根据自己的需要修改这个代码，例如改变车轮的形状、调整约束参数等等。