精灵闪烁

Question

所以基本上我有一些汽车的代码有速度和一切，现在我想要另一个具有相同属性的汽车。所以我复制了整个代码并更改了其他汽车网址和所有变量。我的画布上有2辆车但是它们一直闪烁着。我不知道为什么会这样。

你可以在JsBin上看到它：

http://jsbin.com/fekutakime/3/edit

我复制的汽车代码是：

//Setting the canvas and context
var canvas = document.getElementById('gameCanvas');
var context = canvas.getContext('2d');

//Uploading car
var car = new Image();
car.src = "http://images.clipartpanda.com/car-top-view-clipart-red-racing-car-top-view-fe3a.png";

//Setting properties of car
var x = 450;
var y = 730;
var speed = 10;
var angle = 990;
var mod = 0;

//Event listeners for keys
window.addEventListener("keydown", keypress_handler, false);
window.addEventListener("keyup", keyup_handler, false);

//Interval for animation
var moveInterval = setInterval(function () {
    draw();
}, 30);

//Drawing the car turning and changing speed
function draw() {
    context.clearRect(0, 0, canvas.width, canvas.height);

    x += (speed * mod) * Math.cos(Math.PI / 180 * angle);
    y += (speed * mod) * Math.sin(Math.PI / 180 * angle);

    context.save();
    context.translate(x, y);
    context.rotate(Math.PI / 180 * angle);
    context.drawImage(car, -(car.width / 2), -(car.height / 2));
    context.restore();
}

//Setting the keys
function keyup_handler(event) {
    if (event.keyCode == 38 || event.keyCode == 40) {

        mod = 0;
    }
}

//Setting all of the keys
function keypress_handler(event) {
    console.log(x, y);
    if (event.keyCode == 38) {
        mod = 1;
    }
    if (event.keyCode == 40) {
        mod = -1;
    }
    if (event.keyCode == 37) {
        angle -= 5;
    }
    if (event.keyCode == 39) {
        angle += 5;
    }
}