html5 canvas一群鸟飞行

代码片段：

canvas = document.getElementsByTagName('canvas')[0];
        engine = canvas.getContext('2d');

        width  = window.innerWidth;
        height = window.innerHeight;

        canvas.setAttribute('width', width);
        canvas.setAttribute('height', height);

        bgTop = engine.createLinearGradient(0, 0, 0, height / 2);
        bgTop.addColorStop(0, '#000');
        bgTop.addColorStop(1, '#110');

        bgBot = engine.createLinearGradient(0, height / 2, 0, height);
        bgBot.addColorStop(0, '#000');
        bgBot.addColorStop(1, '#001');