摘要：簡易版時鐘時鐘清除畫布，每次執行重新繪圖，解決時鐘模糊，邊框有鋸齒。

window.requestAnimFrame=(function(){
    return window.requestAnimationFrame||window.webkitRequestAnimationFrame||window.mozRequestAnimationFrame||function(callback){
        window.setTimeout(callback,1000/60);
    };
})();

    var can=document.getElementById("canvas");
    // can.width=document.body.clientWidth ;
    // can.height=document.body.clientHeight;
    var cw=window.innerWidth;
    var ch=window.innerHeight;
    can.width=cw ;
    can.height=ch;
    var cxt=can.getContext("2d");

    var circles=[];        //存放圓形粒子池
    var rects=[];        //存放正方形粒子池
    var triangles=[];    //存放三角形粒子池
    var i=0;
    var count=100;
    var x;                //鼠標的位置
    var y;

    /**
     * 圓形粒子對象
     * @param {[type]} x [description] 中心點
     * @param {[type]} y [description] 中心點
     * @param {[type]} r [description] 半徑
     */
    function Circle(x,y,r){
        this.x=x;
        this.y=y;
        this.r=r;
        this.speed=Math.random()*0.5+1;    //速度
        this.direction=Math.random()*Math.PI*2;    //方向
    }
    /**
     * 更新圓心坐標
     * @return {[type]} [description]
     */
    Circle.prototype.update=function(){
        this.x+=Math.cos(this.direction)*this.speed;
        this.y+=Math.sin(this.direction)*this.speed;
        if(this.x=(cw-this.r)){
            this.x=cw-this.r;
            this.direction=Math.PI-this.direction;
        }
        if(this.y=(ch-this.r)){
            this.y=ch-this.r;
            this.direction=-this.direction;
        }
    };

    /**
     * 繪制圓形粒子
     * @return {[type]} [description]
     */
    Circle.prototype.draw=function(){
        cxt.beginPath();
        cxt.arc(this.x,this.y,this.r,0,360,false);
        cxt.closePath();
        cxt.fillStyle="#f66";
        if(this.x>x-50&&this.xy-50&&this.y
    /**
     * 矩形粒子對象
     * @param {[type]} x 起點位置
     * @param {[type]} y 起點位置
     * @param {[type]} w 矩形寬
     * @param {[type]} h 矩形長
     */
    function Rect(x,y,w,h){
        this.x=x;
        this.y=y;
        this.w=w;
        this.h=h;
        this.x0=Math.random()*cw;    //正方形的中心坐標    為了旋轉而設定
        this.y0=Math.random()*ch;    //正方形的中心坐標
        this.r=Math.sqrt(Math.pow(this.w/2,2)+Math.pow(this.h/2,2));
        this.speed=Math.random()*0.5+1;    //速度
        this.direction=Math.random()*Math.PI*2;    //方向
    }
    /**
     * 更新粒子坐標
     * @return {[type]} [description]
     */
    Rect.prototype.update=function(){
        this.x0+=Math.cos(this.direction)*this.speed;
        this.y0+=Math.sin(this.direction)*this.speed;
        if(this.x0cw-this.r){
            this.x0=cw-this.r;
            this.direction=Math.PI-this.direction;
        }
        if(this.y0ch-this.r){
            this.y0=ch-this.r;
            this.direction=-this.direction;
        }

    };

    /**
     * 繪制正方形粒子
     * @return {[type]} [description]
     */
    Rect.prototype.draw=function(){
        cxt.save();
        cxt.fillStyle="#06c";
        cxt.translate(this.x0,this.y0);        //將坐標原點移至圓心  方便旋轉
        cxt.rotate(i*Math.PI/180);
        cxt.beginPath();
        if(this.x0>x-50&&this.x0y-50&&this.y0
    /**
     * 等邊三角形粒子 
     * @param  {[type]} h 三角形高
     * @return {[type]}   [description]
     */
    function Triangle(b){
        this.b=b;
        this.x0=Math.random()*cw;
        this.y0=Math.random()*ch;
        this.speed=Math.random()*0.5+1;
        this.direction=Math.random()*Math.PI*2;
        this.r=this.b/2*Math.tan(30*Math.PI/180);
        console.log(this.r+"   "+this.b);
    }
    /**
     * 中心更新
     * @return {[type]} [description]
     */
    Triangle.prototype.update=function(){
        this.x0+=Math.cos(this.direction)*this.speed;
        this.y0+=Math.sin(this.direction)*this.speed;
        if(this.x0cw-this.r){
            this.x0=cw-this.r;
            this.direction=Math.PI-this.direction;
        }
        if(this.y0ch-this.r){
            this.y0=ch-this.r;
            this.direction=-this.direction;
        }
    };

    Triangle.prototype.draw=function(){
        cxt.save();
        cxt.fillStyle="#86c";
        cxt.translate(this.x0,this.y0);
        cxt.rotate(i*Math.PI/180);
        cxt.beginPath();
        cxt.moveTo(-this.b/2,-this.r);
        cxt.lineTo(0,Math.sin(60*Math.PI/180)*this.b-this.r);
        cxt.lineTo(this.b/2,-this.r);
        
        
        cxt.closePath();
        if(this.x0>=x-50&&this.x0<=x+50&&this.y0>=y-50&&this.y0<=y+50){
            cxt.globalAlpha=1;
        }else{
            cxt.globalAlpha=0.2;
        }
        cxt.fill();
        cxt.restore();

    };
    while(count--){
        var w=Math.random()*cw;
        var h=Math.random()*ch;
        circles.push(new Circle(w,h,5));
        rects.push(new Rect(-5,5,10,10));
        triangles.push(new Triangle(10));
    }

    canvas.onmousemove=function(ev){
        var ev=ev||window.event;
        x=ev.pageX;
        y=ev.pageY;
        console.log(x+"..."+y);
    };

    /**
     * 在畫布中繪制圓形粒子
     * @return {[type]} [description]
     */
    function loop(){
        requestAnimFrame(loop);
        i++;
        if(i>10000){
            i=0;
        }
        cxt.globalCompositeOperation="destination-out";
        cxt.fillStyle="rgba(0,0,0,1)";
        //透明度
        cxt.globalAlpha=1;
        cxt.fillRect(0,0,cw,ch);
        //顯示重疊的部分
        cxt.globalCompositeOperation="lighter";
        var n=circles.length;
        while(n--){
            circles[n].draw();
            circles[n].update();
        }
        var n=rects.length;
        while(n--){
            rects[n].draw();
            rects[n].update();
        }

        var n=triangles.length;
        while(n--){
            triangles[n].draw();
            triangles[n].update();
        }
    }
    window.onload=loop;