canvas探照灯效果

　　也可以在使用clip()方法前通过使用save()方法对当前画布区域进行保存，并在以后的任意时间通过restore()方法对其进行恢复

　　接下来使用clip()方法实现一个探照灯效果

<button id="btn">变换</button><button id="con">暂停</button><canvas id="canvas" width="400" height="290" style="border:1px solid black">当前浏览器不支持canvas，请更换浏览器后再试</canvas><script>btn.onclick = function(){history.go();}
con.onclick = function(){    if(this.innerHTML == '暂停'){        this.innerHTML = '恢复';
        clearInterval(oTimer);
    }else{        this.innerHTML = '暂停'; 
        oTimer = setInterval(fnInterval,50);
    }
}var canvas = document.getElementById('canvas');//存储画布宽高var H=290,W=400;//存储探照灯var ball = {};//存储照片var IMG;//存储照片地址var URL = 'http://www.gxlcms.com/';function initial(){    if(canvas.getContext){        var cxt = canvas.getContext('2d');        var tempR = Math.floor(Math.random()*30+20);        var tempX = Math.floor(Math.random()*(W-tempR) + tempR);        var tempY = Math.floor(Math.random()*(H-tempR) + tempR)        
        ball = {
            x:tempX,
            y:tempY,
            r:tempR,
            stepX:Math.floor(Math.random() * 21 -10),
            stepY:Math.floor(Math.random() * 21 -10)
        };
        IMG = document.createElement('img');
        IMG.src=URL;
        IMG.onload = function(){
            cxt.drawImage(IMG,0,0);
        }   
    }    
}function update(){
    ball.x += ball.stepX;
    ball.y += ball.stepY; 
    bumpTest(ball);
}function bumpTest(ele){    //左侧
    if(ele.x <= ele.r){
        ele.x = ele.r;
        ele.stepX = -ele.stepX;
    }    //右侧
    if(ele.x >= W - ele.r){
        ele.x = W - ele.r;
        ele.stepX = -ele.stepX;
    }    //上侧
    if(ele.y <= ele.r){
        ele.y = ele.r;
        ele.stepY = -ele.stepY;
    }    //下侧
    if(ele.y >= H - ele.r){
        ele.y = H - ele.r;
        ele.stepY = -ele.stepY;
    }
}function render(){    //重置画布高度，达到清空画布的效果    canvas.height = H;    
    if(canvas.getContext){        var cxt = canvas.getContext('2d');
        cxt.save();        //将画布背景涂黑        cxt.beginPath();
        cxt.fillStyle = '#000';
        cxt.fillRect(0,0,W,H);        //渲染探照灯        cxt.beginPath();
        cxt.arc(ball.x,ball.y,ball.r,0,2*Math.PI);
        cxt.fillStyle = '#000';
        cxt.fill(); 
        cxt.clip();      
        //由于使用了clip()，画布背景图片会出现在clip()区域内        cxt.drawImage(IMG,0,0);
        cxt.restore();
    }

}
initial();
clearInterval(oTimer);function fnInterval(){    //更新运动状态    update();    //渲染    render();    
}var oTimer = setInterval(fnInterval,50);</script>

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