canvas实现探照灯效果

<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://sandbox.runjs.cn/uploads/rs/26/ddzmgynp/chunfen.jpg';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>