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>