js图片放大镜

<!doctype html>
<html>
<head>
<meta charset="utf-8">
<style>
#small {
   300px;
   height:300px;
   border: #000 1px solid;
   float:left;
   position: relative;
}
#small img {
   300px;
   height: 300px;
}

#small span {
   display: block;
   120px;
   height: 120px;
   background: red;
   opacity: .5;
   border: #333 1px solid;
   position: absolute;
   left: 0;
   top: 0;
   display: none;
}

#big {
   300px;
   height: 300px;
   border: #000 1px solid;
   float: left;
   margin-left: 20px;
   overflow: hidden;
   position: relative;
   display: none;
}

#big img {
   position: absolute;
}
</style>
<script>
window.onload = function(){
   var oSmall = document.getElementById('small');
   var oMask = document.getElementById('mask');
   var oBig = document.getElementById('big');
   var oImg = document.getElementById('bigimg');

   oSmall.onmouseover = function(){
       oMask.style.display = 'block';
       oBig.style.display ='block';
   }

   oSmall.onmouseout = function(){
       oMask.style.display = 'none';
       oBig.style.display = 'none';
   }

   oSmall.onmousemove = function(ev){
       var oEvent = ev || event;
       var l = oEvent.clientX - oMask.offsetWidth/2;
       var t = oEvent.clientY - oMask.offsetHeight/2;

       if(l < 0){
           l = 0;
       }else if( l > oSmall.offsetWidth - oMask.offsetWidth){
           l = oSmall.offsetWidth - oMask.offsetWidth;
       }

       if(t < 0){
           t = 0;
       }else if(t > oSmall.offsetHeight - oMask.offsetHeight){
           t = oSmall.offsetHeight - oMask.offsetHeight;
       }
       oMask.style.left = l + 'px';
       oMask.style.top = t + 'px';

       oImg.style.left = l * (oBig.offsetWidth - oImg.offsetWidth) / (oSmall.offsetWidth - oMask.offsetWidth) + 'px';

       oImg.style.top = t * (oBig.offsetHeight - oImg.offsetHeight) / (oSmall.offsetHeight - oMask.offsetHeight) + 'px';
   }
}
</script>
</head>

<body>
<div id="small">
   <img src="s.jpg" alt=""/>
    <span id="mask"></span>
</div>
<div id="big">
   <img src="b.jpg" alt="" id="bigimg"/>
</div>
</body>
</html>