<!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>