js放大镜效果

css如下：

        .demo{ width:350px;margin:30px auto; text-align:left; padding:0; }
        #div{border:1px solid #ccc; position:relative; }
        #div .small_pic{ width:350px; height:350px; background:#eee; position:relative; }
        #div .float_layer{ width:100px; height:100px; border:1px solid #000; background:#fff; filter:alpha(opacity:30); opacity:0.3; position:absolute; top:0; left:0; display:none; }
        #div .mark{ width:100%; height:100%; position:absolute; z-index:2; left:0px; top:0px; background:red; filter:alpha(opacity:0); opacity:0; }
        #div .big_pic{ position:absolute; top:-1px; left:360px; width:250px; height:250px; overflow:hidden; border:2px solid #CCC; display:none; }
        #div .big_pic img{ position:absolute; top:-30px; left:-80px; }    

html如下

<div id="div" class="demo">
    <div class="small_pic" style="cursor: move;">
        <span class="mark"></span>
        <span class="float_layer" style="display: none; top: 250px; left: 30px;"></span>
        <img src="images/zard.jpg" alt="放大镜图片一" style="350px; height:350px;">
    </div>

    <div class="big_pic" style="display: none;">
        <img src="images/zard.jpg" alt="放大镜图片二" style="left: -66px; top: -550px;">
    </div>
</div>

script如下：

    (function(){
        var div = document.getElementById('div');
        var small = getByCls(div, 'small_pic')[0];
        var big = getByCls(div, 'big_pic')[0];
        var img = big.getElementsByTagName('img')[0];
        var mark = small.getElementsByTagName('span')[0];
        var layer = small.getElementsByTagName('span')[1];
        small.onmousemove = function(e){
            e = e || event;
            this.style.cursor = 'move';
            layer.style.display = big.style.display = 'block';
            var t = e.clientY - div.offsetTop - layer.clientHeight/2;
            var l = e.clientX - div.offsetLeft - layer.clientWidth/2;
            if(t < 0){
                t = 0;
            } else if(t > div.clientHeight - layer.clientHeight){
                t = div.clientHeight - layer.clientHeight;
            }
            if(l < 0){
                l = 0;
            } else if(l > div.clientWidth - layer.clientWidth){
                l = div.clientWidth - layer.clientWidth;
            }
            var scaleX = l / (div.clientHeight - layer.clientHeight);
            var scaleY = t / (div.clientWidth - layer.clientWidth);
            layer.style.top = t + 'px';
            layer.style.left = l + 'px';
            img.style.left = -scaleX * (img.clientWidth - big.clientWidth) + 'px';
            img.style.top = -scaleY * (img.clientHeight - big.clientHeight) + 'px';
        }
        small.onmouseout = function(e){
            layer.style.display = big.style.display = 'none';
        }
        
        function getByCls(obj, cls){
            if(obj.getElementsByClassName){
                return obj.getElementsByClassName(cls);
            } else {
                var res = [],
                    reg = new RegExp('^|\s+' + cls + '\s+|$'),
                    all = obj.all;
                for(var i = 0; i < all.length; i++){
                    if(reg.test(all[i].className)){
                        res.push(all[i]);
                    }
                }
                return res;
            }
        }
    }());