JS实现图片上传时的本地预览，兼容IE和firefox谷歌

旁白：

　　一般来说如果要实现在上传前预览图片的话，用纯服务器端的语言必定是办不到的，需要先传到服务器上，哪怕是只是服务器上的临时文件，这也是个很麻烦的事情，不过可以用javascript来做这件事。下面的javascript就可以是现在服务器上预览本地图片。（原作者说代码仅支持IE和 firefox，经本人试验验证ie，火狐，谷歌均ok，谷歌用html时候是不太好用，不过jsp没问题的）。浏览器一向我行我素，非要自己再弄套标准，着实让编程者脑袋疼。期待天下统一吧。IE是用了滤镜。

来人吖，代码伺候吖：

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head>     
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />     
<title>图片上传本地预览</title>     
<style type="text/css">
#preview{width:100px;height:100px;border:1px solid #000;overflow:hidden;}
#imghead {filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod=image);}
</style>
<script type="text/javascript">
function previewImage(file)
{
  var MAXWIDTH  = 100;
  var MAXHEIGHT = 100;
  var div = document.getElementById('preview');
  if (file.files && file.files[0])
  {
      div.innerHTML = '<img id=imghead>';
      var img = document.getElementById('imghead');
      img.onload = function(){
        var rect = clacImgZoomParam(MAXWIDTH, MAXHEIGHT, img.offsetWidth, img.offsetHeight);
        img.width = rect.width;
        img.height = rect.height;
        img.style.marginLeft = rect.left+'px';
        img.style.marginTop = rect.top+'px';
      }
      var reader = new FileReader();
      reader.onload = function(evt){img.src = evt.target.result;}
      reader.readAsDataURL(file.files[0]);
  }
  else
  {
    var sFilter='filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod=scale,src="';
    file.select();
    var src = document.selection.createRange().text;
    div.innerHTML = '<img id=imghead>';
    var img = document.getElementById('imghead');
    img.filters.item('DXImageTransform.Microsoft.AlphaImageLoader').src = src;
    var rect = clacImgZoomParam(MAXWIDTH, MAXHEIGHT, img.offsetWidth, img.offsetHeight);
    status =('rect:'+rect.top+','+rect.left+','+rect.width+','+rect.height);
    div.innerHTML = "<div id=divhead style='"+rect.width+"px;height:"+rect.height+"px;margin-top:"+rect.top+"px;margin-left:"+rect.left+"px;"+sFilter+src+"\"'></div>";
  }
}
function clacImgZoomParam( maxWidth, maxHeight, width, height ){
    var param = {top:0, left:0, width, height:height};
    if( width>maxWidth || height>maxHeight )
    {
        rateWidth = width / maxWidth;
        rateHeight = height / maxHeight;
        
        if( rateWidth > rateHeight )
        {
            param.width =  maxWidth;
            param.height = Math.round(height / rateWidth);
        }else
        {
            param.width = Math.round(width / rateHeight);
            param.height = maxHeight;
        }
    }
    
    param.left = Math.round((maxWidth - param.width) / 2);
    param.top = Math.round((maxHeight - param.height) / 2);
    return param;
}
</script>     
</head>     
<body>
<div id="preview">
    <img id="imghead" width=100 height=100 border=0 src='<%=request.getContextPath()%>/images/defaul.jpg'>
</div>
    <br/>     
    <input type="file" onchange="previewImage(this)" />     
</body>     
</html>