固定网页背景图同时保持图片比例的思路代码

提供一个背景图片策略：

1，背景图片固定

2，随窗口大小改变而改变大小

3，保持比例不变而缩放

支持浏览器:IE 6,7,8,9+ ,FF,Chrome

演示地址：http://www.einino.net/bg_image.html

<style> 
body{margin:0; padding:0;height:2000px; } 
#background_img{ 
top:expression(documentElement.scrollTop); /*we need this expression to fixed the top*/ 
} 
</style>

/** 
*@desc make a fixed background image. resize the image to fit the window size besides do not lost the image's proportion 
*@author EI Nino 
*2013/8/15 
*/ 
var imgBackground=function(_img_obj) 
{ 
this.img = _img_obj; 
this.init(); 
} 
imgBackground.prototype={ 
init:function(){ 
this.img.style.top="0"; 
this.img.style.left="0"; 
if(navigator.appVersion.indexOf('MSIE 6.0')>0){ 
this.img.style.position="absolute"; 
this.img.style.bottom="auto"; 
if(!document.body.style.backgroundImage){//esacpe the flash when scroll the window in IE 6 
document.body.style.backgroundImage="url(about:blank)"; 
document.body.style.backgroundAttachment="fixed"; 
} 
} 
else{ 
this.img.style.position="fixed"; 
} 
this.ra = this.img.width/this.img.height; 
this.resize(); 
this.BindEvent(); 
}, 
resize:function() { 
var self=this; 
if((document.documentElement.clientWidth /document.documentElement.clientHeight )>=self.ra) 
{ 
self.img.style.width=document.documentElement.clientWidth+"px"; 
self.img.style.height=""; 
} 
else{ 
self.img.style.width=""; 
self.img.style.height=document.documentElement.clientHeight +"px"; 
} 
}, 
GetStyle:function(_obj,_style){ 
var obj = _obj; 
return obj.currentStyle? obj.currentStyle[_style] :window.getComputedStyle(obj, null)[_style]; 
}, 
BindEvent:function(){ 
var self = this; 
$(window).resize(function(){//use jquery lib 
self.resize(); 
}); 
} 
}; 
new imgBackground(document.getElementById("background_img"));