固定网页背景图同时保持图片比例

<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 6document.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 libself.resize();});}};new imgBackground(document.getElementById("background_img"));