jquery opacity demo

http://stackoverflow.com/questions/11948803/making-one-div-scroll-in-front-of-another

HTML

<div id="header">
        <img src="http://davesizer.com/blogs/wp-content/uploads/2010/03/eva_jump.jpg" alt="Dog">
    </div>
    <div id="wrapper">
    <div id="outer">
           <div id="content"></div>
    </div>w</div>

CSS

html,
body
{
    margin:0;
    width:100%;
}
#header
{
    position:fixed;
    top:0;
    left:0;
    width:100%;
    height:auto;
    background-size:100%;
}
#header img
{
    width:100%;
    display:block;
    z-index:99;
}
#wrapper
{
    width:100%;
    background-color:lightgrey;
    z-index:100;
    position:absolute;
}
#outer
{
    margin:30px auto;
    padding-top:20px;
    position:relative;
    width:90%;
    height:1500px;
    background-color:red;
}
#inner
{
    margin:0 auto;
    width:200px;
    height:250px;
    background-color:lightblue;
    top:20px
}

JavaScript

$(document).ready(function() {

    var header_h = $('#header').height();
    $('#wrapper').css('margin-top', header_h + 'px');

    $(window).scroll(function() {
        var header = $('#header');
        var opac = Math.max(0, 1 - $(window).scrollTop() / header.height());
        header.css('opacity', opac);
        //console.log(opac, $(window).scrollTop(), header.height());
    });


});