background image scroll animation when page scroll

Demo

EXPLANATION

Note that there are visually 3 movements: The window scroll, the first shoes pair and the second one.
Shoes pair movements are independent, each one has a single coefficient for the scrolling movement

2HTML & CSS code

/* CSS */
.section {        width:540px; height:500px;
                  background:url(images/bg_freext.jpg) no-repeat #000; }
.section .inner { width:540px; height:500px;
                  background:url(images/fg_freext.png) no-repeat; }

<!-- html -->
<div class="section">
    <div class="inner">
        <!-- your content here -->
    </div>
</div>

So we need two div with background for the parent, and a transparent foreground for the child.

3JQuery code

// The plugin code
(function($){
    $.fn.parallax = function(options){
        var $$ = $(this);
        offset = $$.offset();
        var defaults = {
            "start": 0,
            "stop": offset.top + $$.height(),
            "coeff": 0.95
        };
        var opts = $.extend(defaults, options);
        return this.each(function(){
            $(window).bind('scroll', function() {
                windowTop = $(window).scrollTop();
                if((windowTop >= opts.start) && (windowTop <=                                    opts.stop)) {
                    newCoord = windowTop * opts.coeff;
                    $$.css({
                        "background-position": "0 "+ newCoord + "px"
                    });
                }
            });
        });
    };
                  })(jQuery); 
             
          // call the plugin
            $('.section').parallax({ "coeff":-0.65 });
               $('.section .inner').parallax({ "coeff":1.15 });