simparallax/parallax.js

var Parallax = {
    initialise: function(createEvents) {
        $(this.ident).each(function(){
            // Calculate Offset
            inset = $(this).attr('parallax-inset');
            image_height = $(this).height();
            range = image_height / inset;
            $(this).attr('parallax-offset', range / 2);
        });
        console.log("Parallax Initialised.");
        if (createEvents) {this.createEvents();}
        this.calculate();
    },
    calculate: function() {
        window_top = $(window).scrollTop();
        window_bottom = window_top + $(window).height();
        $(this.ident).each(function(){
            if (!is_on_screen(this) || !$(this).attr('parallax-offset')) { return; }
            element = $(this);
            center_pos = (element.offset().top - $(window).scrollTop()) + (element.height() / 2);
            perc = -(0.5 - ((window_top - center_pos) / window_bottom));

            pos = Math.round((perc * element.attr('parallax-offset')) * 100) / 100;
            element.css('background-position',"0px " + pos + "px");
        });
    },
    createEvents: function() {
        $(window).scroll(function() { Parallax.calculate(); });
        $(window).resize(function() { Parallax.initialise(false); })
    },
    ident: '.parallax'
}

function is_on_screen(elm) {
    var vpH = $(window).height(),
        st = $(window).scrollTop(),
        y = $(elm).offset().top,
        elementHeight = $(elm).height();
    return ((y < (vpH + st)) && (y > (st - elementHeight)));
}