simparallax/parallax.js

var Parallax = {
    initialise: function(createEvents) {
        $(this.ident).each(function(){
            // Calculate Offset
            if ($(this).attr('parallax-inset') && $(this).attr('parallax-mode') == 'inset') {
                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();}
        //Calculate the initial positions of the images.
        this.calculate();
    },
    calculate: function() {
        // Get the window dimentions.
        window_top = $(window).scrollTop();
        window_bottom = window_top + $(window).height();

        $(this.ident).each(function(){
            // If the element isn't on the screen, or wasn't initialised, ignore it.
            if (!is_on_screen(this)) { return; }
            element = $(this);
            //Calculate the center position of the image
            center_pos = (element.offset().top - window_top) + (element.height() / 2);
            // Calculate the position of the center point, relative to the center.
            perc = -(0.5 - ((window_top - center_pos) / window_bottom));

            switch(element.attr('parallax-mode')) {
                case 'inset':
                    pos = Math.round((perc * element.attr('parallax-offset')) * 100) / 100;
                    console.log(pos)
                    break;
                case 'linear':
                default:
                    pos = perc*(element.height()/2);
                    break;
            }
            // Apply the calculated value.
            element.css('background-position',"0px " + pos + "px");
        });
    },
    createEvents: function() {
        // Create scroll and resize events for Parallax
        $(window).scroll(function() { Parallax.calculate(); });
        $(window).resize(function() { Parallax.initialise(false); })
    },
    ident: '.parallax' //The identifier for Parallax elements
}

// Check if the element is on the screen, not necessarily all of it.
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)));
}