(function($) {
    'use strict';
    var $window = $(window);
    var windowHeight = $window.height();
    $window.resize(function() {
        windowHeight = $window.height()
    });
    $.fn.parallax = function(xpos, speedFactor, outerHeight) {
        var $this = $(this);
        var getHeight;
        var firstTop;
        var paddingTop = 0;

        function update() {
            $this.each(function() {
                firstTop = $this.offset().top
            });
            if (outerHeight) {
                getHeight = function(jqo) {
                    return jqo.outerHeight(true)
                }
            } else {
                getHeight = function(jqo) {
                    return jqo.height()
                }
            }
            if (arguments.length < 1 || xpos === null) xpos = "50%";
            if (arguments.length < 2 || speedFactor === null) speedFactor = 0.5;
            if (arguments.length < 3 || outerHeight === null) outerHeight = true;
            var pos = $window.scrollTop();
            $this.each(function() {
                var $element = $(this);
                var top = $element.offset().top;
                var height = getHeight($element);
                if (top + height < pos || top > pos + windowHeight) {
                    return
                }
                $this.css('backgroundPosition', xpos + " " + Math.round((firstTop - pos) * speedFactor) + "px")
            })
        }
        $window.bind('scroll', update).resize(update);
        update()
    }
})(jQuery);