jQuery.extend({
    //dims the screen
    dimScreen: function(speed, opacity, callback) {
        if (jQuery('#__dimScreen').size() > 0) return;

        if (typeof speed == 'function') {
            callback = speed;
            speed = null;
        }

        if (typeof opacity == 'function') {
            callback = opacity;
            opacity = null;
        }

        if (speed < 1) {
            var placeholder = opacity;
            opacity = speed;
            speed = placeholder;
        }

        if (opacity >= 1) {
            var placeholder = speed;
            speed = opacity;
            opacity = placeholder;
        }

        speed = (speed > 0) ? speed : 500;
        opacity = (opacity > 0) ? opacity : 0.5;
        return jQuery('<div></div>').attr({
            id: '__dimScreen'
                , fade_opacity: opacity
                , speed: speed
        }).css({
            background: '#000'
            , height: '2000px'
            , left: '0px'
            , opacity: 0
            , position: 'absolute'
            , top: '0px'
            , width: '100%'
            , zIndex: 100
        }).appendTo(document.body).fadeTo(speed, opacity, callback);
    },

    //stops current dimming of the screen
    dimScreenStop: function(callback) {
        var x = jQuery('#__dimScreen');
        var opacity = x.attr('fade_opacity');
        var speed = x.attr('speed');
        x.fadeOut(speed, function() {
            x.remove();
            if (typeof callback == 'function') callback();
        });
    }
});

jQuery.fn.center = function() {
    this.css("position", "absolute");
    this.css("top", ($(window).height() - this.height()) / 2 + $(window).scrollTop() + "px");
    this.css("left", ($(window).width() - this.width()) / 2 + $(window).scrollLeft() + "px");
    return this;
}