背景淡入淡出切换

//图片简单切换：$('xxx').imgtransition({speed: 3000, animate: 1000});
$.fn.imgtransition = function(o){
    var defaults = {
        speed : 5000,
        animate : 1000
    };
    o = $.extend(defaults, o);
    return this.each(function(){
        var arr_e = $("li", this);
        arr_e.css({position: "absolute"});
        arr_e.parent().css({margin: "0", padding: "0", "list-style": "none", overflow: "hidden"});
        function shownext(){
            var active = arr_e.filter(".active").length ? arr_e.filter(".active") : arr_e.first();
            var next =  active.next().length ? active.next() : arr_e.first();
            active.css({"z-index": 9});
            next.css({opacity: 0.0, "z-index": 10}).addClass('active').animate({opacity: 1.0}, o.animate, function(){
                active.removeClass('active').css({"z-index": 8});
            });
        }

        arr_e.first().css({"z-index": 9});
        setInterval(function(){
            shownext();
        },o.speed);
    });
};