jQuery-based vertical rotation slider with fade gradient

vertical_slider_fade_gradient.js

function subjectCrawl() {

    var $list = $('.subject-crawl ul');
    var itemHeight = $list.children('li:first').height();
    var listLength = $list.children().length;

    // position center item
    $list.css('top', -(($list.height() - itemHeight) / 2));
    scrollList();

    setInterval(function() {
        scrollList();
    }, 3000);

    function scrollList() {
        $list.animate({
            'paddingTop': itemHeight
        }, 350, function() {
            var $last = $list.children('li:last-of-type');
            $last.detach();
            $list.prepend($last);
            $list.css('paddingTop', 0);
        });

        listOpacity();
    }

    function listOpacity() {
        var midpoint = ((listLength - 1) / 2);
        for (var i = 0; i < $list.children().length; i++) {
            var pos = i + 1;
            var opacity = pos / midpoint;
            if (opacity > 1) {
                opacity = (pos - (pos - midpoint) * 2) / midpoint;
            }

            if (opacity != 1) {
                opacity = opacity * .25;
            }

            var $item = $list.children().eq(i);
            $item.css('opacity', opacity);
        }
    }
}

subjectCrawl();