如何使物化视差图像响应

Question

我正在使用materialize.css视差图像。但是，图像没有响应。我正在使用v0.100.2

css代码：

.parallax-bg {
    padding: 300px 0;
    background-image: url("http://www.saraads.com/img/site/basic-ecommerce.jpg");
    background-size: cover;
    background-attachment: fixed;
    -webkit-transition: all 0.2s ease;
    -moz-transition: all 0.2s ease;
    transition: all 0.2s ease;
}

javascript代码：

$(document).ready(function(){
    var documentEl = $(document),
    parallaxBg = $('div.parallax-bg');
    documentEl.on('scroll', function() {
        var currScrollPos = documentEl.scrollTop();
        parallaxBg.css('background-position', '0 ' + -currScrollPos/4 + 'px');
    });
});