带有滚动的Jquery / Javascript不透明度动画

我在这里复制粘贴基本代码

    var fadeStart=100 // 100px scroll or less will equiv to 1 opacity    ,fadeUntil=200 // 200px scroll or more will equiv to 0 opacity    ,fading = $('#fading');$(window).bind('scroll', function(){    var offset = $(document).scrollTop()        ,opacity=0    ;    if( offset<=fadeStart ){        opacity=1;    }else if( offset<=fadeUntil ){        opacity=1-offset/fadeUntil;    }    fading.css('opacity',opacity).html(opacity);});