(function () {

/*

memoize video, window and document so you don't have to create and

garbage-collect new objects every time you scroll

*/

var video = $('#trees').get(0),

    $window = $(window),

    $document = $(document);

​

function updateVideo() {

    var duration = video.duration,

         scrollPosition = window.scrollY;

    if (duration) {

        video.currentTime = (scrollPosition / ($document.height() - $window.height())) * duration;

    }

}

​

// update video every time you scroll

$window.scroll(updateVideo);

​

// update video when metadata has loaded

$(video).on('loadedmetadata', updateVideo);

}());