[问]obj.style.backgroundPosition只改变背景图的横向位置?解决方案 _JavaScript

[问]obj.style.backgroundPosition只改变背景图的横向位置?
只改变背景图的横向位置(或者纵向位置).javascript怎么写
两个同时改又怎么写

------解决方案--------------------

HTML code


<!DOCTYPE HTML>
<html>
    <head>
        <meta charset="gb2312" />
        <title></title>
        <style>
            div {
                margin:0 auto;
                width:800px; /*1320*/
                height:513px;
                background:url(http://whoisleon.com/wp-content/themes/whoisleon/images/n_cloud1.jpg) repeat;
            }
        </style>
    </head>
    <body>
        <div id="test"></div>
        <script>
            var $ = function(id){
                return document.getElementById(id);
            };
            var obj = $('test');
            var i = 0;
            setInterval(function(){
                obj.style.backgroundPosition = '0 '+(--i)+'px';
            }, 30)
        </script>        
    </body>
</html>

------解决方案--------------------
Object.style.backgroundPositionX=position //横向
Object.style.backgroundPositionY=position //竖向