急,如何把弹出窗口定位在屏幕的中央????解决了马上给分

//居中弹出窗体
function getwindow(URL,width,height)
{
//根据屏幕居中
window.open(URL,'','width='+width+',height='+height+',top='+(screen.height-height)/2+',left='+(screen.width-width)/2);
//下面是根据页面居中
//window.open(URL,'','width='+width+',height='+height+',top='+(document.body.offsetHeight-height)/2+',left='+(document.body.offsetWidth-width)/2);
//window.open(URL,'','width='+width+',height='+height+',top=250,left='+document.body.offsetWidth/3);
}