可拖动窗口，附带鼠标控制渐变透明，开启关闭(2)

function doit(img,mode){
  clearTimeout(timer);
  if(img.style.filter.indexOf("alpha")==-1)img.style.filter+="alpha(opacity="+org_opacity+")";
  var cur_level=img.filters.alpha.opacity;
  if(mode){
    cur_level+=step;
    if(cur_level>100)cur_level=100;
  }else{
    cur_level-=step;
    if(cur_level<org_opacity)cur_level=org_opacity;
  }
  img.filters.alpha.opacity=cur_level;
  if(cur_level!=100 && cur_level!==ext_time)
    timer=setTimeout("doit("+img.id+","+mode+")",ext_time)
}