function dd_on() {
    opaque_menu(0.5);
  }
  
  function dd_off() {
    opaque_menu(1);
  }
   
  
  function opaque_menu(opacity) {
    var e=document.getElementById('headerImg');
    var o=e.style;
    o.opacity=(opacity); //Opera
    o.MozOpacity=(opacity); //Mozilla+Firefox
    o.KhtmlOpacity=(opacity); //Konqueror
    o.filter='alpha(opacity='+opacity+')'; //IE
  }