function centra(p) {
  with (document) {
    if (getElementById('schermo').clientWidth-getElementById('capo').clientWidth >= 0)
      getElementById('capo').style.left = (getElementById('schermo').clientWidth-getElementById('capo').clientWidth)/2+"px";
    else
      getElementById('capo').style.left = "0px";
	  if (getElementById('schermo').clientHeight-getElementById('capo').clientHeight >= 0)
	    getElementById('capo').style.top = (getElementById('schermo').clientHeight-getElementById('capo').clientHeight)/2+"px";
    else
      getElementById('capo').style.top = "0px";
  }
  document.getElementById('schermo').style.visibility = "visible";
}