function getScrollXY() {
  var scrOfX = 0, scrOfY = 0;
  if( typeof( window.pageYOffset ) == 'number' ) {
    //Netscape compliant
    scrOfY = window.pageYOffset;
    scrOfX = window.pageXOffset;
  } else if( document.body && ( document.body.scrollLeft || document.body.scrollTop ) ) {
    //DOM compliant
    scrOfY = document.body.scrollTop;
    scrOfX = document.body.scrollLeft;
  } else if( document.documentElement && ( document.documentElement.scrollLeft || document.documentElement.scrollTop ) ) {
    //IE6 standards compliant mode
    scrOfY = document.documentElement.scrollTop;
    scrOfX = document.documentElement.scrollLeft;
  }
  return [ scrOfX, scrOfY ];
}

function mostra_sfondo(layer)  {
	var scroll = getScrollXY();
	var offset = parseInt($(document).width())/2 - $('#'+layer).width()/2;
	var offscroll = scroll[1]+200;
	$('#'+layer).css('left',offset+'px');
	$('#'+layer).css('top',offscroll+'px');
	$('#sfondo').css('height',$(document).height()+scroll[1]);
	$('#sfondo').show();
	$('#'+layer).show();
}

function nascondi_sfondo(layer)  {
	$('#'+layer).html('');
	$('#'+layer).hide();
	$('#sfondo').css('height','100%');
	$('#sfondo').hide();	
}