function blackout(op, force){
  if(typeof(force) == "undefined") var force = false;
  
  if(force == true){
    cortina.maximizeToWindow().stop(true, false).css("opacity", op);
    return;
  }

  if($(".cortina").size()<=0){
    cortina = $("<div class='cortina'></div>");
    cortina.appendTo(".tela").css({"opacity": 1, "z-index":80})
  } else {
    cortina = $(".cortina");
  }
  cortina.maximizeToWindow().stop(true, false).animate({
    "opacity": op
  }, {
    duration: "200",
    complete: function(){
      if(op<=0){
        cortina.remove();
      }
    }
  });
  $(window).resize(function(){
    $(".cortina").maximizeToWindow();
  });
}