MainController.rb

class MainController < UIViewController 
....

  def display_login
    animation = CATransition.animation
    animation.setDuration(1)
    animation.setType("kCATransitionFade")
    self.view.layer.addAnimation(animation, forKey:nil)

    @login = LoginController.alloc.init
    @login.delegate = self
    self.presentViewController(@login, animated:false, completion:nil)
  end
...
end