@@

module Audio
  @@bgm_fadein_thread = nil
  def Audio.bgm_play_fade(filename, time, volume=100, pitch=100)
    if !FileTest.exist?(filename)
      p "Audio.bgm_play_fade Error : The file \"#{filename}\" does not exist."
      return
    end
    time /= ( 1000/20 )
    dvol = 1.0 * volume / time
    @@bgm_fadein_thread = Thread.new do
      for i in 0 ... time
        Audio.bgm_play(filename, (dvol*i).to_i, pitch)
      end
    end
  end
end