text: It follows on from the hugely successful first movie, reuniting stars including Robert Downey Jr as Iron Man and Chris Hemsworth as Thor.
  Marvel villain Ultron is a rebellious artificial intelligence which creates an android that attacks the Avengers.
  Whedon revealed the name after footage of the latest Thor film was shown.
  The footage was introduced by actor Tom Hiddleston - who played villain Loki in the first Avengers film - in full costume, according to Hollywood Reporter.
  Hiddleston stayed in character, making his speech and rallying members of the audience as they chanted the villain's name.
  Comic-Con, which has been running since 1970, celebrates "the historic and ongoing contribution of comics to art and culture".
  It holds conventions and events featuring comics and related popular artforms - not least films featuring comic-book characters and computer games.
target: Director Joss Whedon has revealed the title of his 2015 Avengers sequel - Avengers: Age of Ultron - at the Comic-Con convention in San Diego.