I am a great fan of 'fiducial marker', namely, the tag used to be recognized by Augmented Reality systems to display virtual stuff on glasses or screen. I like it because it tells me that the area I visit has some related content (since somebody tagged it). Of course I would need glasses to access this content (would it be a monster? a postit?). It can also be a simple trick related to nothing (absurd tagging!). Would this be a street art artifact in the near future? Anyhow, we should take into account that the marker as shown below are to be seens by both users: on the one hand, the one with augmented glasses and on the other hand, the regular passersby who will have to learn mor about this weird black and white dotted pictures... Picture taken from a paper about ARQuake