An interactive ad.

I just read about this in The New York Times. There is no link to the article (written by Christopher Shea) so I've pasted the whole thing here. It seems like a startlingly good use of a new technology. Not a gratuitous cool-for-cool's sake use.

"It happens when nobody is watching." As the tagline on a poster raising awareness about domestic violence, that's not bad. But it was the poster itself that was truly attention-grabbing — for it brought the issue of being watched (or not) to life.

The poster, placed in a bus shelter in Berlin, was a one-time installation sponsored by Amnesty International. When a person in the shelter was looking at the poster, he saw, along with the words, a photograph of an amiable couple: a stocky, professional-looking man in a blue oxford-cloth shirt, his arm around the shoulders of his girlfriend or wife. If no one in the shelter was paying attention to the poster, though, the image switched: now the man was raising his fist against the woman as she leaned away and protected her face. (There was a slight lag in the switch, so viewers could notice that the poster was changing its image.)

Designed by the Hamburg-based firm Jung von Matt (which bills itself as being in the business of "attention warfare"), the ad worked via a camera attached to a computer outfitted with face-tracking software with a working range of about 16 feet. A Potsdam company called Vis-à-pix created the technology. Jung von Matt described the ad as the first of its kind, and it won a silver prize at the 2009 Cannes Lions International Advertising Festival and a gold prize at the New York Festivals International Advertising Awards.

The technology has since improved, according to Vis-à-pix. New posters can even identify the sex of onlookers. Consider a poster created for the service counters of the rental-car company Sixt: when a man gets close, he is tempted with an image of a limousine; if the customer is a woman, she sees, instead, a spunky Cabriolet.