|Thiago De Moraes|
Starting after dark, images of people looking up were projected inside pedestrians' shadows. The images are of volunteers -- recorded in six U.K. cities -- and appear to be looking straight at the person walking.
It's all very simple, but very engaging and good fun. Beside being a very popular public space, Trafalgar Square is also where the National Gallery is, and it's quite interesting to see an art installation happening in front of it led by open-source technology.
The artist's site has a lot of other interesting interactive art projects. Here are a few videos on YouTube showing how the whole thing worked.