By Gareth Mitchell
Digital Planet, BBC World Service
A huge interactive art installation has been launched in Trafalgar Square, London.
Under Scan was previously tried out in Northampton's Market Square in 2006
The concept is the brainchild of Mexican-Canadian artist Rafael Lozano-Hemmer, who depicts animated videos in the shadows of people as they walk through the famous city landmark.
Mr Lozano-Hemmer uses theatre lighting, surveillance technology and about 1,000 moving video portraits to create the interactive experience.
"This is an installation which is basically a big shadow play," he said.
"We have a couple of powerful projectors which cast bright light and our shadow is projected on the floor.
"As you walk around, these portraits are projected inside of your shadow and they look up at you,.
"When you are no longer interested in them, you walk away and they disappear."
Entitled Under Scan, the installation has previously been shown in five East Midlands cities - Nottingham, Leicester, Lincoln, Northampton and Derby.
The Mexican-Canadian artist feels that we should embrace technology
"For each city we have around 200 different people from the community being portrayed," said Mr Lozano-Hemmer.
"Here in London we recorded in the Tate Modern and have almost 300 Londoners who have been part of the project."
Mr Lozano-Hemmer and his team wanted to film real people in order to make these video projections come to life.
"People were free to represent themselves how they wanted," he said.
"The only thing that we asked them to do in the workshop, was to look straight at the camera.
"Thatís the moment when we trigger, when your shadow is on top."
The art installation is controlled from a temporary cabin set at the edge of Trafalgar Square.
The project in London involved around 300 people
ďWe have computer vision interface, which is basically a way for the computer to observe who is going by, itís typically used in robotics", said Mr Lozano-Hemmer.
Driving the project is open source software, used by project engineer Conroy Badger to get the system tracking people as they walk and working out where they will be in the next three seconds.
"It acquires a person as a location and then it looks at how that target is moving," Mr Lozano-Hemmer said.
"It then says, in three seconds time, this person may be over here and thatís where the computer chooses to put a portrait.
"The idea is that as you are walking around, the portraits just appear in your path."
Apple computers loaded with 80GB of recordings act as the video servers that project the portraits onto the shadows.
The artist was driven to create the installation by a feeling that technology has become inseparable from modern identity.
"Even if you are a painter today, you are part of this technological culture, it is like a language rather than a tool," he added.
The installation will operate from dusk until midnight until 23 November.