My friends Tony, Sam, Rudro, Brendan and I built a multi-touch table and showed it at the Maker Faire in May 2008. The table looks like a large computer screen, but you can touch it with as many fingers as you want to interact with it. Hundreds of people came through our booth and tried out the table. It was really fun watching people use it, answering questions and explaining how the table works.
UPDATE: We got mentioned in a Popular Mechanics article about the faire. Neat!
My friend Tony and his son, Sam, built the table by following some DIY instructions from Instructables. Tony, Rudro, Brendan and I designed and built a variety of demo applications to experiment with how multi-touch interaction works. We built a piano, a music sequencer, a plasma/lightning simulation, an image zoomer, a fractal exploration program and a physics simulation.
Behind the projection screen there is a camera that is sensitive to only infra-red light. Around the edges of the plexiglass screen there is a string of infra-red LEDs. The light from the LEDs is trapped within the plane of the glass until you touch the surface. The area where your finger touches the screen scatters the IR light back towards the camera. This is called Frustrated Total Internal Reflection (FTIR). The camera doesn’t see the projected image, it only sees a black frame with a white blob at each contact point. Using some computer vision algorithms the computer can turn those blobs into mouse clicks.
This technique was developed by a super smart guy named Jeff Han at NYU a few years ago. Since then the technique has been adapted and used in a variety of amazing products, installations and art projects. See for example, Perceptive Pixel, Microsoft Surface, Cubit, Reactable and many others.
Recently the DIY community has figured out cheaper and cheaper ways of building multi-touch displays using similar techniques. There is even a kit of parts you can order to build one yourself.
Many of the cheaper designs, including ours, suffer from instability in the face of varying lighting conditions. Luckily, our friend Alex loaned us a bunch of Duvateen so we were able to block out stray light that disrupted the table during the show. Everything worked great except when the nearby Tesla coil was active - but really, no one can compete with something that cool