PingPongPlus: design of an athletic-tangible interface for computer-supported cooperative play
Proceedings of the SIGCHI conference on Human Factors in Computing Systems
The magic carpet: physical sensing for immersive environments
CHI EA '97 CHI '97 Extended Abstracts on Human Factors in Computing Systems
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We have developed a very simple retrofit to a large display surface that enables knocks or taps to be located and characterized (e.g., determining type of hit --- metallic tap, knuckle tap, or bash --- and intensity) in real time. We do this by analyzing the waveforms captured by 4 piezoelectric transducers (one mounted in each corner of the surface) and a dynamic microphone (mounted anywhere on the glass) in a digital signal processor. Differential timing yields the position, frequency content infers the kind of hit, and peak amplitude reflects the intensity. This technique was first explored in collaboration between Paradiso and Ishii [Ishii et. al. 1999] to make an interactive ping-pong table. Moving to glass display surfaces introduced significant problems, however --- knuckle taps are low-frequency impulses that vary considerably hit-to-hit, and the bending waves propagating through the glass are highly dispersive. A heuristically-guided cross-correlation algorithm [Paradiso et al. 2002] was developed to counteract these effects and provide spatial measurements that can resolve knuckle impacts to within σ = 2-4 cm (depending on the material thickness) across a 2-meter sheet of glass. As the requisite hardware is minimal, and everything is mounted on the inside sheet of glass, this is a very simple retrofit to, for example, store window displays, ushering in an entirely new concept of interactive window browsing, where passers-by can interact with information on the store's products by simply knocking. We have explored this concept in retail, where one of our trackers was installed on the main display window of an American Greetings store near Rockefeller Center in Manhattan for this year's Christmas-Valentine's Day season (right figure), and in museums (e.g., left figure, which shows the system running at the Ars Electronica Center in Linz, Austria).