Figure 2. SmartSkin, an interactive surface system based on capacitive sensing [ 7]. (a) Collaborative table system allowing multi-hand, multi-person interaction; (b) object movement using arm motion; (c) and (d) results of sensing showing hand shape and multiple finger points; and (e) the algorithm As-Rigid-As-Possible Shape Manipulation [ 4] in SmartSkin multi-touch interaction.

 

finger reflects IR light and thus becomes visible to the camera. With a simple image-processing technique (such as frame subtraction), the finger shape is separated from the background.

Using this sensing principle, HoloWall distin-guishes multiple hand and finger contact points, enabling typical multi-touch interactions (such as zooming with two hands, as in Figure 1c). Moreover, it also recognizes the human hand, arm, body, physical objects (such as rods), and visual patterns (such as 2D barcodes attached to the object), as in Figure 1c and Figure 1d).

Figure 1c shows two users playing a ping-pong game using HoloWall demonstrated at the SIGGRAPH conference in 1998. Although the system was originally designed for hand and body gestures, some participants used other physical objects as instruments for interaction; the system recognizes any light-reflecting object. Such dynamic expandability is

an interesting feature of organic user interfaces.

Note that a sensing principle similar to that of Holo Wall is also used in other interactive-surface systems (such as the Microsoft Surface, www.microsoft. com/surface/). Perceptive Pixels [ 2] is another optical multi-touch input system, though it is based on a sensing principle different from the one used by HoloWall.

 

SMARTSKIN

SmartSkin (see Figure 2) is a multi-touch interactive surface system based on capacitive sensing [ 7] that uses a grid-shaped antenna to measure hand and finger proximity. The antenna consists of a transmitter and receiver electrodes (copper wires). The vertical wires are transmitter electrodes; the horizontal wires are receiver electrodes. When one transmitter is excited by a wave signal (typically several hundred kHz), the receiver receives the signal because each crossing point (transmitter/receiver pair) functions as a capacitor. The magnitude of the received signal is proportional to the frequency and voltage of the transmitted signal, as well as to the capacitance between the two electrodes. When a conductive, grounded object approaches a crossing point, it