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
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