Figure 1. By giving tangible (physical) representation to digital information, tangible user interfaces make information directly graspable and manipulable through haptic feedback. Intangible representation (such as video projection) may complement tangible representation, synchronizing with it.

architectural buildings to configure and control an underlying urban simulation of shadow, light reflection, wind flow, and traffic congestion (see Figure 2). In addition to a set of building models, Urp provides interactive tools for querying and controlling the parameters of the urban simulation, most notably position and rotation control via the physical models. Also included are a clock tool to change the position of the sun, a material wand to change the building surface between bricks and glass (with light reflection), a wind tool to change wind direction, and an anemometer to measure wind speed.

The physical building
models in Urp cast digital
shadows onto the work-
bench surface (via video
projection) corresponding
to solar shadows at a partic-
ular time of day. This time,
representing the position of
the sun, can be controlled
by turning the physical
hands of a “clock tool,” like
the one in Figure 2. The
building models can be
moved and rotated, with
the angle of their corre-
sponding shadows trans-
formed depending on
position and time of day.
Moving the hands of the
clock tool can cause Urp to
simulate a day of shadow
movement among the
buildings. Urban planners
can identify and isolate
intershadowing problems
(shadows cast on adjacent
buildings) and reposition buildings to avoid areas that
are needlessly dark; alternatively, they can maximize
light among the buildings.

In Urp, the physical models of buildings are tangi-

The Tangible Media Group at the MIT Media Laboratory moved from GUIs to tangible user interfaces (TUIs) in the mid-1990s. TUIs represented a new way to embody Mark Weiser’s (former chief scientist at Xerox PARC) vision of ubiquitous computing by weaving digital technology into the fabric of the physical environment, rendering the technology invisible [ 9]. Rather than make pixels melt into an interface, TUIs use physical forms that fit seamlessly into a user’s physical environment. TUIs aim to take advantage of these haptic-interaction skills, an approach significantly different from GUIs. The key TUI idea remains: give physical form to digital information [ 3], letting serve as the representation and controls for its digital counterparts. TUIs make digital information directly manipulatable with our hands and perceptible through our peripheral senses through its physical embodiment (see Figure 1).

Figure 2. Urp and shadow simulation. Physical building models that cast digital shadows and a clock tool to control time of day (position of the sun).

URP: FIRST-GENERATION TUI

To illustrate basic TUI concepts, I start with the Urban Planning Workbench, or Urp (developed by the Tangible Media Group in 1999), as an example early TUI [ 8]. Urp uses scaled physical models of