Characteristics of tangible interfaces

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Two proposals seem most promising for an understanding of the characteristics of tangible interfaces. Both are relevant. Ullmer and Ishii stress seamless integration of representation and control.
There are 4 characteristics concerning representation and control:

  1. Physical representations are computationally coupled to underlying digital information.
  2. Physical representations embody mechanisms for interactive control.
  3. Physical representations are perceptually coupled to actively mediated digital representations. (visual augmentation via projection, sound…)
  4. Physical state of tangibles embodies key aspects of the digital state of a system. (TUIs are persistent: turn off the electrical power and there is still something meaningfull there what can be interpreted)

And:

  • Tangible interfaces rely on a balance between physical and digital representations.
  • Digital representations are needed to mediate dynamic information.
  • The elements of TUIs are spatially re-configurable (in contrast to tangible digital appliances) (Ishii, H., and Ullmer, B, 1997).

Whereas Ullmer’s characterisation focuses on issues of representation and its computational coupling, Brauer’s perspective is one of human-computer interaction, comparing GUI interaction with graspable interfaces. Brauer defines as special qualities of graspable interfaces the following two key characteristics:

  • a) Physical spatiality describes the co-presence of user, objects and other users in one interaction space. This space is a hybrid. Physical objects have a double affiliation to real/physical and virtual/digital space, but must still obey laws of the physical world. Real and virtual parts are each enhanced by the other. Because of co-presence of users and objects, interaction takes place IN the user interface. Therefore input and output space coincide. The user experiences a bodily shared space, his/her body is in the same space as the interaction objects. Following [17, 22, 35, 41] physical spatiality, by preserving physical laws and sharing of space, results in well-understood visibility of objects and of gestures. Strictly speaking this characteristic is a prerequisite for the next characteristic.
  • b) Haptic directness denotes direct manipulation where the physical, graspable objects themselves are the interface. The user has direct contact with the interface elements and has an embodied experience of manipulation, using his/her hands and body movements. Interaction is unmediated and intuitive, leading to ‘direct engagement’. Because hands interact directly with interface elements, two-handed or parallel interaction is possible. Unmediated, direct manipulation results in isomorphic and structure-preserving operations.

A motivation for the Tangible Media Group is that our ancestors developed in the past a range of specialized physical arifacts with different functionality, for instance to measure the passage of time, to predict the movement of planets or to compute. By grasping and manipulating these instruments, they developed rich languages to interact with real physical objects.

Newly arrived and quite ignorant of the languages of the Levant, Marco Polo could express himself only by drawing objects from his baggage-drums, salt fish, necklaces of wart hogs’ teeth-and pointing to them with gestures, leaps, cries of wonder or of horror, imitating the bay of a jackal, the hoot of the owl… Italo Calvino, Invisible Cities

But many of these artifacts disappeared and were replaced by the most common of devices: the Personal Computer. In consequence, human computer Interaction is currently limited to the use of a screen (desk-mounted, head-mounted, hand-held, etc.), a mouse and a keyboard. The Tangible Media Group wants to reject this traditional way of HCI and wants to use real physical objects for representation and control of digital information instead.
Most research on tangible interfaces has been focused on implementation. This concentrates on defining concepts, building category systems (B. Ullmer and H. Ishii, 2000), evaluating usability (M. Fjeld,et al, 1999) or potential interaction metaphors. The focus is on single user interaction, with questions of cooperative use largely left out of consideration. As requirements for cooperative use are not identical with usability requirements for single user settings a deeper understanding seems essential in order to deliberately design for cooperative use.
I did some elementary investigation into cooperative use as part of the tangible work I was doing about 3 years ago.
Ishii, H., and Ullmer, B. “Tangible Bits: Towards Seamless Interfaces between People, Bits, and Atoms”, Proc. of CHI’97, pp. 234-241, ACM 1997.
Emerging Frameworks for Tangible User Interfaces Brygg Ullmer and Hiroshi Ishii.