Reflections on Tangible User Interfaces

INTRODUCTION

The Token and Constraints (TAC) Paradigm described by Shaer, Leland, Calvillo-Gamez and Jacob in their paper The TAC Paradigm: Specifying Tangible User Interfaces includes several concepts and themes which illuminate certain Tangible User Interfaces. Based on Ullmer’s Token + Constraint approach to TUIs, the paradigm describes TUIs with “a set of relationships between physical objects and digital information…[where] a user may manipulate physical objects in order to access or manipulate digital information.” Its five key properties are Coupling, Relative Definition, Association, Computational Interpretation and Manipulation, which I apply in a case study of the MIT Media Lab’s Senseboard.

COUPLING

Coupling a physical token, or “pyfo,” with a variable creates a token by definition in the TAC paradigm. As an example in Senseboard, coupling information with a data “puck” creates a token. Using RFID technology, the data variable is associated with the given puck, and at run the data is projected onto its coupled puck.

RELATIVE DEFINITION

According to the researchers, this property declares that any “pyfo” may be defined as a token, a constraint or both. In Senseboard, command pucks and pucks without associated information are none of the above. Pucks that have been coupled to data are tokens, as defined above. Constraints in the Senseboard system include pucks that have already been placed, as they prevent other pucks from occupying that space, and also include any other structure controlling where the pucks can be placed on the board.

ASSOCIATION

An association describes the case when a token is physically associated with a constraint. This type of TAC relationship is exemplified by the association between a command puck and a given command in the Senseboard. When a command puck is being applied, not only does it impact the underlying puck but it temporarily obscures it, constraining its movement and preventing it from having other commands applied to it simultaneously.

COMPUTATIONAL INTERPRETATION

Manipulating a token within its constraints changes the state of the application via computational interpretation. Outside of the constraints, manipulation has no computational interpretation and, to the application, is irrelevant. In the case of Senseboard, when pucks are being maneuvered on the board it changes the state of the application, as the puck’s RFID sensors are interacting with the different RFID sensors across different spaces on the board. Similarly, applying command pucks on the board changes  the state of the pucks they are applied to, as their RFID sensor communicates with those of the puck below it and then the space on the Senseboard below that. However, when pucks (command or not) are maneuvered outside of the board (the main constraint of the system), the information and changes are not recorded and have no computational interpretation, as they are not communicated to the application.

MANIPULATION

This property describes the potential for each TAC to be manipulated discretely, continuously or in both ways. This manipulation is permitted within the application’s constraints and their physical properties. In Senseboard, pucks can be manipulated by adding, removing or moving the pucks across the board. Command pucks can be manipulated by placing them over the pucks on which they are to impact, and then removing them from said pucks. These discrete manipulations can be derived from the physical properties of the magnetized pucks and the encompassing size of the command pucks, which fit over the pucks they are meant to alter.

SUMMARY