Zoomable Graphical Sketchpad
Exploration of Pad++ is part of a research program to develop alternative strategies for interface design. Our goal is to move beyond mimicking the mechanisms of earlier media and to start to more fully exploit radical new computer-based mechanisms. We propose an information physics view of interface objects that we think provides an effective complement to traditional metaphor-based approaches.
While an informational physics strategy for interface design certainly involves metaphor, we think there is much that is distinctive about a physics-based approach. Traditional metaphor-based approaches map at the level of high-level objects and functionality. They yield interfaces with objects such as windows, trash cans, and menus, and functions like opening and closing windows and choosing from menus. While there are ease-of-use benefits from such mappings, they also orient designers towards mimicking mechanisms of earlier media rather than towards exploring potentially more effective computer-based mechanisms. Semantic zooming is but one example mechanism that we think more naturally arises from adopting an informational physics strategy. Even geometric zooming, especially with the orders of magnitude possible in Pad++, is not a mechanism that traditional metaphors would lead designers to investigate.
We are not the first to following a physics-inspired course in thinking about interface design. It derives, like most interesting interface ideas, from the seminal work of Sutherland [32] on Sketchpad. Simulations and constraint-based interfaces that led to the development of direct manipulation style interfaces are other examples of this general approach. They too derive from Sutherland and continue to inspire developments. Recent examples include the work of Borning and his students [5] [6]. Witkin [18] [34] in particular has taken a physics-as-interface approach to construction of dynamic interactive interfaces.
Smith's Alternate Reality Kit [29] [30] and languages such as Self [33] are also examples of following a physics-based strategy for interface design. These systems make use of techniques normally associated with simulation to help "blur the distinction between data and interface by unifying both simulation objects and interface objects as concrete objects" [9]. More importantly, they are based on implementation of mechanisms at a different level than is traditional. Smith, for example, gives users access to control of parameters of the underlying physics in his Alternate Reality Kit. With this approach comes the realization that one can do much more than just mimic reality. As Chang and Unger [9] point out about their use of cartoon animation mechanisms in Self, "adhering to what is possible in the physical world is not only limiting, but also less effective in achieving realism."
It is important to look at the costs as well as the benefits of traditional metaphor-based strategies. They can lead away from exploration of new mechanisms and limit views of possible interfaces in at least four ways.
First, metaphors necessarily pre-exist their use. Pre-Copernicans could never have used the metaphor of the solar system for describing the atom. In designing interfaces, one is limited to the metaphorical resources at hand. In addition, the metaphorical reference must be familiar in order to work. An unfamiliar interface metaphor is functionally no metaphor at all. One can never design metaphors the way one can design self-consistent physical descriptions of appearance and behavior. Thus, as an interface design strategy, physics, in the sense described above, offers more design options than traditional metaphor-based approaches.
Second, metaphors are temporary bridging concepts. When they become ubiquitous, they die. In the same way that linguistic metaphors lose their metaphorical impact (e.g., foot of the mountain or leg of table), successful metaphors also wind up as dead metaphors (e.g. file, menu, window, desktop). The familiarity provided by the metaphor during earlier stages of use gives way to a familiarity with the interface due to actual experience.
Thus, after a while, it is the actual details of appearance and behavior (i.e. the physics) rather than any overarching metaphor that form much of the substantive knowledge of an experienced user. Any restrictions that are imposed on the behaviors of the entities of the interface to avoid violations of the initial metaphor are potential restrictions of functionality that may have been employed to better support the users' tasks and allow the interface to continue to evolve along with the users' increasing competency.
Similarly the pervasiveness of dead metaphors such as files, menus, and windows may well restrict us from thinking about alternative organizations of interaction with the computer. There is a clash between the dead metaphor of a file and newer concepts of persistent distributed object hierarchies.
Third, since the sheer amount and complexity of information with which we need to interact continues to grow, we require interface design strategies that scale. A traditional metaphor-based strategy does not scale. A physics approach, on the other hand, scales to organize greater and greater complexity by uniform application of sets of simple laws. In contrast, the greater the complexity of the metaphorical reference, the less likely it is that any particular structural correspondence between metaphorical target and reference will be useful. We see this often as designers start to merge the functionality of separate applications to better serve the integrated nature of complex tasks. Metaphors that work well with the individual simple component applications typically do not smoothly integrate to support the more complex task.
Fourth, it is clear that metaphors can be harmful as well as helpful since they may well lead users to import knowledge not supported by the interface. Our point is not that metaphors are not useful but that as the primary design strategy they may well restrict the range of interfaces designers consider and impose less effective trade-offs than designers might come to if they were led to consider a larger space of possible interfaces.
There are, of course, also costs associated in following a physics-based design strategy. One cost is that designers can no longer rely as heavily on users' familiarity with the metaphorical reference (at least at the level of traditional objects and functionality) and so, physics-based designs may take longer to learn. However, the power of metaphor comes early in usage and is rapidly superceded by the power of actual experience. One might want to focus on easily discoverable physics. As is the case with metaphors, all physics are not created equally discoverable or equally fitted to the requirements of human cognition.
Copyright Computer Science Department, The University of New Mexico