A good user-interface is an essential element of every application that requires even a minimum of interaction between user and computer. As such, a user-interface must be "intelligent" enough to perceive the requirements of a user depending on the current situation and the application specific context. While providing the right tools to the right time and such leading the user to the application procedure the user interface has to be non-obtrusive, because it must support, but not interfere with the user’s flow of thoughts. Hence, the ideal user-interface is simple, intuitive, and it adapts itself depending on the context. This simplicity comes, however, at a price: The simpler and more intuitive a user-interface gets, the more complex and difficult is its implementation.

User-Interface Metaphor

The design of the user-interface used for the SQbS prototype is based on a simple sketchpad metaphor. That is, the user-interface should have the same functionality provided by a simple piece of paper, a pencil, and an eraser. This original set of tools is to be extended by non-obtrusive features and functions that today’s computerized environments support. Such capabilities include non-destructive editing, multiple views of the same data, analyzing tools, as well as polymorphous characteristics of the input device. However, this complex functionality should be hidden from the user, for whom an interaction with a sketch-based device should be as natural and intuitive as scribbling on a notebook.

There is, however, an important difference between a computer user-interface and an ordinary piece of paper. Because the area of interaction is at the same time input as well as output device we can establish an extended two way communication between user and computer. That is, the system keeps the user up to date about the current status so that the user has an opportunity to react accordingly. To reflect different processing states and provide appropriate feedback, we have implemented three abstraction levels of a sketch. However, only one level is mandatory to draw and generate the formal model of a sketch. All abstraction levels are implemented as different views and the exchange of information between user and system occurs on all three levels.

Application Toolbox

Use of a Pen

The primary tool of our prototype application is a pen with a functionality that depend on the actual task. Our pen has a rocker switch with two positions like the on the figure below.

The major goal of alternative user-interface elements is to enhance the interface between user and computer and support an interaction without being obtrusive (Blaser, Sester, Egenhofer 1999). A promising approach for doing this is to copy from frequently used methods that people use to communicate with each other. For a sketch-based application this implies that we have to take drawing gestures into account. Our prototype has three typical drawing gestures implemented that could be used for any drawing application.

The communication from computer to user is primarily conducted on a visual base. Although is possible—and for some applications even adequate—to give acoustical feedback and instructions, it is often considered distracting or unwanted, for instance when more than one user is working in the same environment. Sound can only hardly be channeled into a specific direction, it is transitory, and people are not capable to interpret more than one or two acoustical signals at the same time. This is much different with the visual approach that can be directed and that provides the base for a broadband interaction between user and system, which is, because the output device can be scanned and because it is perpetual. We have, therefore, implemented various visual clues and forms of interaction that people already are familiar with in their everyday life.

The readiness of the sketch, finally, is indicated by three lights with different colors (red, yellow, green, and blue) that are situated in the lower right corner of the user-interface. The lights indicate the status of sketched objects, the formal network, and the spatial database query (provided but not yet functional).