Summary

Ben Shneiderman coined the term direct manipulation interfaces for inter-active systems that give the user direct control of objects via physical ac-tions [109, 110]. Shneiderman identified key properties of highly usable human-computer interfaces. His analysis provides the necessary criteria to evaluate the achievements and shortcomings of constraint-drawing interfaces.

The three principles of direct manipulation interfaces are

1. Continuous representation of the objects and actions of interest;

2. Physical actions or presses of labeled buttons instead of complex syn-tax;

3. Rapid incremental reversible operations whose effect on the object of interest is immediately visible [110].

Not every graphical user interface fulfills these requirements and, con-versely, direct manipulation interfaces do not have to be graphical interfaces.

Window managers on all operating systems allow the user to manipulate the position and size of application windows interactively. Until a few years ago, the window content did not update while the user dragged a window. The manipulated object was thus not continuously represented, only its outline would have followed the mouse cursor. Direct manipulation systems should give the user the feeling of interacting with objects themselves, and continu-ously representing the objects on the screen is important in maintaining this illusion.

Shneiderman suggests that application interfaces that incorporate these principles have many beneficial attributes:

1. Novices can learn basic functionality quickly, usually through a demon-stration by a more experienced user.

2.2. INTERACTIVE APPLICATIONS 23 2. Experts can work extremely rapidly to carry out a wide range of tasks, even defining new functions and features, because they can explore ar-eas of interest interactively and do not have to learn a complex syntax.

3. Knowledgeable intermittent users can retain operational concepts.

4. Error messages are rarely needed.

5. Users can see immediately if their actions are furthering their goals, and if not, they can simply change the direction of their activity.

6. Users have reduced anxiety because the system is comprehensible and because actions are so easily reversible. (See [109], p. 251)

Hutchins, Hollan, and Norman [72] analyze how direct manipulation in-terfaces can reduce the user’s cognitive effort both in performing a task and in understanding the presented result. Direct manipulation interfaces are supposed to reduce the distance between the task the user has in mind and the way the task can be achieved using an application interface. Hutchins et al. call this thegulf of execution. Direct manipulation interfaces also have to minimize the distance between a system’s output and the user’s under-standing, termed the gulf of evaluation by the authors. Results should be presented in a way that corresponds with the user’s mental model so she can easily perceive, interpret and evaluate them.

Adapting a user interface to the mental concepts of its users reduces the application’s generality as Hutchins et al. describe. Common tasks can be made extremely simple, even for novice users, but tasks other than those foreseen become impossible to accomplish. This lack of generality is one of the most important disadvantages of direct manipulation interfaces that the authors mention.

The constraint-based drawing applications presented in this chapter were general-purpose drawing applications, supporting typical geometric primi-tives like points, lines, and rectangles. Their interfaces implemented many of the principles of direct manipulation outlined above. Juno and Juno-2 extended the graphical interface with a constraint equation editor. The pow-erful new concept of constraints they introduced, significantly widened the gulf of execution and the gulf of evaluation.

Expressing Intentions Using Constraints

The user has to understand how to express his intentions in terms of con-straints, i.e., she has to bridge the gulf of execution. In very few application

scenarios do users actually think in terms of constraints. Engineers who use CAD applications to simulate mechanical systems, to engineer car engines, or to analyze the structural properties of a building understand what a con-straint is and can think in terms of concon-straints. The user proficient in using a document processor and a presentation program who wishes to draw a flow chart almost certainly does not know what a constraint is.

Many of the presented applications used constraints to create new graph-ical objects from geometrgraph-ical primitives by connecting lines, fixing their length, and constraining their angles. Most interactive applications today achieve the same goal bygrouping several objects, effectively creating a new, compound object in the process. The object group behaves like a single ob-ject and can be dragged, rotated and scaled. Constraints have also been used to declare that two objects in a diagram be connected by a line. A con-necting line has several important properties: It should not intersect other objects, it should be rectilinear, and have as few bends as possible. It is difficult to express these requirements as generic constraints. A procedural algorithm, on the other hand, can recompute optimal routings of connecting lines on-the-fly while an attached object is dragged.

To a user drawing a flow chart, the concept of grouping objects and of drawing a rectilinear connecting line are immediately obvious. These con-cepts correspond to the user’s needs in a way that generic constraints do not.

Constraints could be seen as a low-level technique that could be employed to maintain desired properties like the grouping of primitives. But exposing them to the user demands too much prior knowledge that most users do not possess.

Understanding Constrained Drawings

Constraints can affect the behavior of drawn objects in ways difficult to predict. Constrained drawings thus widen the gulf of evaluation. In the presented applications, constraints were used to define object behavior. If a single object was selected by the user and subsequently moved, other ob-jects were moved too if they were connected by a chain of constraints. In studies, users have reported that such a behavior is highly unintuitive [117].

Although the user has manipulated only a single object, the effects of his action propagate through the whole drawing. At best, the cognitive effort required to predict such effects is reduced by visualizing every constraint as Briar did. This approach increased the visual clutter and made drawings hard to understand in another way.

All presented constraint-drawing applications restricted user interactions.

Briar, e.g., enforced all constraints during a user action. A user could not

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