David A. Carr

Extending tree-maps to three dimensions : a comparative study

This paper presents StepTree, an information visualization tool designed for depicting hierarchies, such as directory structures. StepTree is similar to the hierarchy-visualization tool, Treemap, in that it uses a rectangular, space-filling methodology, but differs from Treemap in that it employs three-dimensional space, which is used to more clearly convey the structural relationships of the hierarchy. The paper includes an empirical study comparing typical search and analysis tasks using StepTree and Treemap. The study shows that users perform significantly better on tasks related to interpreting structural relationships when using StepTree. In addition, users achieved the same performance with StepTree and Treemap when doing a range of other common interpretative and navigational tasks.

The effect of animated transitions on user navigation in 3D tree-maps

This paper describes a user study conducted to evaluate the use of smooth animated transitions between directories in a three-dimensional, tree-map visualization. We looked specifically at the task of returning to a previously visited directory after either an animated or instantaneous return to the root location. The results of the study show that animation is a double-edged sword. Even though users take more shortcuts, they also make more severe navigational errors. It seems as though the promise of a more direct route to the target directory, which animation provides, somehow precludes users who navigate incorrectly from applying a successful recovery strategy.

Browsing thumbnails: a comparison of three techniques

We conducted a usability study comparing three different browser alternatives (iconic, zoom-and-pan, and fisheye) for an electronic bulletin board containing thumbnails of notes. The iconic browser behaves in the manner of a standard windowing system. The zoom-and-pan browser allows the user to pan over a note and zoom to a readable size. The fisheye browser was an implementation of the bifocal view. We conducted an experiment where 21 subjects performed browsing tasks with each browser. We found that users performed best with the iconic browser and that young people (20-25) were significantly faster than older (31-53)

Evaluating graph theoretic clustering algorithms for reliable multicasting

In reliable multicast protocols, each data packet being sent must be acknowledged. Collecting the acknowledgments centrally at the sources can cause ACK-implosion and can result in poor scalability. To overcome this, clustering algorithms which use virtual structures to gather acknowledgments were proposed. In this work, we analyze the complexities of three such clustering algorithms: Lorax, k-degree, and Self-adjust. We compare the quality of the virtual structures produced by these! algorithms, focusing on the number of clusters, cluster size, cluster radius, and the optimal positioning of cluster leaders. Our simulation showed that the virtual structure produced by Self-adjust is better in terms of cluster radius and the location of cluster leaders. However, due to the self-adjusting nature of the algorithm, it might take longer time to compute than the other two algorithms.

Toward more understandable user interface specifications

Many different methods have been used to specify user interfaces: algebraic specification, grammars, task description languages, transition diagrams with and without extensions, rule-based systems, and by demonstration. However, none of these methods has been widely adopted. Current user interfaces are still built by writing a program, perhaps with the aid of a UIMS. There are two principal reasons for this. First, specification languages are difficult to use. Reading a specification and understanding its exact meaning is very difficult. Writing a correct specification is even more difficult. Second, most specification languages are not executable. This means that after the user interface programmer makes the effort to write a specification, the user interface must still be coded. As a consequence, most programmers have little incentive to do a specification. A pilot study into the comprehensibility of specifications is described. The results of this study suggest that user interface specifications are difficult to interpret manually. A possible solution to this problem, specification animation, is also described.

A cluster-tree protocol for reliable multicasting

Distributed simulations and conferences require a reliable multicast protocol that guarantees safe data delivery in a reasonably short time. Such high-quality service demands substantial network resources. As these applications grow in use, scalability becomes an important issue. One way to achieve scalability is through clustering. The overall load is distributed among clusters so that large multicast groups avoid overloading the network. We propose a protocol for reliable multicasting, based on a cluster structure. We prove that in the cluster, the leader is an ancestor of all cluster members with respect to the multicast routing tree. This relationship yields an efficient acknowledgement structure. We also describe an acknowledgement algorithm based on a pulsing mechanism and prove that it has constant latency for acknowledging data. Finally, we show that the protocol is scalable by proving that it generates a constant load for all nodes.