Clifford A. Shaffer

Flexible collaboration transparency: supporting worker independence in replicated application-sharing systems

This article presents a critique of conventional collaboration transparency systems, also called “application-sharing” systems, which provide the real-time shared use of legacy single-user applications. We find that conventional collaboration transparency systems are inefficient in their use of network resources and lack support for key groupware principles: concurrent work, relaxed WYSIWIS, and group awareness. Next, we present an alternative approach to implementing collaboration transparency that provides many features previously seen only in collaboration-aware applicaitons. Our approach is based on a replicated architecture where selected single-user interface components are dynamically replaced by multiuser versions. The replacement occurs at run-time and is transparent to the single-user application and its developers.. As an instance of this approach, we describe its incorporation into a Java-based collaboration transparency system for serializable, Swing-based Java applications, called Flexible JAMM (Java Applets Made Multiuser). To validate that the flexible collaboration transparency system is truly an improvement over conventional systems, we conducted an empirical study of collaborators performing both tightly and loosely coupled tasks using Flexible JAMM versus a representative conventional collaboration transparency system, Microsoft NetMeeting. Completion times were significantly faster in the loosely coupled task using Flexible JAMM and were not adversely affected in the tightly coupled task. Accuracy was equivalent for both systems. Participants greatly preferred Flexible JAMM.

Transparent sharing of Java applets: a replicated approach

People interact together in all aspects of life and, as computers have become prevalent, users seek computer support for their interactions. The WWW provides an unprecedented opportunity for users to interact with each other, and the advent of JavaThfl has created a consistent computing environment to support synchronous collaboration. We describe JAMM, a prototype Java runtime environment that supports the shared use of existing Java applets, thus leveraging the existing base of software for synchronous collaboration. Our approach is based on a replicated architecture, where each user maintains their own copy of the Java applet, and the users’ input events are broadcast to each applet copy. We discuss solutions to certain key problems, such as unanticipated sharing, supporting late-joiners, and replicating input sources other than user inputs (e.g., files, sockets, and random number generators).

Optimal quadtree construction algorithms

An algorithm is presented that builds a quadtree in time proportional to the number of blocks in the image. In particular, this algorithm constructs a linear quadtree from a raster image stored on disk in time proportional to the number of nodes in the output quadtree plus the (relatively minor) amount of time to read the input data. Empirical tests show that for typical 512 x 512 pixel images, the new algorithm results in an order of magnitude or better improvement over traditional algorithms which insert each pixel separately and require a merge routine to form larger nodes. These traditional algorithms have an execution time that is proportional to the number of pixels in the image. Thus, when using the number of insertions into the output quadtree as a metric, our algorithm is optimal to within a constant factor. The algorithm is also adapted to build a pointer-based quadtree, again without a need to merge any nodes.

A GEOGRAPHIC INFORMATION SYSTEM USING QUADTREES

We describe the current status of an ongoing research effort to develop a geographic information system based on quadtrees. Quadtree encodings were constructed for area, point and line features for a small area in Northern California. The encoding used was a variant of the linear quadtree. The implementation used a B-tree to organize the list of leaves and allow management of trees too large to fit in core memory. Several database query functions have been implemented, including set operations, region property computations, map editing functions and map subset and windowing functions. A user of the system may access the database via an English-like query language.

A real-time robot arm collision avoidance system

A data structure and update algorithm are presented for a prototype real-time collision avoidance safety system simulating a multirobot workspace. The data structure is a variant of the octree, which serves as a spatial index. The authors use the N-objects octree, which indexes a collection of 3D primitive solids. These primitives make up the two seven-degree-of-freedom robot arms and workspace modeled by the system. Octree nodes containing mor than a predetermined number N of primitives are decomposed. This rule keeps the octree small, as the entire world model for the application can be implemented using a few dozen primitives. As robot arms move, the octree is updated to reflect their changed positions. Modification to the octree is rarely required. Incidents in which one robot arm comes too close to another arm or an object are reported. Cycle time for interpreting current arm joint angles, updating the octree to reflect new positions, and detecting/reporting imminent collisions averages 30 ns on an Intel 80386 processor running at 20 MHz. >

QUILT: a geographic information system based on quadtrees†

This paper describes QUILT, a prototype geographic information system (GIS) that uses the quadtree data structure as the underlying representation for cartographic data. While QUILT contains many features typically available in a GIS, its primary purpose is to serve as a testbed for the design and testing of new data structures and algorithms for use in computer cartography. Quadtree variants for region, point and line data are implemented using the linear quadtree, organized on disk by a B-tree. QUILT provides a simple attribute attachment system which associates non-spatial data with geographic objects. The user views QUILT as an augmented LISP environment. QUILTs geographic functions include conversion of rasters to and from quadtrees; subset operations to select specified geographic objects; map editing, display, windowing, intersection and union operations; polygon expansion; and computation of geographic object properties such as the centroid, area, perimeter and bounding rectangle for sets of geog...

Algorithm visualization: a report on the state of the field

We present our findings on the state of the field of algorithm visualization, based on extensive search and analysis of links to hundreds of visualizations. We seek to answer questions such as how content is distributed among topics, who created algorithm visualizations and when, the overall quality of available visualizations, and how visualizations are disseminated. We have built a wiki that currently catalogs over 350 algorithm visualizations, contains the beginnings of an annotated bibliography on algorithm visualization literature, and provides information about researchers and projects. Unfortunately, we found that most existing algorithm visualizations are of low quality, and the content coverage is skewed heavily toward easier topics. There are no effective repositories or organized collections of algorithm visualizations currently available. Thus, the field appears in need of improvement in dissemination of materials, informing potential developers about what is needed, and propagating known best practices for creating new visualizations.

Effective features of algorithm visualizations

Many algorithm visualizations have been created, but little is known about which features are most important to their success. We believe that pedagogically useful visualizations exhibit certain features that hold across a wide range of visualization styles and content. We began our efforts to identify these features with a review that attempted to identify an initial set of candidates. We then ran two experiments that attempted to identify the effectiveness for a subset of features from the list. We identified a small number of features for algorithm visualizations that seem to have a significant impact on their pedagogical effectiveness, and found that several others appear to have little impact. The single most important feature studied is the ability to directly control the pace of the visualization. An algorithm visualization having a minimum of distracting features, and which focuses on the logical steps of an algorithm, appears to be best for procedural understanding of the algorithm. Providing a good example for the visualization to operate on proved significantly more effective than letting students construct their own data sets. Finally, a pseudocode display, a series of questions to guide exploration of the algorithm, or the ability to back up within the visualization did not show a significant effect on learning.

Supporting worker independence in collaboration transparency

Conventional collaboration-transparency systems are inefficient in their use of network resources and lack support for key groupware principles: concurrent work, relaxed WYSIWIS, and group awareness. We present an alternative implementation approach to collaboration transparency that provides many features previously seen only in collaboration-aware applications. Our approach is based on an object-oriented replicated architecture where selected single-user interface objects are dynamically replaced by multi-user extensions. The replacement is transparent to the single-user application and its developer. As an instance of this approach, we describe its incorporation into a new Java-based collaboration-transparency system, called Flexible JAMM. We conducted an empirical study to evaluate the effectiveness of Flexible JAMM versus a representative conventional collaboration-transparency system, NetMeeting. Completion times were significantly faster in a loosely-coupled task using Flexible JAMM, and were not adversely affected in a tightly-coupled task. Accuracy was unaffected by the system used. Participants greatly preferred Flexible JAMM. The evaluation validates our aim of supporting multiple styles of collaboration.

In-building wideband multipath characteristics at 2.5 and 60 GHz

This paper contains measured data for 2.5 and 60 GHz in-building partition loss. Path loss measurements were recorded using a broadband sliding correlator channel sounder which recorded over 39000 power delay profiles (PDP) in 22 separate locations in a modern office building. Transmitters and receivers were separated by distances ranging from 3.5 to 27.4 meters, and were separated by a variety of obstructions, in order to emulate future single-cell-per-room wireless networks. These measurements may aid in the development of future in-building wireless networks in the unlicensed 2.4 GHz and 60 GHz bands.