Sign In

Communications of the ACM

101 - 110 of 225 for bentley

The skip quadtree: a simple dynamic data structure for multidimensional data

We present a new multi-dimensional data structure, which we call the skip quadtree (for point data in R2) or the skip octree (for point data in Rd, with constant d > 2). Our data structure combines the best features of two well-known data structures, in that it has the well-defined "box"-shaped regions of region quadtrees and the logarithmic-height search and update hierarchical structure of skip lists. Indeed, the bottom level of our structure is exactly a region quadtree (or octree for higher dimensional data). We describe efficient algorithms for inserting and deleting points in a skip quadtree, as well as fast methods for performing point location, approximate range, and approximate nearest neighbor queries.


Why the high attrition rate for computer science students: some thoughts and observations

This paper investigates the possible causes for high attrition rates for Computer Science students. It is a serious problem in universities that must be addressed if the need for technologically competent professionals is to be met.


Photon mapping made easy

This paper presents the authors' introduction of photon mapping in an undergraduate computer graphics course, Software was designed as a pedagogical and demonstration tool which permitted students to practice and learn photon mapping. Classroom experience and examples that illustrate soft shadows, color bleeding, indirect illumination and caustic are also discussed.


Evaluating computer-supported cooperative work: models and frameworks

Evaluating distributed CSCW applications is a difficult endeavor. Frameworks and methodologies for structuring this type of evaluation have become a central concern for CSCW researchers. In this paper we describe the problems involved in evaluating remote collaborations, and we review some of the more prominent conceptual frameworks of group interaction that have driven CSCW evaluation in the past. A multifaceted evaluation framework is presented that approaches the problem from the relationships underlying joint awareness, communication, collaboration, coordination, and work coupling. Finally, recommendations for carrying out multifaceted evaluations of remote interaction are provided.


A laboratory method for studying activity awareness

Many failures in long-term collaboration occur because of a lack of activity awareness. Activity awareness is a broad concept that involves awareness of synchronous and asynchronous interactions over extended time periods. We describe a procedure to evaluate activity awareness and collaborative activities in a controlled setting. The activities used are modeled on real-world collaborations documented earlier in a field study. We developed an experimental method to study these activity awareness problems in the laboratory. Participants worked on a simulated long-term project in the laboratory over multiple experimental sessions with a confederate, who partially scripted activities and probes. We present evidence showing that this method represents a valid model of real collaboration, based on participants' active engagement, lively negotiation, and awareness difficulties. We found that having the ability to define, reproduce, and systematically manipulate collaborative situations allowed us to assess the effect of realistic conditions on activity awareness in remote collaboration.


Binary space partitions of orthogonal subdivisions

We consider the problem of constructing binary space partitions (BSPs) for orthogonal subdivisions (space filling packings of boxes) in d-space. We show that a subdivision with n boxes can be refined into a BSP of size O(n d+1/3), for all d ≥ 3, and that such a partition can be computed in time O(K log n), where K is the size of the BSP produced. Our upper bound on the BSP size is tight for 3-dimensional subdivisions in higher dimensions, this is the first nontrivial result for general full-dimensional boxes. We also present a lower bound construction for a subdivision of n boxes in d-space that requires a BSP of size Ω(n946;(d)), where β(d) converges to (1+ √5 )/2 as d → ∞.


Assessing the effect of failure severity, coincident failures and usage-profiles on the reliability of embedded control systems

The increasingly ubiquitous use of embedded systems to manage and control our technologically (ever-increasing) complex lives makes us more vulnerable than ever before. Knowing how reliable such systems are is absolutely necessary especially for safety, mission and infrastructure critical applications. This paper presents a structured compositional modeling method for assessing reliability based on characteristic data and stochastic models. We illustrate this using a classic embedded control system (sensor-inputs | processing | actuator-outputs), Anti-lock Braking System (ABS) and empirical data. Special emphasis is laid on modeling extra-functional characteristics of severity of failures, coincident failures and usage-profiles with the goal of developing a modeling strategy that is realistic, generic and extensible. The validation approach compares the results from the two separate models. The results are comparable and indicate the effect of coincident failures, failure severity and usage-profiles is predictable.


Computing curricula 2004: the overview project

In 2001, the ACM and the IEEE-CS published Computing Curricula 2001 which contains curriculum recommendations for undergraduate programs in computer science. That report also called for additional discipline-specific volumes for each of computer engineering, information systems, and software engineering. In addition, it called for an Overview Volume to provide a synthesis of the various volumes. The Computing Curricula 2004 Task Force has undertaken a two-pronged strategy to fulfill the latter charge. The purpose of this session is to present an overview of the Task Force's work and to generate feedback from the SIGCSE membership to the Task Force about the direction and plans we have undertaken.


A multi-modal approach for determining speaker location and focus

This paper presents a multi-modal approach to locate a speaker in a scene and determine to whom he or she is speaking. We present a simple probabilistic framework that combines multiple cues derived from both audio and video information. A purely visual cue is obtained using a head tracker to identify possible speakers in a scene and provide both their 3-D positions and orientation. In addition, estimates of the audio signal's direction of arrival are obtained with the help of a two-element microphone array. A third cue measures the association between the audio and the tracked regions in the video. Integrating these cues provides a more robust solution than using any single cue alone. The usefulness of our approach is shown in our results for video sequences with two or more people in a prototype interactive kiosk environment.


On translating geometric solids to functional expressions

Language for Structured Design (LSD)is a high level, visual, logic programming language for design of structured objects. LSD combines the design and programming activities in a homogeneous programming/design environment by extending Lograph, a visual logic programming language, with the notion of solids and operations on them. At the back-end, however, a solid modeling kernel for maintaining low level description of solids and operations is required.In this paper, we report on our progress towards employing PLaSM, a functional programming language for solid modeling, as the solid modeling kernel of LSD. This is achieved through the use of a translator engine which transforms the high level object description produced by LSD programs to PLaSM programs.