Joaquim Madeira

An algorithm for the ranking of shortest paths

An efficient computational implementation of a path deletion K shortest paths algorithm and a new algorithm for the same problem are presented. In a path deletion K shortest paths algorithm a sequence {g1, g2,…, gK} of networks is is defined, such that g1 is given network and its k-th shortes path is trivially determined from the shortest path in gk. In essence, as soon as the shortest path in gK is determined it is excluded from gk in such a way that no new paths are formed and no more paths are deleted. So, far each gk two procedures are executed: a shortest path algorithm and a path deletion algorithm. In the presented computational implementation, all the information resulting from the determination of the k-th shortest path is carried throughout gk + 1, gk + 2,…, gK. The new algorithm not only keeps this characteristics but also avoids the last K−1 executions of a shortest path algorithm, which results in a suprising and very substantial reduction in the execution time. In fact, for randomly generated networks with 104 nodes and 105 arcs, once the shortest the new algorithm computes the next 100 shortest paths in times of the order of 10−1 seconds. To illustrate the efficiency of this algorithm, comparative computational experiments are reported.

Head-mounted display versus desktop for 3D navigation in virtual reality: a user study

Virtual Reality (VR) has been constantly evolving since its early days, and is now a fundamental technology in different application areas. User evaluation is a crucial step in the design and development of VR systems that do respond to users’ needs, as well as for identifying applications that indeed gain from the use of such technology. Yet, there is not much work reported concerning usability evaluation and validation of VR systems, when compared with the traditional desktop setup. The paper presents a user study performed, as a first step, for the evaluation of a low-cost VR system using a Head-Mounted Display (HMD). That system was compared to a traditional desktop setup through an experiment that assessed user performance, when carrying out navigation tasks in a game scenario for a short period. The results show that, although users were generally satisfied with the VR system, and found the HMD interaction intuitive and natural, most performed better with the desktop setup.

Technical Section: Using color in visualization: A survey

Color mapping is an important technique used in visualization to build visual representations of data and information. With output devices such as computer displays providing a large number of colors, developers sometimes tend to build their visualization to be visually appealing, while forgetting the main goal of clear depiction of the underlying data. Visualization researchers have profited from findings in adjoining areas such as human vision and psychophysics which, combined with their own experience, enabled them to establish guidelines that might help practitioners to select appropriate color scales and adjust the associated color maps, for particular applications. This survey presents an overview on the subject of color scales by focusing on important guidelines, experimental research work and tools proposed to help non-expert users.

A computational improvement for a shortest paths ranking algorithm

Abstract In this short note we present a computational improvement for the shortest paths ranking algorithm due to some of the authors. According to its computational complexity, this new version of the algorithm entails a very significant reduction in its execution time and an even larger reduction in the required computer memory space. Computational experiments are reported.

There is More to Color Scales than Meets the Eye: A Review on the Use of Color in Visualization

The appropriate use of color in Visualization is a very important subject. The choice of the proper color scale to use with a particular data set is not just a matter of choosing the prettiest representation. Throughout the years researchers have studied this subject and managed to propose guidelines which help users along the process of color scale selection. This article presents a brief overview on the subject focusing on the desired properties for color scales, the guidelines that should drive their choice, the advantages of applying those guidelines, the experimental research work on the field, and the tools proposed to help non-expert users.

Efficiency of boundary evaluation for a cellular model

Feature modeling systems usually employ a boundary representation (b-rep) to store the shape information on a product. It has, however, been shown that a b-rep has a number of shortcomings, and that a cellular representation can be a valuable alternative. A cellular model stores additional shape information on features, including the feature faces that are not on the boundary of the product. Such information can be profitably used for several purposes. A major operation in every feature modeling system is boundary evaluation, which computes the geometric model of a product, i.e. either the b-rep or the cellular model, from the features that have been specified by the user. Since boundary evaluation has to be executed each time a feature is added, removed or modified, its efficiency is of paramount importance. In this paper, boundary evaluation for a cellular model is described in some detail. Its efficiency is compared to the efficiency of boundary evaluation for a b-rep, on the basis of both complexity analysis and performance measurements for the two types of evaluation. It turns out that boundary evaluation for a cellular model is, in fact, more efficient than for a b-rep, which makes cellular models even more attractive as an alternative to b-reps.

PolyMeCo - a polygonal mesh comparison tool

Polygonal meshes are used in many areas to model different objects and structures. Depending on their applications, they sometimes have to be processed to, for instance, reduce their complexity (simplification). This mesh processing introduces error, whose evaluation is important when choosing the kind of processing that is to be done for a particular application. Although some mesh comparison tools are described in the literature, little attention has been given to the way results are presented. A tool is presented which enhances the way users perform mesh analysis and comparison, by providing an environment where several visualization options are available and can be used in a coordinated way.

Detection and 3D representation of pulmonary air bubbles in HRCT volumes

Bubble emphysema is a disease characterized by the presence of air bubbles within the lungs. With the purpose of identifying pulmonary air bubbles, two alternative methods were developed, using High Resolution Computer Tomography (HRCT) exams. The search volume is confined to the pulmonary volume through a previously developed pulmonary contour detection algorithm. The first detection method follows a slice by slice approach and uses selection criteria based on the Hounsfield levels, dimensions, shape and localization of the bubbles. Candidate regions that do not exhibit axial coherence along at least two sections are excluded. Intermediate sections are interpolated for a more realistic representation of lungs and bubbles. The second detection method, after the pulmonary volume delimitation, follows a fully 3D approach. A global threshold is applied to the entire lung volume returning candidate regions. 3D morphologic operators are used to remove spurious structures and to circumscribe the bubbles. Bubble representation is accomplished by two alternative methods. The first generates bubble surfaces based on the voxel volumes previously detected; the second method assumes that bubbles are approximately spherical. In order to obtain better 3D representations, fits super-quadrics to bubble volume. The fitting process is based on non-linear least squares optimization method, where a super-quadric is adapted to a regular grid of points defined on each bubble. All methods were applied to real and semi-synthetical data where artificial and randomly deformed bubbles were embedded in the interior of healthy lungs. Quantitative results regarding bubble geometric features are either similar to a priori known values used in simulation tests, or indicate clinically acceptable dimensions and locations when dealing with real data.

A 3d tool for left ventricle segmentation editing

Image segmentation has a very important role in many application areas, such as medical imaging. Even robust segmentation methods cannot deal with the wide range of variation observed, for example, in shape and orientation of an anatomical structure. Given the need to accomplish accurate segmentations in order to perform quantitative measurements or compare structures in different time instances, it is important to have tools which allow easy segmentation editing/correction by experts. In 3D images (e.g., obtained using CT scanners) performing segmentation editing of regions which span several slices might be a tiresome task if it has to be done slice-by-slice with a 2D tool. This article presents a 3D segmentation editing tool, to be applied to left ventricle segmentations, which enables radiographers to correct segmentations provided by an automatic method.

Education: PolyMeCo-An integrated environment for polygonal mesh analysis and comparison

Polygonal meshes are used in several application areas to model different objects and structures. Depending on the application, mesh models sometimes have to be processed to, for instance, reduce their complexity (mesh simplification). Such operations introduce differences regarding the original mesh, whose evaluation is of paramount importance when choosing the processing methods to be applied for a particular purpose. Although some mesh analysis and comparison tools are described in the literature, little attention has been given to the way mesh features and mesh comparison results can be visualized. Moreover, particular functionalities have to be made available to enable systematic use and proper data analysis and exploration. PolyMeCo-a tool for polygonal mesh analysis and comparison-was designed and developed taking the above objectives into account. It enhances the way users analyze features and compare meshes by providing an integrated environment where various mesh quality measures and several visualization options are available and can be used in a coordinated way, thus leading to greater insight into the visualized data.