Abstract
Precomputed Radiance Transport (PRT) was introduced as a technique to enable interactive navigation and distant environmental real time relighting of rigid scenes. Evaluating radiance transport is, however, a computationally very demanding task, which precludes PRT's utilization during the model design phase, since the user must wait for long periods of time before being able to light and navigate within the model. This paper proposes and validates an approach to provide visual feedback to the user as soon as possible, within PRT context. By resorting to parallel processing and progressive refinement, the user is quickly presented with a lower lighting resolution of the virtual model. This is then progressively refined by incrementally increasing the number of incident directions taken into account on transport computations. PRT is, however, a complex algorithm that requires frequent collective communications of huge volumes of data, thus constraining the maximum achievable speedup on a parallel system. This issue is analysed and an alternative workload distribution is proposed and evaluated on a 12 node dual processor cluster. The final solution ensures a good resource utilization rate, reducing response times from dozens of seconds to a few hundred milliseconds. Copyright © 2006 by the Association for Computing Machinery, Inc.

Abstract
Physically based global illumination rendering at interactive frame rates would enable users to navigate within complex virtual environments, such as archaeological models. These algorithms, however, are computationally too demanding to allow interactive navigation on current PCs. A technique based on image subsampling and spatiotemporal coherence among successive frames is exploited, while resorting to progressive refinement whenever there is available computing power. A physically based ray tracer (Radiance) is used to compute reflected radiance at the model's triangles vertices. Progressive refinement is achieved increasing the sampling frequency by subdividing certain triangles and requesting shading information for the resulting vertices. This paper proposes and evaluates different criteria for selecting which triangles to subdivide. A random criterium and two criteria based on Normalized Luminance Differences are evaluated: one operating on image space, the other on object space. Results, obtained with a model of an old roman town, show that the object space criterium is able to locate and represent visual discontinuities, such as shadows, and does so requiring less triangle subdivisions than the other two.

Abstract
The 2006 Eurographics Symposium on Parallel Graphics and Visualization took place at Universidade do Minho, Barga, Portugal. There were a total of 47 registered participants which represents a significant increase in the number of attendees compared to previous editions of the event. The goal was to enable people to attend all events in an attempt to answer a reciprocal interest on research works. It was believed that by organizing related events in the same geographical area it is able to attract more participants for each events. The first keynote was on Rendering on Demand by Alan Chalmers from the University of Bristol. The second keynote was devoted entirely to commodity visualization clusters and many other papers generated results using clusters.

Abstract
Database replication based on group communication systems has recently been proposed as an efficient and resilient solution for large-scale data management. However, its evaluation has been conducted either on simplistic simulation models, which fail to assess concrete implementations, or on complete system implementations which are costly to test with realistic large-scale scenarios. This paper presents a tool that combines implementations of replication and communication protocols under study with simulated network, database engine, and traffic generator models. Replication components can therefore be subjected to realistic large scale loads in a variety of scenarios, including fault-injection, while at the same time providing global observation and control. The paper shows first how the model is configured and validated to closely reproduce the behavior of a real system, and then how it is applied, allowing us to derive interesting conclusions both on replication and communication protocols and on their implementations.

Abstract
Several techniques for database replication using group communication have recently been proposed, namely, the Database State Machine, PostgresR, and the NODO protocol. Although all rely on a totally ordered multicast for consistency, they differ substantially on how multicast is used. This results in different performance trade-offs which are hard to compare as each protocol is presented using a different load scenario and evaluation method. In this paper we evaluate the suitability of such protocols for replication of On-Line Transaction Processing (OLTP) applications in clusters of servers and over wide area networks. This is achieved by implementing them using a common infra-structure and by using a standard workload. The results allows us to select the best protocol regarding performance and scalability in a demanding but realistic usage scenario.

Abstract
This paper explores sonic ideas concerning the time-analysis of functional programs defined by instantiating typical recursion patterns such as folds, unfolds. and hylomorphisms. The concepts in this paper are illustrated through a rich set of examples in the Haskell programming language. We concentrate on unfolds and folds (also known as anamorphisms and catamorphisms respectively) of recursively defined types, as well as the more general hylomorphism pattern. For the latter, we use as case-studies two famous sorting algorithms, mergesort and quicksort. Even though time analysis is not compositional, we argue that splitting functions to expose the explicit construction of the recursion tree and its later consumption helps with this analysis.