Percorrer por data de Publicação, começado por "2010-12"
A mostrar 1 - 10 de 15
Resultados por página
Opções de ordenação
- A consulta de apoio psicológico no Serviço de Apoio ao Estudante do Instituto Politécnico de Leiria (SAPE/IPL)Publication . Pereira, Ana Patrícia; Seco, Graça Maria dos Santos Batista; Filipe, Luís Pereira; Alves, Sandra
- Multiple description video transcoding with temporal drift controlPublication . Correia, Pedro; Assunção, Pedro; Silva, VitorThis paper proposes a multiple description (MD) transcoding scheme capable of preventing drift by distortion accumulation in temporally predicted motion compensated slices. Drift compensation is achieved by generating a controlled amount of side information to be used for decoding whenever a description fails to reach the end user terminal. The side information is generated by re-encoding the transcoding residue with an independent quantisation parameter which also controls redundancy. A simplified architecture is devised to reduce transcoding complexity in regard to the number of processing functions and buffer requirements. The experimental results show that temporally predicted frames do not suffer from drift and their quality is significantly improved at reduced redundancy cost in comparison with a classic MD transcoding scheme.
- Subjective assessment of frame loss concealment methods in 3D videoPublication . Carreira, J.; Pinto, L.; Rodrigues, N.; Faria, S.; Assuncao, P.This paper investigates the subjective impact resulting from different concealment methods for coping with lost frames in 3D video communication systems. It is assumed that a high priority channel is assigned to the main view and only the auxiliary view is subject to either transmission errors or packet loss, leading to missing frames at the decoder output. Three methods are used for frame concealment under different loss ratios. The results show that depth is well perceived by users and the subjective impact of frame loss not only depends on the concealment method but also exhibits high correlation with the disparity of the original sequence. It is also shown that under heavy loss conditions it is better to switch from 3D to 2D rather than presenting concealed 3D video to users.
- Multiscale recurrent pattern matching approach for depth map codingPublication . Graziosi, Danillo B.; Rodrigues, Nuno M. M.; Pagliari, Carla L.; Silva, Eduardo A. B. da; Faria, Sérgio M. M. de; Perez, Marcelo M.; Carvalho, Murilo B. deIn this article we propose to compress depth maps using a coding scheme based on multiscale recurrent pattern matching and evaluate its impact on depth image based rendering (DIBR). Depth maps are usually converted into gray scale images and compressed like a conventional luminance signal. However, using traditional transform-based encoders to compress depth maps may result in undesired artifacts at sharp edges due to the quantization of high frequency coefficients. The Multidimensional Multiscale Parser (MMP) is a pattern matching-based encoder, that is able to preserve and efficiently encode high frequency patterns, such as edge information. This ability is critical for encoding depth map images. Experimental results for encoding depth maps show that MMP is much more efficient in a rate-distortion sense than standard image compression techniques such as JPEG2000 or H.264/AVC. In addition, the depth maps compressed with MMP generate reconstructed views with a higher quality than all other tested compression algorithms.
- Characterization of Titan’s Ontario Lacus region from Cassini/VIMS observationsPublication . Moriconi, M. L.; Lunine, J. I.; Adriani, A.; D’Aversa, E.; Negrão, A.; Filacchione, G.; Coradini, A.Liquid hydrocarbons were long predicted on Titan's surface before the RADAR instrument onboard Cassini detected lakes poleward of 70°N in July 2006. Before that the Cassini Imaging Science Subsystem (ISS) observed a lake-like feature in the South Pole, named Ontario Lacus, in July 2004. Here we analyze one observation of Ontario Lacus taken by the Visual and Infrared Mapping Spectrometer (VIMS) on 2007 December 5, during the T 38 flyby. This is the best spatially resolved image of a Titan lake to date by an imaging spectrometer, and has been previously reported in Brown et al. (Brown, R.H., Soderblom, L.A., Soderblom, J.M., Clark, R.N., Jaumann, R., Barnes, J.W., Sotin, C., Buratti, B., Baines, K.H., Nicholson, P.D. [2008]. Nature 454, 607-610) and in Barnes et al. (Barnes, J.W. et al. [2009]. Icarus 201, 217-225). The observing geometry and our data processing will be explained, followed by a discussion of the main characteristics of the image. The analyzed image covers a small portion of Ontario Lacus and shows what appears from RADAR data to be a region of modest slope (" ramp" ) adjacent to the dark lake itself. Our analysis of 5.0 μm spectral data suggests that the previously reported absorption feature of ethane seen at shorter wavelengths may be produced by damp sediments adjacent to the main liquid basin. The latter appears to be absorbing all of the photons incident upon it in the 5 μm spectral region and shows no discernible absorption features. A characterization of the basin composition and morphology is developed with the help of ISS and RADAR observations. The simplest model consistent with the data is an optically deep lake surrounded by a region in which ethane, propane, possibly methane, and other, less volatile hydrocarbons and nitriles are present mixed into spectroscopically neutral sediments. The dominance of relatively low vapor pressure organics outside the lake itself suggests a retreat of Ontario Lacus associated with evaporation on seasonal or longer timescales, consistent with analysis of RADAR and ISS images.
- Parallel AlineaGA: An island parallel evolutionary algorithm for multiple sequence alignmentPublication . Silva, Fernando José Mateus da; Pérez, Juan Manuel Sánchez; Pulido, Juan Antonio Gómez; Ródriguez, Miguel A. VegaMultiple sequence alignment is the base of a growing number of Bioinformatics applications. This does not mean that the accuracy of the existing methods corresponds to biologically faultless alignments. Searching for the optimal alignment for a set of sequences is often hindered by the size and complexity of the search space. Parallel Genetic Algorithms are a class of stochastic algorithms which can increase the speed up of the algorithms. They also enhance the efficiency of the search and the robustness of the solutions by delivering results that are better than those provided by the sum of several sequential Genetic Algorithms. AlineaGA is an evolutionary method for solving protein multiple sequence alignment. It uses a Genetic Algorithm on which some of its genetic operators embed a simple local search optimization. We have implemented its parallel version which we now present. Comparing with its sequential version we have observed an improvement in the search for the best solution. We have also compared its performance with ClustalW2 and T-Coffee, observing that Parallel AlineaGA can lead the search for better solutions for the majority of the datasets in study.
- Alternative Zigbee codes derived from orthogonal perfect DFT sequencesPublication . Pereira, João S.; Silva, Henrique J. A. daThe performance of a code-division multiple access (CDMA) system is directly related with the choice of adequate codes. The codes for use in CDMA communication systems should have a perfect aperiodic (or periodic) autocorrelation function and should be orthogonal to each other at all time shifts. A mathematical property that provides a way to find large sets of real perfect DFT (discrete Fourier transform) sequences is presented. These sequences can be transformed into real orthogonal perfect DFT sequences and also into bipolar codes that have better properties than the codes used in the Zigbee communication system. These new bipolar codes are approximately orthogonal and have nearly optimum periodic and aperiodic autocorrelation functions. We also evaluate our bipolar codes with an improved error probability model.
- Using the semantic web to define a language for modelling controlled flexibility in software processesPublication . Martinho, R.; Varajão, J.; Domingos, D.Software processes and corresponding models are dynamic entities that must evolve to cope with changes occurred in the enacting process, the software development organisation, the market and the methodologies used to produce software. However, in the everyday practice, software team members do not want total flexibility. They rather prefer to learn about and follow previously defined controlled flexibility, that is, advices on which, where, how and by whom process models and related instances can change/adapt. Process engineers can express these advices within a process model with a domain-specific language (DSL), which complements the core process modelling language with additional controlled flexibility information. Then, software team members can browse and learn on this information in process models and instances, and be guided when performing changes. In this study, the authors propose the use of the semantic web and associated ontology-based technologies to develop and evolve their controlled flexibility DSL for software processes. They use an ontology-based format to define the controlled flexibility-related concepts, descriptions and axioms that specify the formal semantics of their DSL. In addition, the authors provide concrete mappings between these ontology concepts and a unified modelling language class-based DSL metamodel and describe how it supports changes made in the ontology.
- On the quantification of the controlling regimes in automotive catalytic convertersPublication . Santos, H.; Costa, M.The conversion of pollutants in automotive catalytic converters is influenced by a number of physical and chemical processes that take place in the gaseous and solid phases as the exhaust gases flow through the converter. A detailed understanding of the complex processes involving flow dynamics, heat and mass transport and heterogeneous surface reactions is of crucial importance to improve the converter design. The main objective of the present study is to quantify the magnitudes of the external and internal mass transfer as well as chemical reaction limiting processes as a function of the converter operating temperature. To this end, experimental data, obtained for a three way catalyst (TWC) under real world operating conditions, are analyzed and compared against analytical expressions that allow for the quantification of the different limiting processes involved. The results demonstrate that (i) the external mass transfer resistance overlaps the reaction resistance only at moderate operating temperatures and not immediately above the ignition temperature as generally considered in the literature, (ii) the transport phenomena (external and internal mass transfer) represents 90% of the total resistance for temperatures higher than 792 K, (iii) the internal mass transfer in the porous washcoat presents a larger resistance than the external mass transfer from the bulk fluid to the washcoat wall even at high operating temperatures, and (iv) based on the quantification of the individual resistances as a function of the TWC operating temperature, it was demonstrated both the influence of the substrate cell density and of the effective diffusivity on the TWC conversions.
- Dynamics on certain sets of stochastic matricesPublication . Ramos, C. Correia; Martins, Nuno; Baptista, A. NascimentoWe study iteration of polynomials on symmetric stochastic matrices. In particular, we focus on a certain one-parameter family of quadratic maps which exhibits chaotic behavior for a wide range of the parameters. The well-known dynamical behavior of the quadratic family on the interval, and its dependence on the parameter, is reproduced on the spectrum of the stochastic matrices. For certain subclasses of stochastic matrices the referred dynamical behavior is also obtained in the matrix entries. Since a stochastic matrix characterizes a Markov chain, we obtain a discrete dynamical system on the space of reversible Markov chains. Therefore, depending on the parameter, there are initial conditions for which the corresponding reversible Markov chains will lead under iteration to a fixed point, to a periodic point, or to an aperiodic point. Moreover, there are sensitivity to initial conditions and the coexistence of infinite repulsive periodic orbits, both features of chaos.