Improving the Performance of K-Means for Color Quantization

Color quantization is an important operation with many applications in graphics and image processing. Most quantization methods are essentially based on data clustering algorithms. However, despite its popularity as a general purpose clustering algorithm, k-means has not received much respect in the color quantization literature because of its high computational requirements and sensitivity to initialization. In this paper, we investigate the performance of k-means as a color quantizer. We implement fast and exact variants of k-means with several initialization schemes and then compare the resulting quantizers to some of the most popular quantizers in the literature. Experiments on a diverse set of images demonstrate that an efficient implementation of k-means with an appropriate initialization strategy can in fact serve as a very effective color quantizer.Comment: 26 pages, 4 figures, 13 table

Materiais termocrómicos de filme fino

Dissertação para obtenção do Grau de Mestre em Engenharia de MateriaisA cor é um elemento que não pode ser dissociado do nosso dia-a-dia, uma vez que se encon-tra presente no mundo nas mais diversas formas, aparecendo na alimentação, vestuário, natureza e podendo mesmo ter influência no estado de espírito de uma pessoa. Com o gradual aumento do conhecimento sobre a ciência que está por detrás da cor, surge uma interdisciplinaridade entre diversos campos de estudo, sendo que a sua utilização no contexto da tecnologia actual tem vindo a ser bastante desenvolvida, permitindo o aparecimento de conceitos como o termocromismo. Este trabalho visou o desenvolvimento de materiais termocrómicos de filme fino. O material de base utilizado foi o vanádio (V) e o pentóxido de vanádio (V2O5). Foi produzido óxido de vanádio através da utilização de evaporação térmica resistiva e de RF magnetron sputtering, como material singular ou na forma de liga (com NPs de Sn, Ag e Au e óxidos como WO3 e TiO2). Realizou-se tam-bém um estudo das propriedades de filmes depositados a partir de um alvo de tungsténio (W), por RF magnetron sputtering. O estudo das propriedades dos filmes finos obtidos consistiu na análise das características eléctricas, ópticas, estruturais e morfológicas dos filmes, tal qual depositados e sujeitos a recozimen-tos de três tipos, a três temperaturas distintas. Como técnicas de caracterização dos filmes foram usadas a espectroscopia do visível e infravermelho próximo, difracção de raios-X, microscopia elec-trónica de varrimento, perfilometria, condutividade eléctrica em função da temperatura e termogra-fia. Os resultados obtidos permitem verificar a influência da espessura e dos diversos tipos de re-cozimento nas propriedades dos filmes depositados. De destacar, entre os resultados alcançados, que para filmes depositados por RF magnetron sputtering, a partir do alvo de vanádio, obteve-se diferentes tipos de estrutura para o óxido de vanádio, quando recozidos à pressão atmosférica ou sobre vácuo. Os filmes estudados apresentam boas propriedades semi-condutoras e, embora não se tenha verificado qualquer tipo de mudança na temperatura de transição semicondutor-isolante, não inviabiliza o material de futuramente poder ser usado em dispositivos electro-ópticos

New variants of variable neighbourhood search for 0-1 mixed integer programming and clustering

Many real-world optimisation problems are discrete in nature. Although recent rapid developments in computer technologies are steadily increasing the speed of computations, the size of an instance of a hard discrete optimisation problem solvable in prescribed time does not increase linearly with the computer speed. This calls for the development of new solution methodologies for solving larger instances in shorter time. Furthermore, large instances of discrete optimisation problems are normally impossible to solve to optimality within a reasonable computational time/space and can only be tackled with a heuristic approach. In this thesis the development of so called matheuristics, the heuristics which are based on the mathematical formulation of the problem, is studied and employed within the variable neighbourhood search framework. Some new variants of the variable neighbourhood searchmetaheuristic itself are suggested, which naturally emerge from exploiting the information from the mathematical programming formulation of the problem. However, those variants may also be applied to problems described by the combinatorial formulation. A unifying perspective on modern advances in local search-based metaheuristics, a so called hyper-reactive approach, is also proposed. Two NP-hard discrete optimisation problems are considered: 0-1 mixed integer programming and clustering with application to colour image quantisation. Several new heuristics for 0-1 mixed integer programming problem are developed, based on the principle of variable neighbourhood search. One set of proposed heuristics consists of improvement heuristics, which attempt to find high-quality near-optimal solutions starting from a given feasible solution. Another set consists of constructive heuristics, which attempt to find initial feasible solutions for 0-1 mixed integer programs. Finally, some variable neighbourhood search based clustering techniques are applied for solving the colour image quantisation problem. All new methods presented are compared to other algorithms recommended in literature and a comprehensive performance analysis is provided. Computational results show that the methods proposed either outperform the existing state-of-the-art methods for the problems observed, or provide comparable results. The theory and algorithms presented in this thesis indicate that hybridisation of the CPLEX MIP solver and the VNS metaheuristic can be very effective for solving large instances of the 0-1 mixed integer programming problem. More generally, the results presented in this thesis suggest that hybridisation of exact (commercial) integer programming solvers and some metaheuristic methods is of high interest and such combinations deserve further practical and theoretical investigation. Results also show that VNS can be successfully applied to solving a colour image quantisation problem.EThOS - Electronic Theses Online ServiceMathematical Institute, Serbian Academy of Sciences and ArtsGBUnited Kingdo