09302 Abstracts Collection -- New Developments in the Visualization and Processing of Tensor Fields

From 19.07. to 24.07.2009, the Dagstuhl Seminar 09302 ``New Developments in the Visualization and Processing of Tensor Fields \u27\u27 was held in Schloss Dagstuhl~--~Leibniz Center for Informatics. During the seminar, several participants presented their current research, and ongoing work and open problems were discussed. Abstracts of the presentations given during the seminar as well as abstracts of seminar results and ideas are put together in this paper. The first section describes the seminar topics and goals in general. Links to extended abstracts or full papers are provided, if available

Improving the Tractography Pipeline: on Evaluation, Segmentation, and Visualization

Recent advances in tractography allow for connectomes to be constructed in vivo. These have applications for example in brain tumor surgery and understanding of brain development and diseases. The large size of the data produced by these methods lead to a variety problems, including how to evaluate tractography outputs, development of faster processing algorithms for tractography and clustering, and the development of advanced visualization methods for verification and exploration. This thesis presents several advances in these fields. First, an evaluation is presented for the robustness to noise of multiple commonly used tractography algorithms. It employs a Monte–Carlo simulation of measurement noise on a constructed ground truth dataset. As a result of this evaluation, evidence for obustness of global tractography is found, and algorithmic sources of uncertainty are identified. The second contribution is a fast clustering algorithm for tractography data based on k–means and vector fields for representing the flow of each cluster. It is demonstrated that this algorithm can handle large tractography datasets due to its linear time and memory complexity, and that it can effectively integrate interrupted fibers that would be rejected as outliers by other algorithms. Furthermore, a visualization for the exploration of structural connectomes is presented. It uses illustrative rendering techniques for efficient presentation of connecting fiber bundles in context in anatomical space. Visual hints are employed to improve the perception of spatial relations. Finally, a visualization method with application to exploration and verification of probabilistic tractography is presented, which improves on the previously presented Fiber Stippling technique. It is demonstrated that the method is able to show multiple overlapping tracts in context, and correctly present crossing fiber configurations

Robust perceptual organization techniques for analysis of color images

Esta tesis aborda el desarrollo de nuevas técnicas de análisis robusto de imágenes estrechamente relacionadas con el comportamiento del sistema visual humano. Uno de los pilares de la tesis es la votación tensorial, una técnica robusta que propaga y agrega información codificada en tensores mediante un proceso similar a la convolución. Su robustez y adaptabilidad han sido claves para su uso en esta tesis. Ambas propiedades han sido verificadas en tres nuevas aplicaciones de la votación tensorial: estimación de estructura, detección de bordes y segmentación de imágenes adquiridas mediante estereovisión.El mayor problema de la votación tensorial es su elevado coste computacional. En esta línea, esta tesis propone dos nuevas implementaciones eficientes de la votación tensorial derivadas de un análisis en profundidad de esta técnica.A pesar de su capacidad de adaptación, esta tesis muestra que la formulación original de la votación tensorial (a partir de aquí, votación tensorial clásica) no es adecuada para algunas aplicaciones, dado que las hipótesis en las que se basa no se ajustan a todas ellas. Esto ocurre particularmente en el filtrado de imágenes en color. Así, esta tesis muestra que, más que un método, la votación tensorial es una metodología en la que la codificación y el proceso de votación pueden ser adaptados específicamente para cada aplicación, manteniendo el espíritu de la votación tensorial.En esta línea, esta tesis propone un marco unificado en el que se realiza a la vez el filtrado de imágenes y la detección robusta de bordes. Este marco de trabajo es una extensión de la votación tensorial clásica en la que el color y la probabilidad de encontrar un borde en cada píxel se codifican mediante tensores, y en el que el proceso de votación se basa en un conjunto de criterios perceptuales relacionados con el modo en que el sistema visual humano procesa información. Los avances recientes en la percepción del color han sido esenciales en el diseño de dicho proceso de votación.Este nuevo enfoque ha sido efectivo, obteniendo excelentes resultados en ambas aplicaciones. En concreto, el nuevo método aplicado al filtrado de imágenes tiene un mejor rendimiento que los métodos del estado del arte para ruido real. Esto lo hace más adecuado para aplicaciones reales, donde los algoritmos de filtrado son imprescindibles. Además, el método aplicado a detección de bordes produce resultados más robustos que las técnicas del estado del arte y tiene un rendimiento competitivo con relación a la completitud, discriminabilidad, precisión y rechazo de falsas alarmas.Además, esta tesis demuestra que este nuevo marco de trabajo puede combinarse con otras técnicas para resolver el problema de segmentación robusta de imágenes. Los tensores obtenidos mediante el nuevo método se utilizan para clasificar píxeles como probablemente homogéneos o no homogéneos. Ambos tipos de píxeles se segmentan a continuación por medio de una variante de un algoritmo eficiente de segmentación de imágenes basada en grafos. Los experimentos muestran que el algoritmo propuesto obtiene mejores resultados en tres de las cinco métricas de evaluación aplicadas en comparación con las técnicas del estado del arte, con un coste computacional competitivo.La tesis también propone nuevas técnicas de evaluación en el ámbito del procesamiento de imágenes. En concreto, se proponen dos métricas de filtrado de imágenes con el fin de medir el grado en que un método es capaz de preservar los bordes y evitar la introducción de defectos. Asimismo, se propone una nueva metodología para la evaluación de detectores de bordes que evita posibles sesgos introducidos por el post-procesado. Esta metodología se basa en cinco métricas para estimar completitud, discriminabilidad, precisión, rechazo de falsas alarmas y robustez. Por último, se proponen dos nuevas métricas no paramétricas para estimar el grado de sobre e infrasegmentación producido por los algoritmos de segmentación de imágenes.This thesis focuses on the development of new robust image analysis techniques more closely related to the way the human visual system behaves. One of the pillars of the thesis is the so called tensor voting technique. This is a robust perceptual organization technique that propagates and aggregates information encoded by means of tensors through a convolution like process. Its robustness and adaptability have been one of the key points for using tensor voting in this thesis. These two properties are verified in the thesis by applying tensor voting to three applications where it had not been applied so far: image structure estimation, edge detection and image segmentation of images acquired through stereo vision.The most important drawback of tensor voting is that its usual implementations are highly time consuming. In this line, this thesis proposes two new efficient implementations of tensor voting, both derived from an in depth analysis of this technique.Despite its adaptability, this thesis shows that the original formulation of tensor voting (hereafter, classical tensor voting) is not adequate for some applications, since the hypotheses from which it is based are not suitable for all applications. This is particularly certain for color image denoising. Thus, this thesis shows that, more than a method, tensor voting can be thought of as a methodology in which the encoding and voting process can be tailored for every specific application, while maintaining the tensor voting spirit.By following this reasoning, this thesis proposes a unified framework for both image denoising and robust edge detection.This framework is an extension of the classical tensor voting in which both color and edginess the likelihood of finding an edge at every pixel of the image are encoded through tensors, and where the voting process takes into account a set of plausible perceptual criteria related to the way the human visual system processes visual information. Recent advances in the perception of color have been essential for designing such a voting process.This new approach has been found effective, since it yields excellent results for both applications. In particular, the new method applied to image denoising has a better performance than other state of the art methods for real noise. This makes it more adequate for real applications, in which an image denoiser is indeed required. In addition, the method applied to edge detection yields more robust results than the state of the art techniques and has a competitive performance in recall, discriminability, precision, and false alarm rejection.Moreover, this thesis shows how the results of this new framework can be combined with other techniques to tackle the problem of robust color image segmentation. The tensors obtained by applying the new framework are utilized to classify pixels into likely homogeneous and likely inhomogeneous. Those pixels are then sequentially segmented through a variation of an efficient graph based image segmentation algorithm. Experiments show that the proposed segmentation algorithm yields better scores in three of the five applied evaluation metrics when compared to the state of the art techniques with a competitive computational cost.This thesis also proposes new evaluation techniques in the scope of image processing. First, two new metrics are proposed in the field of image denoising: one to measure how an algorithm is able to preserve edges, and the second to measure how a method is able not to introduce undesirable artifacts. Second, a new methodology for assessing edge detectors that avoids possible bias introduced by post processing is proposed. It consists of five new metrics for assessing recall, discriminability, precision, false alarm rejection and robustness. Finally, two new non parametric metrics are proposed for estimating the degree of over and undersegmentation yielded by image segmentation algorithms

09302 Summary -- New Developments in the Visualization and Processing of Tensor Fields

This Dagstuhl Seminar was concerned with the visualization and processing of tensor fields, like its two predecessors: seminar 04172 organized by Hans Hagen and Joachim Weickert in April 2004, and the follow-up seminar 07022 in January 2007 with David Laidlaw and Joachim Weickert as organizers. Both earlier meetings were successful, resulting in well received books and triggering fruitful scientific interaction and exchange of experience across interdisciplinary boundaries. We believe that the 2009 seminar will prove to have been equally successful