## J.2 PHYSICAL SCIENCES AND ENGINEERING

- Applications of Computational Topology to the Visualization of Scalar Fields (2014)
- This thesis discusses several applications of computational topology to the visualization of scalar fields. Scalar field data come from different measurements and simulations. The intrinsic properties of this kind of data, which make the visualization of it to a complicated task, are the large size and presence of noise. Computational topology is a powerful tool for automatic feature extraction, which allows the user to interpret the information contained in the dataset in a more efficient way. Utilizing it one can make the main purpose of scientific visualization, namely extracting knowledge from data, a more convenient task. Volume rendering is a class of methods designed for realistic visual representation of 3D scalar fields. It is used in a wide range of applications with different data size, noise rate and requirements on interactivity and flexibility. At the moment there is no known technique which can meet the needs of every application domain, therefore development of methods solving specific problems is required. One of such algorithms, designed for rendering of noisy data with high frequencies is presented in the first part of this thesis. The method works with multidimensional transfer functions and is especially suited for functions exhibiting sharp features. Compared with known methods the presented algorithm achieves better visual quality with a faster performance in presence of mentioned features. An improvement on the method utilizing a topological theory, Morse theory, and a topological construct, Morse-Smale complex, is also presented in this part of the thesis. The improvement allows for performance speedup at a little precomputation and memory cost. The usage of topological methods for feature extraction on a real world dataset often results in a very large feature space which easily leads to information overflow. Topology simplification is designed to reduce the number of features and allow a domain expert to concentrate on the most important ones. In the terms of Morse theory features are represented by critical points. An importance measure which is usually used for removing critical points is called homological persistence. Critical points are cancelled pairwise according to their homological persistence value. In the presence of outlier-like noise homological persistence has a clear drawback: the outliers get a high importance value assigned and therefore are not being removed. In the second part of this thesis a new importance measure is presented which is especially suited for data with outliers. This importance measure is called scale space persistence. The algorithm for the computation of this measure is based on the scale space theory known from the area of computer vision. The development of a critical point in scale space gives information about its spacial extent, therefore outliers can be distinguished from other critical points. The usage of the presented importance measure is demonstrated on a real world application, crater identification on a surface of Mars. The third part of this work presents a system for general interactive topology analysis and exploration. The development of such a system is motivated by the fact that topological methods are often considered to be complicated and hard to understand, because application of topology for visualization requires deep understanding of the mathematical background behind it. A domain expert exploring the data using topology for feature extraction needs an intuitive way to manipulate the exploration process. The presented system is based on an intuitive notion of a scene graph, where the user can choose and place the component blocks to achieve an individual result. This way the domain expert can extract more knowledge from given data independent on the application domain. The tool gives the possibility for calculation and simplification of the underlying topological structure, Morse-Smale complex, and also the visualization of parts of it. The system also includes a simple generic query language to acquire different structures of the topological structure at different levels of hierarchy. The fourth part of this dissertation is concentrated on an application of computational geometry for quality assessment of a triangulated surface. Quality assessment of a triangulation is called surface interrogation and is aimed for revealing intrinsic irregularities of a surface. Curvature and continuity are the properties required to design a visually pleasing geometric object. For example, a surface of a manufactured body usually should be convex without bumps of wiggles. Conventional rendering methods hide the regions of interest because of smoothing or interpolation. Two new methods which are presented here: curvature estimation using local fitting with B´ezier patches and computation of reflection lines for visual representation of continuity, are specially designed for assessment problems. The examples and comparisons presented in this part of the thesis prove the benefits of the introduced algorithms. The methods are also well suited for concurrent visualization of the results from simulation and surface interrogation to reveal the possible intrinsic relationship between them.

- Visualization methods for sustainable planning (2012)
- In urban planning, both measuring and communicating sustainability are among the most recent concerns. Therefore, the primary emphasis of this thesis concerns establishing metrics and visualization techniques in order to deal with indicators of sustainability. First, this thesis provides a novel approach for measuring and monitoring two indicators of sustainability - urban sprawl and carbon footprints – at the urban neighborhood scale. By designating different sectors of relevant carbon emissions as well as different household categories, this thesis provides detailed information about carbon emissions in order to estimate impacts of daily consumption decisions and travel behavior by household type. Regarding urban sprawl, a novel gridcell-based indicator model is established, based on different dimensions of urban sprawl. Second, this thesis presents a three-step-based visualization method, addressing predefined requirements for geovisualizations and visualizing those indicator results, introduced above. This surface-visualization combines advantages from both common GIS representation and three-dimensional representation techniques within the field of urban planning, and is assisted by a web-based graphical user interface which allows for accessing the results by the public. In addition, by focusing on local neighborhoods, this thesis provides an alternative approach in measuring and visualizing both indicators by utilizing a Neighborhood Relation Diagram (NRD), based on weighted Voronoi diagrams. Thus, the user is able to a) utilize original census data, b) compare direct impacts of indicator results on the neighboring cells, and c) compare both indicators of sustainability visually.

- Distributed Compliant Control for Biologically Inspired Behavior-Based Robots (2009)
- In its rather short history robotic research has come a long way in the half century since it started to exist as a noticeable scientic eld. Due to its roots in engineering, computer science, mathematics, and several other 'classical' scientic branches,a grand diversity of methodologies and approaches existed from the very beginning. Hence, the researchers in this eld are in particular used to adopting ideas that originate in other elds. As a fairly logical consequence of this, scientists tended to biology during the 1970s in order to nd approaches that are ideally adapted to the conditions of our natural environment. Doing so allows for introducing principles to robotics that have already shown their great potential by prevailing in a tough evolutionary selection process for millions of years. The variety of these approaches spans from efficient locomotion, to sensor processing methodologies and all the way to control architectures. Thus, the full spectrum of challenges for autonomous interaction with the surroundings while pursuing a task can be covered by such means. A feature that has proven to be amongst the most challenging to recreate is the human ability of biped locomotion. This is mainly caused by the fact that walking,running and so on are highly complex processes involving the need for energy efficient actuation, sophisticated control architectures and algorithms, and an elaborate mechanical design while at the same time posting restrictions concerning stability and weight. However, it is of special interest since our environment is favoring this specic kind of locomotion and thus promises to open up an enormous potential if mastered. More than the mere scientic interest, it is the fascination of understanding and recreating parts of oneself that drives the ongoing eorts in this area of research. The fact that this is not at all an easy task to tackle is not only caused by the highly dynamical processes but also has its roots in the challenging design process. That is because it cannot be limited to just one aspect like e.g. the control architecture, actuation, sensors, or mechanical design alone. Each aspect has to be incorporated into a sound general concept in order to allow for a successful outcome in the end. Since control is in this context inseparably coupled with the mechanics of the system, both has to be dealt with here.

- Models and Algorithms for Image-Based Analysis of Microstructures (2009)
- Modern digital imaging technologies, such as digital microscopy or micro-computed tomography, deliver such large amounts of 2D and 3D-image data that manual processing becomes infeasible. This leads to a need for robust, flexible and automatic image analysis tools in areas such as histology or materials science, where microstructures are being investigated (e.g. cells, fiber systems). General-purpose image processing methods can be used to analyze such microstructures. These methods usually rely on segmentation, i.e., a separation of areas of interest in digital images. As image segmentation algorithms rarely adapt well to changes in the imaging system or to different analysis problems, there is a demand for solutions that can easily be modified to analyze different microstructures, and that are more accurate than existing ones. To address these challenges, this thesis contributes a novel statistical model for objects in images and novel algorithms for the image-based analysis of microstructures. The first contribution is a novel statistical model for the locations of objects (e.g. tumor cells) in images. This model is fully trainable and can therefore be easily adapted to many different image analysis tasks, which is demonstrated by examples from histology and materials science. Using algorithms for fitting this statistical model to images results in a method for locating multiple objects in images that is more accurate and more robust to noise and background clutter than standard methods. On simulated data at high noise levels (peak signal-to-noise ratio below 10 dB), this method achieves detection rates up to 10% above those of a watershed-based alternative algorithm. While objects like tumor cells can be described well by their coordinates in the plane, the analysis of fiber systems in composite materials, for instance, requires a fully three dimensional treatment. Therefore, the second contribution of this thesis is a novel algorithm to determine the local fiber orientation in micro-tomographic reconstructions of fiber-reinforced polymers and other fibrous materials. Using simulated data, it will be demonstrated that the local orientations obtained from this novel method are more robust to noise and fiber overlap than those computed using an established alternative gradient-based algorithm, both in 2D and 3D. The property of robustness to noise of the proposed algorithm can be explained by the fact that a low-pass filter is used to detect local orientations. But even in the absence of noise, depending on fiber curvature and density, the average local 3D-orientation estimate can be about 9° more accurate compared to that alternative gradient-based method. Implementations of that novel orientation estimation method require repeated image filtering using anisotropic Gaussian convolution filters. These filter operations, which other authors have used for adaptive image smoothing, are computationally expensive when using standard implementations. Therefore, the third contribution of this thesis is a novel optimal non-orthogonal separation of the anisotropic Gaussian convolution kernel. This result generalizes a previous one reported elsewhere, and allows for efficient implementations of the corresponding convolution operation in any dimension. In 2D and 3D, these implementations achieve an average performance gain by factors of 3.8 and 3.5, respectively, compared to a fast Fourier transform-based implementation. The contributions made by this thesis represent improvements over state-of-the-art methods, especially in the 2D-analysis of cells in histological resections, and in the 2D and 3D-analysis of fibrous materials.

- Analyse und Visualisierung raumplanerischer Prozesse mit Hilfe von Voronoidiagrammen (2008)
- Die vorliegende Arbeit beschäftigt sich mit der visuellen Kontrolle raumplanerischer Entwürfe. Grundlage der Überlegungen ist das gegenwärtige Verfahren, der Planungsprozess, das zur Erstellung der Entwürfe führt. Der Entscheidungsweg hin zum endgültigen Ergebnis erfolgt zurzeit noch ohne Rechnerunterstützung. Die in den Planungsprozess Involvierten stützen ihre Entscheidungen bspw. auf Pläne, eigene Erfahrungen und Statistiken und fertigen im Verlauf von Diskussionsrunden verschiedene Entwürfe an. Dieser Ablauf ist komplex, aufgrund der eingehenden Daten und der damit zusammenhängenden Diskussionen, und langwierig da erst nach einigen Iterationsschritten ein Ergebnis vorliegt. Die Arbeit verfolgt das Ziel, die Akteure durch eine Rechnerunterstützung schneller und zielgerichtet zu einer Entscheidungsfindung zu führen. Meine Untersuchung des Anwendungsumfeldes hat ergeben, dass dies nur möglich ist, wenn zum Einen das entstehende System in der Lage ist, die großen, heterogenen Datenmengen zu verarbeiten und andererseits die Visualisierung der Ergebnisse in einer Form erfolgt, die den Akteuren vom bisherigen Planungsprozess her bekannt ist. Die Visualisierung darf dabei keine bewertende Aussage treffen, sondern muss die Informationen der Analyse neutral in einem dem Nutzer bekannten Format abbilden. Als Ansatzpunkt stellt sich der informelle Bereich der Entscheidungsfindung dar. Es werden zwei Lösungswege aus dem Bereich der Clusteringalgorithmen verfolgt, die die großen Datenmengen verarbeiten und analysieren. Als Ergebnis erhalten die Akteure durch das Voronoi-Diagramm direkt einen Entwurf, der die Einschätzungen aller Akteure widerspiegelt und durch ein Übereinanderlegen mit der Karte des Plangebietes dem klassischen Format im Rahmen des Planungsprozesses entspricht. Dadurch wird die Akzeptanz der Rechnerunterstützung bei den Beteiligten des Planungsprozesses gesteigert. Sollte dieser Entwurf noch keine direkte Zustimmung finden, kann über die entwickelte Informationsvisualisierung eine Anzeige und in der Folge eine Anpassung der Eingangsgrößen erfolgen und somit sehr schnell ein neuer Entwurf entwickelt werden. Die Visualisierung übernimmt dabei die Funktion der bisher in Papierform erstellten Pläne im Entscheidungsprozess und bietet damit auch fachfremden Beteiligten eine visuelle Kontrollmöglichkeit der Qualität des Entwurfes. Insgesamt werden mit dem Tool IKone die Akteure in Anlehnung an die standardmäßigen Abläufe und visuellen Darstellungen mittels eines rechnergestützten Systems unterstützt.

- Visualization of Complex Three-Dimensional Flow Structures (2007)
- The visualization of numerical fluid flow datasets is essential to the engineering processes that motivate their computational simulation. To address the need for visual representations that convey meaningful relations and enable a deep understanding of flow structures, the discipline of Flow Visualization has produced many methods and schemes that are tailored to a variety of visualization tasks. The ever increasing complexity of modern flow simulations, however, puts an enormous demand on these methods. The study of vortex breakdown, for example, which is a highly transient and inherently three-dimensional flow pattern with substantial impact wherever it appears, has driven current techniques to their limits. In this thesis, we propose several novel visualization methods that significantly advance the state of the art in the visualization of complex flow structures. First, we propose a novel scheme for the construction of stream surfaces from the trajectories of particles embedded in a flow. These surfaces are extremely useful since they naturally exploit coherence between neighboring trajectories and are highly illustrative in nature. We overcome the limitations of existing stream surface algorithms that yield poor results in complex flows, and show how the resulting surfaces can be used a building blocks for advanced flow visualization techniques. Moreover, we present a visualization method that is based on moving section planes that travel through a dataset and sample the flow. By considering the changes to the flow topology on the plane as it moves, we obtain a method of visualizing topological structures in three-dimensional flows that are not accessible by conventional topological methods. On the same algorithmic basis, we construct an algorithm for the tracking of critical points in such flows, thereby enabling the treatment of time-dependent datasets. Last, we address some problems with the recently introduced Lagrangian techniques. While conceptually elegant and generally applicable, they suffer from an enormous computational cost that we significantly use by developing an adaptive approximation algorithm. This allows the application of such methods on very large and complex numerical simulations. Throughout this thesis, we will be concerned with flow visualization aspect of general practical significance but we will particularly emphasize the remarkably challenging visualization of the vortex breakdown phenomenon.

- SIMERO: Sichere Mensch-Roboter-Koexistenz (2003)
- Den in der industriellen Produktion eingesetzten Manipulatoren fehlt in der Regel die Möglichkeit, ihre Umwelt wahrzunehmen. Damit Mensch und Roboter in einem gemeinsamen Arbeitsraum arbeiten können, wird im SIMERO-System die Transferbewegung des Roboters durch Kameras abgesichert. Dieses Kamerasystem wird auf Ausfall überprüft. Dabei werden Fehler in der Bildübertragung und Positionierungsfehler der Kameras betrachtet.