Kolloquiumszyklus - previous dates

Visually pleasant image reconstruction has important role in computer graphics. In our work we explore the applicability of triangulations for image reconstruction. Two new algorithms are introduced for generation of data-dependent triangulation. The new deterministic algorithm entitled as image partitioning algorithm (IPA) shifts this reconstruction method closer to real usage. We present a new modification of the optimization technique simulated annealing with generalized look-ahead process (SALA). Also a new way of utilization of color information is presented, to achieve qualitative course of reconstruction of color images. Results show both theoretical and practical superiority over another methods. This work is a part of the APVT project Virtual Bratislava.

Over the past years, impostors and image based simplification have been proposed to replace complex geometry with simpler meshes and appropriate replacement textures. Along this path, the Billboard Clouds approach approximates the global shape of an objects with a small set of planes and uses semi-transparent textures to capture finer details such as silouhettes for example. The problem is cast as a geometric cover, where a minimal set of planes is searched to intersect "regions of validity" of the model's faces. In view-independent billboard clouds (where the BC simplification must be usable for any viewpoint), those regions are defined by spheres around the vertices indicating the maximum displacement allowed during simplification. In view-independent cases (where the BC simplification is to be used for a given viewcell), the definition of the validity region involves an accurate computation of the reprojection error. In this talk, we will briefly present the view-independent billboard clouds and will then introduce our recent results on their extension to the view-dependent case.

Hypermedia enables the use of associative modeling and management of information. One of the topics studied in the ATELIER -project was hypermedia's utilization possibilities with location-based data. Special tool called E-Diary was developed for collecting multimedia data and location information during remote visits and for storing them in a hypermedia database. Hypermedia combined with physical input devices provided interesting new ways to browse through the collected data, including gesture based navigation. This presentation illustrates the framework and tools used to study the use of hypermedia with location-based data.

The multimedia and virtual reality projects of our Laboratory during last ten years can be summarized as follows:
Tutorial and entertainment programs for handicapped children
Rehabilitation programs for stroke patients and persons with phobias
We have developed multimedia software for handicapped children having different impairments: partial-vision, hearing difficulties, loco-motive difficulties, mental retardation, dyslexia, etc. We show the advantages of multimedia software to develop handicapped children.
 

What are the advantages of multimedia software to develop handicapped children's skills?

It is an audiovisual medium
It is interactive
The treatment or situation can be reproduced, the same condition can be repeated several times.
The display presentation can be set according to the visus. The size, form, contrast, colour, size of line width, etc. of the objects and the background can be selected for best suiting the patient.
It can be adjusted to the individual needs.
Multimedia systems have an effect on more then one sense, and can be more effective.
It can help creativity, it can be varied.
It is like a game: (the child does not find the exercise as penitence, he/she likes it.
The child feels the success.
One can use motivating audio feed-back.
It can be used both in individual and small-group therapy.
Also the parent can use it with success.
Most important is that the child should get interested and his/her interest is kept for long periods of time. This is not an easy task, but multimedia presentations are very effective in this respect too.
One can include in "games" into the multimedia programs.We show the special needs of the handicapped children that have to be considered when developing multimedia software.

What are the user interface design questions in developing multimedia software for handicapped children?

To draw the picture with thick contour lines for low vision -,
Give short sentences for mentally handicapped -,
Design the navigation tools for children with loco-motive difficulties.
The hearing impaired children want the sounds too.

Rehabilitation programs for stroke patients and persons with phobias.

We developed a computer controlled method which enables - as a difference to methods used internationally - not only the establishment of the diagnosis, but permits measurement of the effectiveness of the therapy. It allows
To produce a database of the patients that contains not only their personal data but also the results of the tests, their drawings and audio recordings.
It is an intensive therapeutic test and contains tutorial programs. Now we are collecting the test results in this project. We developed some virtual worlds for treating phobias: virtual balcony, a ten-storied building with an external glass elevator and an internal glass elevator in the virtual Attrium Hyatt hotel. We developed virtual environment for claustrophobia too: a closed lift and a room, where the wall can be moved. For specific phobias (fear of travelling) we modelled the underground travelling in Budapest. For the education we developed virtual shopping software for autistic children too. I show the advantages of Virtual Reality in the investigation, evaluation and therapy of perception, behaviour and neuropsychological studies.

Die Keplerschen Gesetze und Begriffe wie Präzessionsbewegung oder Zeitgleichung werden didaktisch mittels Computeranimationen aufbereitet und damit innerhalb kurzer Zeit anschaulich klar. Wieso dauert das Winterhalbjahr fünf Tage weniger als das Sommerhalbjahr? Wie sind Tag-Nacht-Gleichen geometrisch charakterisiert? Warum kommen wir gerade ins Zeitalter des Wassermanns? Zum Abschluss wird eine realistische Simulation einer minutengenauen Sonnenuhr gezeigt, deren Funktionsweise ohne zu erwähnten Begriffe völlig unverständlich wäre.

We present a new framework to efficiently trade off acuracy for speed in collision detection between deformable objects. It combines a conventional proximity detection based on hierarchical bounding volumes with a stochastic method for collision detection. The hierarchical method selects regions of possible collisions. The stochastic method randomly selects pairs of geometric primitives in these regions and make them iteratively converge to local distance minima. By tuning the number of active pairs, a trade-off between complete detection and computation speed is obtained. Preliminary results exhibit significant speedups over previous approaches.

The dynamic range of many real-world environments exceeds the capabilities current of display technology by several orders of magnitude. In this talk I present the results of a collaborative research project, namely the design of two different display systems that are capable of displaying images with a dynamic range much more similar to that encountered in the real world. One system can be built from off-the-shelf components and the other relies on a custom backlighting system. Software issues as well as the advantages and the disadvantages of the two designs are discussed together with potential applications.

In this talk we will first discuss a case-based educational system for treatment decision-making and intervention planning of liver tumors. We will focus on the appropriateness and development of visualization techniques for exploring patient specific data in a problem-oriented learning environment. In this framework, NPR-techniques such as silhouettes and hatching lines are discussed. In the second part, we outline a method for visualization of anatomic tree structures, such as vascular and bronchial trees by means of convolution surfaces. We will go into detail of the filter design to achieve a correct visualization of the vessel diameter and avoid irritating bulges and unwanted blending. Afterwards examples and validation details are presented and discussed.

When people hear the term "Augmented Reality" (AR) they currently first think of a head-mounted display and a local tracking system which superimposes virtual information into a user's field of view. Recently, research into AR-setups has started to move away from this primary setup. Information is presented not only in head-mounted displays but also on world-registered monitors that can be attached to portable instruments or which are carefully arranged within an ubiquitous environment. Furthermore, the increasing need for mobile AR-applications requests tracking arrangements to be laid out in a more global scheme integrating a number of heterogeneous trackers. To this end, an ubquitously available tracking system needs to be able to let mobile users establish dynamic connections to various tracking services. This talk will present several examples which show the confluence of concepts from ubiquitous computing and augmented reality.

Due to rapid technological progress in high dynamic range (HDR) video capture and display, the efficient storage and transmission of such data is crucial for the completeness of any HDR imaging pipeline. We propose a new approach for inter-frame encoding of HDR video, which is embedded in the well-established MPEG-4 video compression standard. The key component of our technique is luminance quantization that is optimized for the contrast threshold perception in the human visual system. The quantization scheme requires only 10--11 bits to encode 12 orders of magnitude of visible luminance range and does not lead to perceivable contouring artifacts. Besides video encoding, the proposed quantization provides perceptually-optimized luminance sampling for fast implementation of any global tone mapping operator using a lookup table. To improve the quality of synthetic video sequences, we introduce a coding scheme for discrete cosine transform (DCT) blocks with high contrast. We demonstrate the capabilities of HDR video in a player, which enables decoding, tone mapping, and applying post-processing effects in real-time. The tone mapping algorithm as well as its parameters can be changed interactively while the video is playing. We can simulate post-processing effects such as glare, night vision, and motion blur, which appear very realistic due to the usage of HDR data.

The computer graphics industry, and in particular those involved with films, games and virtual reality, continue to demand more realistic multi-sensory computer generated environments. In addition, there is an ever increasing desire for multi-user networked interaction. Despite the ready availability of modern high performance graphics cards, the complexity of the scenes being modelled, the need for interaction and the high fidelity required of the images and sound means that synthesising such scenes is still simply not possible in a reasonable, let alone real time on a single computer. Two approaches do, however, appear to offer the possibility of helping achieve high fidelity virtual environments in real-time: Parallel Processing and Visual Perception. Parallel Processing has a number of computers working together to render a single image, which appears to provide almost unlimited performance, however, enabling many processors to work efficiently together is a significant challenge. Visual Perception, on the other hand, takes into account that it is the human who will ultimately be looking at the resultant images, and while the human eye is good, it is not that good. Exploiting knowledge of the human visual system can save significant rendering time by simply not computing those parts of a scene which the human will fail to notice. This talk will consider how parallel processing and visual perception may be combined to achieve perceptual realism in real-time. The application considered for this approach is the high fidelity reconstruction of archaeological sites.

As if large collections of purely textual documents would not still pose a rich set of research challenges (i.e., robust and reliable algorithms for structuring, content extraction and information filtering) for generations of researchers this presentation advocates a change in the interpretation of the term ’document’: Rather than seeing a document in a classical context of being a ’paper’ predominantly compiled of text with a few figures interspersed we recommend to adopt a more general view which considers a ’document’ as an entity consisting of any media type appropriate to store or exchange information in a given context. Only this shift in the document paradigm will open new application fields to Digital Library (DL) technology for the mutual benefit of DL’s and application domains: DL’s offering an unprecedented level of functionality and (new) application domains (e.g., digital mock-up in engineering) benefiting from a more powerful DL technology. According to a study by Lyman et al [3] the world produces between 1 and 2 exabytes (i.e., 1018 bytes or a billion gigabytes) of unique information per year. From that vast amount of data printed documents of all kinds only comprise 0.003%. The major share being taken by images, animations, sound, 3D models and other numeric data. Of course, a large an increasing proportion of the produced material is created, stored and exchanged in digital form -­ currently ranging at about 90% of the total. Yet, little of this information is accessible through Digital Library collections. This presentation gives a motivation for a ’generalized view’ on the term document and raises several issues stimulating research work in the field of Computer Graphics to make Digital Libraries of the future more accessible.

In this paper a novel volume-rendering technique based on Monte Carlo integration is presented. As a result of a preprocessing, a point cloud of random samples is generated using a normalized continuous reconstruction of the volume as a probability density function. This point cloud is projected onto the image plane, and to each pixel an intensity value is assigned which is proportional to the number of samples projected onto the corresponding pixel area. In such a way a simulated X-ray image of the volume can be obtained. Theoretically, for a fixed image resolution, there exists an M number of samples such that the average standard deviation of the estimated pixel intensities is under the level of quantization error regardless of the number of voxels. Therefore Monte Carlo Volume Rendering (MCVR) is mainly proposed to efficiently visualize large volume data sets. Furthermore, network applications are also supported, since the trade-off between image quality and interactivity can be adapted to the bandwidth of the client/server connection by using progressive refinement.

In closed environment, especially in bright colored interiors, there occurs a significant change of saturation and some shifting of hue of original selected colors. This is due to multiple light interreflections. The human vision mechanism partly reduces this effect thanks to the change of the reference white. We can use multispectral radiosity or other multispectral global illumination models to compute the physical effects. A color appearance model, the new and powerful CIECAM02 model, will be used to compute the perceptual aspects.

The CIECAM02 includes the luminance and chromatic adaptation effects, and it has compact forward and inverse transformation formulas. The input data for the color appearance model is ensured by computing the multispectral radiosity solution. Thereby both the spectral radiance for every viewpoint and view direction and the spectral irradiance on every path of the scene are known. Nearly all earlier global illumination approaches ignored the often strong changes of originally selected colors. Using the presented method the selection or mixture of paints is possible with the same, after physical and perceptual effects, color appearance previously selected under standard viewing condition in a color atlas.

Finally some questions of perceptual metamerism to ensure highly constant color appearance under different viewing conditions and some aesthetical rules of color design will be discussed.

Medical imaging is one of the most established practical fields of visualization. While most used methods deal with individual images from 3D scanners - volumes are seen as stack of images -, 3D visualizations are slowly moving into the daily practice of research hospitals.

Major challenges in this process is the difficult specification of how the features in volume datasets are visualized (transfer functions, etc), occlusion of interesting features by others, and fast increasing size of datasets. While a few years ago 256^3 datasets were the standard size in radiology, the current standard size already increased to 512^2x1000 volumes. Soon, highfield-MRI scanners will even produce volumes of 2048^2 x 1000 in research applications.

In this talk, I will discuss several techniques how to deal with large medical data. In particular, I will present work in the context of virtual endoscopy, a medical procedure oriented visualization technique that provides an environment familiar to physicians.

The status of the current output devices as: display and printers, limit to visualize or print correctly High Dynamic Range images. Tone mapping helps to resolve this problem, but when accurate visualization is requested local operators are required. Local operators are able to capture this goal, but they require high computational costs that reduce their use in real applications. In this talk we propose a speed-up technique in order to reduce the computational costs of an existing local operator derived by retinex. It consists to extract both: global and local information from the existing operator and to extrapolate it on the whole image. We show how to extract the global information sampling the input image and using singular value decomposition (SVD). On the other hand, the local information is extracted selecting a small number of samples for each pixel of the input image and applying directly the local operator. We show the efficiency of our method on several images, and the time performances comparing it with the original local operator.

Many areas in medicine, computational physics and various other disciplines have to deal with large or animated volumetric data sets that demand for an adequate visualization. An important visualization technique for the exploration of volumetric data sets is direct volume rendering: Each point in space is assigned a density for the emission and absorption of light and the volume renderer computes the light reaching the eye along viewing rays. The rendering can be implemented efficiently using texture mapping hardware: the volume is discretized into textured slices that are blended over each other using alpha blending. However the huge amount of data to be processed for rendering large and animated volumes also demands for compression schemes that are both very efficient in terms of compression ratio and in terms of decompression speed.

Geometric algorithms for analyzing and interpreting volumetric data draw from established methods in computational geometry. Geometrically, these algorithms often assume simple geometric primitives, such as tetrahedra. In practice, however, data commonly comes sampled on a cubic grid. Several responses to this are possible, such as modifying the experimental procedure, modifying the data, or modifying algorithm.

I shall discuss various approaches for dealing with this problem: where relevant, I shall use the problem of computing contour trees as a sample algorithm. These approaches include non-cubic sampling, correct analysis of the trilinear interpolant, working directly with marching cubes, and subdividing cubes into tetrahedra.

In particular, I will discuss the side-effects of simplicial subdivision on the final isosurfaces, and how to track the connectivity of the standard marching cubes cases. 

 Volume Graphics is part of Computer Graphics whose main subject of study are points and objects made of points. This seeming lack of descriptiveness turns out to be very powerful in describing many natural and complex phenomena from weather patterns to fuel cells to our human body. Besides the creation of 2D images of complex objects the goal of Volume Graphics or Scientific Visualization at large is the creation of tools that enhance the understanding of the objects under investigation. This typically requires the user to interact with the object in real time by extracting only features of interest, creating images that are accurate and reliable.

This talk will give an overview of the research in Scientific Visualization at the Graphics, Usability and Visualization (GrUVi) Lab at Simon Fraser University. The second half of the talk will focus on recent results of utilizing colour phenomena, such as metamers and colour constancy, for novel data exploration algorithms. 

Verfahren zur Simulation chirurgischer Eingriffe bieten vielfältige Möglichkeiten zur Ergänzung und zur Verbesserung herkömmlicher Ausbildungsmethoden in der Medizin. Eine wichtige Anforderung an die Simulation ist dabei ein realistisch wirkendes und interaktives Verhalten.

Szenarien zur Chirurgiesimulation bestehen in der Regel aus deformierbaren und starren Objekten, die miteinander interagieren. Daraus ergeben sich zwei wesentliche Komponenten der Simulation: Es werden ef fiziente und robuste Verfahren zur Berechnung deformierbarer Objekte sowie schnelle Verfahren zur Kollisionserkennung benötigt.

Im ersten Teil des Vortrags werden Modelle und Verfahren zur interaktiven Berechnung des dynamischen Verhaltens komplexer deformierbarer Objekte vorgestellt. Es wird gezeigt, wie Feder-Masse-Modelle dazu eingesetzt werden können, elastische und plastische Verformung von Objekten mit einer Komplexität von bis zu zehntausend Tetraedern interaktiv zu berechnen.

Im zweiten Teil des Vortrags wird ein Verfahren zur Kollisionserkennung vorgestellt, das insbesondere für verformbare Objekte geeignet ist. Im Gegensatz zu Verfahren, die auf Hierarchien von Begrenzungsvolumina oder auf Raumunterteilung basieren, wird ein neuer Ansatz vorgestellt, der die Stärken heutiger Graphikhardware ausnutzt. Das vorgestellte Verfahren ermöglicht eine interaktive volumetrische Kollisionserkennung deformierbarer Objekte mit einer Gesamtkomplexität von bis zu einhunderttausend Oberflächendreiecken.

Abschließend werden weitere Komponenten für die Chirurgiesimulation zusammenfassend vorgestellt sowie potentielle medizinische Anwendungen erläutert. 

The computer simulation of the human musculoskeletal system is playing an increasingly important role in medical diagnosis as well as in the planning of corrections, physiotherapeutic programs and prosthetic implants. The basic goal of computer simulation is to reproduce mechanical motion within the musculoskeletal system in a biofidelic manner based on individual patient parameters. This makes it possible for physicians to compare functional properties of patients prior and after medical treatment, or even, as a long-term objective, to predict therapeutic effects before grasping the scalpel. In this seminar, the foundations of simulation of mechanical systems based on multibody dynamics and their application to the dynamics of the human leg are presented. Multibody dynamics is a well-known field of research of mechanical engineering that has been developed in the last twenty years and has been applied to a great variety of systems, such as road and rail vehicles, robots, tool machines, etc. In the technical setting, it has become the primary environment of development for innovative systems, as virtual reality methods have proved to reduce costs and design-cycle time significantly. Our approach for multibody dynamics consists of employing object-oriented methods that allow the user to build dynamic models as executable programs, which are then open for extensions and linking to other existing software packages, such as computer graphics, control theory, signal analysis, etc. This is in contrast to existing methods, which use the monolithic, all-inclusive program structure. The basic idea of the object-oriented approach is to mimic real-world mechanical parts by corresponding software objects that transmit motion and forces as in the real system. In this way, a model can be built as an assembly of individual "kinetostatic transmission elements" that can be triggered intuitively at the generic level, i.e., whose transmission properties can be accessed without regard to their internal structure. We show how with these basic functions it is possible to solve all problems of dynamics. The ideas are then applied to the mechanical model of the human lower extremity, displaying a model of hip, upper and lower leg, and foot, consisting of 15 degrees of freedom and 43 individual muscles. Parameters for bones and muscles are taken for a generic case from literature. Simulations involve geometry (muscle extensions during walking), inverse dynamics (joint torques computed from motion capturing systems and force plate output describing contact force at the feet), as well as preliminary results for the dynamics (trajectories of the lower extremity based on muscle activation profiles). The developed software has been extended by a 3D user interface that allows the user to perform simulations online and hence to assess the physical parameters directly at the computer monitor. The software is being applied at the Children's Hospital of the University of Graz for treatment of children with spastic diplegia. Comparison of simulations and measurements at the gait lab show a good agreement of the computed inverse dynamics and experimental data. Further illustrative examples for the concepts developed in this talk are taken from mechanism analysis, rail vehicles, and biomechanics of neck and forearm.