Eurovis: Eurographics Conference on Visualization

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Particle Tracing in σ-Transformed Grids using Tetrahedral 6-Decomposition
(The Eurographics Association, 1998) Sadarjoen, Ari; Boer, Alex J. de; Post, Frits H.; Mynett, Arthur E.; Bartz, Dirk
Particle tracing in curvilinear grids often employs decomposition of hexahedral cells into 5 tetrahedra. This method has shortcomings when applied to sigma-transformed grids, a grid type having strongly sheared cells, commonly used in hydrodynamic simulations. This paper describes an improved decomposition method into 6 tetrahedra. It is shown that this method is robust in sigma-transformed grids, however large the shear- ing. Results are presented of applying the technique to a real world sim- ulation. Comparisons are made between the accuracy and speed of the 5-decomposition and the 6-decomposition methods.
Stereoscopic Volume Rendering
(The Eurographics Association, 1998) Hubbold, Roger J.; Hancock, David J.; Moore, Christopher J.; Bartz, Dirk
In this paper we describe the extension of a parallel, distributed, direct volume renderer for use with a novel auto-stereoscopic display. We begin by brie y describing the target application of our research, radiation therapy planning, why we believe that stereoscopic viewing may be helpful for this, and the design of our DVR system. We then report on some of the problems we have encountered, and the results we have obtained in experiments. These demonstrate that stereoscopic viewing is beneficial for perceiving depth in transparent DVR images. We illustrate the application of the system to the visualisation of prostate cancer treatment plans. Finally, we describe the use of head-tracking to implement 3D stereo look-around.
Visualization of Time-Dependent Velocity Fields by Texture Transport
(The Eurographics Association, 1998) Becker, Joachim; Rumpf, Martin; Bartz, Dirk
Vector field visualization is an important topic in scientific visualization. The aim is to graphically represent field data in an intuitively understandable and precise way, which should be closely related to the physical interpretation. A new tool, the texture transport method is presented, which especially applies to time-dependent velocity fields. It is based on an accurate numerical scheme for convection equations, which is used to compute Lagrangian coordinates in space time. These coordinates are then used as texture coordinates referring to some prescribed texture in the Lagrangian reference space. The method is combined with a reliability indicator. This indicator in uences the final appearance of the texture and thereby leads to reliable visual information. At first the method applies to 2D problems. It can be generalized to 3D.
Data-Dependent Surface Simplification
(The Eurographics Association, 1998) Frank, Karin; Lang, Ulrich; Bartz, Dirk
In Scientific Visualization, surfaces have often attached data, e. g. cutting surfaces or isosurfaces in numerical simulations with multiple data components. These surfaces can be e. g. the output of a marching cubes algorithm which produces a large number of very small triangles. Existing triangle decimation algorithms use purely geometric criteria to simplify oversampled surfaces. This approach can lead to coarse representations of the surface in areas with high data gradients, thus loosing important information. In this paper, a data-dependent reduction algorithm for arbitrary triangulated surfaces is presented using besides geometric criteria a gradient approximation of the data to de ne the order of geometric elements to be removed. Examples show that the algorithm works su ciently fast to be used interactively in a VR environment and allows relatively high reduction rates keeping a good quality representation of the surface.
Data Compression of Multiresolution Surfaces
(The Eurographics Association, 1998) Klein, Reinhard; Gumhold, Stefan; Bartz, Dirk
In this paper we introduce a new compressed representation for multiresolution models (MRM) of triangulated surfaces of 3D-objects. Associated with the representation we present compression and decompression algorithms. Our representation allows us to extract the surface at variable resolution in time linear in the output size. It applies to MRMs generated by di erent simpli cation algorithms like local vertex deletion or edge and triangle collapse. The time required to transmit models over communication lines and the space needed to store the MRMs is signi - cantly reduced.
Enhancing the Visualization of Characteristic Structures in Dynamical Systems
(The Eurographics Association, 1998) Löffelmann, Helwig; Gröller, Eduard; Bartz, Dirk
We present a thread of streamlets as a new technique to visualize dynamical systems in three-dimensional space. A trade-off is made between solely visualizing a mathematical abstraction through lowerdimensional manifolds, i.e., characteristic structures such as fixed points, separatrices, etc., and directly encoding the flow through stream lines or stream surfaces. Bundles of streamlets are selectively placed near characteristic trajectories. An over-population of phase space with occlusion problems as a consequence is omitted. On the other hand, information loss is minimized since characteristic structures of the flow are still illustrated in the visualization.
Mirror, Mirror on the Wall, Who Has the Best Visualization of All?- A Reference Model for Visualization Quality
(The Eurographics Association, 1998) Haase, Helmut; Bartz, Dirk
What is a 'good' visualization, one which leads to desired insights? How can we evaluate the quality of a scientific visualization or compare two visualizations (or visualization systems) to each other? In the following, the importance of considering the 'visualization context' is stressed. It consists of the prior knowledge of the user; the aims of the user; the application domain; amount, structure, and distribution of the data; and the available hardware and software. Then, six subqualities are identified: data resolution quality, semantic quality, mapping quality, image quality, presentation and interaction quality, and multi-user quality. The QV IS reference model de nes a weight value C (i.e., importance) and a quality value Q for each subquality. The QV IS graph is introduced as a compact, easy to perceive representation of the so-de ned visualization quality. An example illustrates all concepts. The reference model and the graph can help to evaluate visualizations and thus to further improve the quality of scientific visualizations.
Adaptively Adjusting Marching Cubes Output to Fit A Trilinear Reconstruction Filter
(The Eurographics Association, 1998) Allamandri, Fabio; Cignoni, Paolo; Montani, Claudio; Scopigno, Roberto; Bartz, Dirk
The paper focuses on the improvement of the quality of isosurfaces fitted on volume datasets with respect to standard MC solutions. The new solution presented improves the precision in the reconstruction process using an approach based on mesh re nement and driven by the evaluation of the trilinear reconstruction filter. The iso-surface reconstruction process is adaptive, to ensure that the complexity of the fitted mesh will not become excessive. The proposed approach has been tested on many datasets; we discuss the precision of the obtained meshs and report data on fitted meshes complexity and processing times.
Particle Tracing on Sparse Grids
(The Eurographics Association, 1998) Teitzel, Christian; Grosso, Roberto; Ertl, Thomas; Bartz, Dirk
These days sparse grids are of increasing interest in numerical simulations. Based upon hierarchical tensor product bases, the sparse grid approach is a very e cient one improving the ratio of invested storage and computing time to the achieved accuracy for many problems in the area of numerical solution of di erential equations, for instance in numerical fluid mechanics. The particle tracing algorithms that are available so far cannot cope with sparse grids. Now we present an approach that directly works on sparse grids. As a second aspect in this paper, we suggest to use sparse grids as a data compression method in order to visualize huge data sets even on small workstations. Because the size of data sets used in numerical simulations is still growing, this feature makes it possible that workstations can continue to handle these data sets.
Fast Generation of Multiresolution Surfaces from Contours
(The Eurographics Association, 1998) Schilling, Andreas; Klein, Reinhard; Bartz, Dirk
Surface reconstruction from contours is an important problem especially in medical applications. Other uses include reconstruction from topographic data, or isosurface generation in general. The drawback of existing reconstruction algorithms from contours is, that they are relatively complicated and often have numerical problems. Furthermore, algorithms to generate multiresolution surface models do not exploit the special situation having contours. In this paper we describe a new robust and fast reconstruction algorithm from contours that delivers a multiresolution surface with controlled distance from the original contours. Supporting selective refinement in areas of interest, this multiresolution model can be handled interactively without giving up accuracy.
Three-Dimensional Visualization of Atomic Collision Cascades
(The Eurographics Association, 1998) Sroubek, Filip; Slavik, Pavel; Bartz, Dirk
The paper describes a new approach to the visualization of atomic collision cascades and using the interaction with visualized data. The collision cascade is a physical phenomenon initiated by bombarding the surface of a solid with accelerated atomic particles. The process evolves in time and therefore it is necessary to develop some tools that would allow to investigate and visualize the dynamics of the process. Such tools are classi ers ( lters) that enable to select and visualize objects with specific dynamic properties. As the visualization has been done in a 3D environment a question arises how to specify effectively and user friendly both the properties and the objects in the 3D space. Several techniques are available that allow interaction in the 3D space. It has been necessary to test some of these techniques and to determine which one is the most suitable for the given application class.
Experiments on the Accuracy of Feature Extraction
(The Eurographics Association, 1998) Reinders, Freek; Spoelder, Hans J.W.; Post, Frits H.; Bartz, Dirk
Feature extraction is an approach to visualization that ex- tracts important regions or objects of interest algorithmically from large data sets. In our feature extraction process, high-level attributes are cal- culated for the features, thus resulting in averaged quantitative measures. The usability of these measures depends on their robustness with noise and their dependency on parameters like the density of the grid that is used. In this paper experiments are described to investigate the accuracy and robustness of the feature extraction method. Synthetic data is gener- ated with prede ned features, this data is used in the feature extraction procedure, and the obtained attributes of the feature are compared to the input attributes. This has been done for several grid resolutions, for di erent noise levels, and with di erent feature extraction parameters. We present the results of the experiments, and also derive a number of guidelines for setting the extraction parameters.
Visualization of Grinding Processes
(Springer and The Eurographics Association, 1999) Fiege, Markus; Scheuermann, Gerik; Münchhofen, Michael; Hagen, Hans; Gröller, E., Löffelmann, H., Ribarsky, W.
In grinding technology, the application ofsupcrabrasivcs and increasing demands for higher productivity and higher quality require an appropriate selection of optimum set-up parameters. An efficient way to determine and test these parameters is modeling and simulating the grinding process. A visualization of the results can support the choice of the parameters and increase the knowledge of the complex grinding process. This paper describes a web-based visualization tool on the basis of a kinematic simulation. The tool allows the visualization of the surface of an already ground workpiece as well as the changing shape of the workpiece during the grinding process. Two methods for the visualization of the grinding-objects are implemented. One method describes the scene with the Virtual Reality Modeling Language, the other one uses a renderer to create the images.
VIVENDI - A Virtual Endoscopy System for Virtual Medicine
(Springer and The Eurographics Association, 1999) Bartz, Dirk; Skalej, Martin; Gröller, E., Löffelmann, H., Ribarsky, W.
Virtual Medicine is an emerging and challenging field in Computer Graphics. Numerous visualization methods are used to model and render data of different modalities. In this paper, we present a new endoscopy system for virtual medicine. The main purpose of this system is to provide support for the planning of complicated endoscopic interventions inside of the ventricular system of the human brain. Although, our current focus is on ventricle endoscopy, this system is applicable to other areas as well. In order to achieve interactive framerates on workstations with medium graphics performance, we apply an efficient implementation of a basic algorithm for general visibility queries.
Real-Time Maximum Intensity Projection
(Springer and The Eurographics Association, 1999) Mroz, Lukas; König, Andreas; Gröller, Eduard; Gröller, E., Löffelmann, H., Ribarsky, W.
Maximum Intensity Projection (MIP) is a volume rendering technique which is used to extract high-intensity structures from volumetric data. At each pixel the highest data value encountered along the corresponding viewing ray is determined. MIP is commonly used to extract vascular structures from medical MRI data sets (angiography). The usual way to compensate for the loss of spatial and occlusion information in MIP images is to view the data from different view points by rotating them. As the generation of MIP is usually non-interactive, this is done by calculating multiple images offline and playing them back as an animation. In this paper a new algorithm is proposed which is capable of interactively generating Maximum Intensity Projection images using parallel projection and templates. Voxels of the data set which will never contribute to a MIP due to their neighborhood are removed during a preprocessing step. The remaining voxels are stored in a way which guarantees optimal cache coherency regardless of the viewing direction. For use on low-end hardware, a preview-mode is included which renders only more significant parts of the volume during user interaction. Furthermore we demonstrate the usability of our data structure for extensions of the MIP technique like MIP with depth-shading and Local Maximum Intensity Projection (LMIP).
Visualization by Examples: Mapping Data to Visual Representations using Few Correspondences
(Springer and The Eurographics Association, 1999) Alexa, Marc; Müller, Wolfgang; Gröller, E., Löffelmann, H., Ribarsky, W.
In this paper we propose a new approach for the generation of visual scales for the visualization of scalar and multivariate data. Based on the specification of only a few correspondences between the data set and elements of a space of visual representations complex visualization mappings are produced. The foundation of this approach is the introduction of a multidimcnsional space of visual representations. The mapping between these spaces can he defined by approximating or satisfying the user defined relations between data values and visual atributes.
A Comparison of Error Indicators for Multilevel Visualization on Nested Grids
(Springer and The Eurographics Association, 1999) Gerstner, Thomas; Rumpf, Martin; Weikard, Ulrich; Gröller, E., Löffelmann, H., Ribarsky, W.
Multiresolution visualization methods have recently become an indispensable ingredient of real time interactive post processing. Here local error indicators serve as criteria where to refine the data representation on the physical domain. In this article we give an overview on different types of error measurement on nested grids and compare them for selected applications in 2D as well as in 3D. Furthermore, it is pointed out that a certain saturation of the considered error indicator plays an important role in multilevel visualization and can he reused for the evaluation of data bounds in hierarchical searching or for a multilevel backface culling of isosurfaces.
VISSION: An Object Oriented Dataflow System for Simulation and Visualization
(Springer and The Eurographics Association, 1999) Telea, Alexandru; Wijk, Jarke J. van; Gröller, E., Löffelmann, H., Ribarsky, W.
Scientific visualization and simulation speciification and monitoring are sometimes addressed by object-oriented environments. Even though object orientation powerfully and elegantly models many application domains, integration of 00 libraries in such systems remains a difficult task. The elegance and simplicity of object orientation is often lost in the integration phase, so combining 00 and dataflow concepts is usually limited. We propose a system for visualization and simulation with a generic object-oriented way to simulation design, control and interactivity, which merges 00 and dataflow modelling in a single abstraction. Advantages of the proposed system over similar tools are presented and illustrated by a comprehensive set of examples.
Geodesic Flow on Polyhedral Surfaces
(Springer and The Eurographics Association, 1999) Polthier, Konrad; Schmies, Markus; Gröller, E., Löffelmann, H., Ribarsky, W.
On a curved surface the front of a point wave evolves in concentric circles which start to overlap and branch after a certain time. This evolution is described by the geodesic flow and helps us to understand the geometry of surfaces. In this paper we compute the evolution of distance circles on polyhedral surfaces and develop a method to visualize the set of circles, their overlapping, branching, and their temporal evolution simultaneously. We consider the evolution as an interfering wave on the surface, and extend isometric texture maps to efficiently handle the branching and overlapping of the wave.
A Methodology for Comparing Direct Volume Rendering Algorithms Using a Projection-Based Data Level Approach
(Springer and The Eurographics Association, 1999) Kim, Kwansik; Pang, Alex; Gröller, E., Löffelmann, H., Ribarsky, W.
Identifying and visualizing uncertainty together with the data is a well recognized problem. One of the culprits that introduce uncertainty in the visualization pipeline is the visualization algorithm itself. Uncertainties introduced in this way usually arise from approximations and manifest themselves as artifacts in the resulting images. In this paper, we focus on comparing different direct volume rendering (DVR) algorithms and their artifacts as a result of DVR algorithm selections and their associated parameter settings. We present a new data level comparison methodology that uses differences in intermediate rendering information. In particular, we extend the traditional image level comparison techniques to include data level comparison techniques. In image level comparisons, quantized pixel values are the starting point for comparison measurements. In contrast, data level comparison techniques have the advantage of accessing and evaluating the intermediate 3D information during the rendering process. Our data level approach overcomes limitations of image level approaches and provide capabilities to compare application dependent details as well as general rendering qualities. One of the key challenges with our data level comparison approach is finding a common base for comparing the rich variety of DVR algorithms. In this paper, we present how a projection algorithm can be used as a base for comparing other DVR algorithms. In addition, a set of projection-based metrics are derived to quantify the comparison measurements among DVR algorithms. The results presented in this paper complement our earlier findings where a ray-based approach was used as the base for comparing other DVR algorithms.

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