VisSym03: Joint Eurographics - IEEE TCVG Symposium on Visualization

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Visual Hierarchical Dimension Reduction for Exploration of High Dimensional Datasets
(The Eurographics Association, 2003) Yang, J.; Ward, M.O.; Rundensteiner, E.A.; Huang, S.; G.-P. Bonneau and S. Hahmann and C. D. Hansen
Traditional visualization techniques for multidimensional data sets, such as parallel coordinates, glyphs, and scatterplot matrices, do not scale well to high numbers of dimensions. A common approach to solving this problem is dimensionality reduction. Existing dimensionality reduction techniques usually generate lower dimensional spaces that have little intuitive meaning to users and allow little user interaction. In this paper we propose a new approach to handling high dimensional data, named Visual Hierarchical Dimension Reduction (VHDR), that addresses these drawbacks. VHDR not only generates lower dimensional spaces that are meaningful to users, but also allows user interactions in most steps of the process. In VHDR, dimensions are grouped into a hierarchy, and lower dimensional spaces are constructed using clusters of the hierarchy. We have implemented the VHDR approach into XmdvTool, and extended several traditional multidimensional visualization methods to convey dimension cluster characteristics when visualizing the data set in lower dimensional spaces. Our case study of applying VHDR to a real data set supports our belief that this approach is effective in supporting the exploration of high dimensional data sets.
Vector Field Visualization using Markov Random Field Texture Synthesis
(The Eurographics Association, 2003) Taponecco, Francesca; Alexa, Marc; G.-P. Bonneau and S. Hahmann and C. D. Hansen
Vector field visualization aims at generating images in order to convey the information existing in the data. We use Markov Random Field (MRF) texture synthesis methods to generate the visualization from a set of sample textures. MRF texture synthesis methods allow generating images that are locally similar to a given example image. We extend this idea for vector field visualization by identifying each vector value with a representative example image, e.g. a strongly directed texture that is rotated according to a 2D vector. The visualization is synthesized pixel by pixel, where each pixel is chosen from the sample texture according to the vector values of the local pixel. The visualization locally communicates the vector information as each pixel is chosen from a sample that is representative of the vector. Furthermore it is smooth, as MRF texture synthesis searches for best fitting neighborhoods. This leads to dense and smooth visualizations with the additional freedom to use arbitrary representation textures for any vector value.
Detecting Critical Regions in Scalar Fields
(The Eurographics Association, 2003) Weber, Gunther H.; Scheuermann, Gerik; Hamann, Bernd; G.-P. Bonneau and S. Hahmann and C. D. Hansen
Trivariate data is commonly visualized using isosurfaces or direct volume rendering. When exploring scalar fields by isosurface extraction it is often difficult to choose isovalues that convey "useful" information. The significance of visualizations using direct volume rendering depends on the choice of good transfer functions. Understanding and using isosurface topology can help in identifying "relevant" isovalues for visualization via isosurfaces and can be used to automatically generate transfer functions. Critical isovalues indicate changes in topology of an isosurface: the creation of new surface components, merging of surface components or the formation of holes in a surface component. Interesting isosurface behavior is likely to occur at and around critical isovalues. Current approaches to detect critical isovalues are usually limited to isolated critical points. Data sets often contain regions of constant value (i.e., mesh edges, mesh faces, or entire mesh cells). We present a method that detects critical points, critical regions and corresponding critical isovalues for a scalar field defined by piecewise trilinear interpolation over a uniform rectilinear grid. We describe how to use the resulting list of critical regions/points and associated values to examine trivariate data.
Isosurfaces on Optimal Regular Samples
(The Eurographics Association, 2003) Carr, Hamish; Theußl, Thomas; Möller, Torsten; G.-P. Bonneau and S. Hahmann and C. D. Hansen
Volumetric samples on Cartesian lattices are less efficient than samples on body-centred cubic (BCC) lattices. We show how to construct isosurfaces on BCC lattices using several different algorithms. Since the mesh that arises from BCC lattices involves a large number of cells, we show two alternate methods of reducing the number of cells by clumping tetrahedra into either octahedra or hexahedra. We also propose a theoretical model for estimating triangle counts for various algorithms, and present experimental results to show that isosurfaces generated using one of our algorithms can be competitive with isosurfaces generated using Marching Cubes on similar Cartesian grids
Post-Convolved Splatting
(The Eurographics Association, 2003) Neophytou, Neophytos; Mueller, Klaus; G.-P. Bonneau and S. Hahmann and C. D. Hansen
One of the most expensive operations in volume rendering is the interpolation of samples in volume space. The number of samples, in turn, depends on the resolution of the final image. Hence, viewing the volume at high magnification will incur heavy computation. In this paper, we explore an approach that limits the number of samples to the resolution of the volume, independent of the magnification factor, using a cheap post-convolution process on the interpolated samples to generate the missing samples. For X-ray, this post-convolution is needed only once, after the volume is fully projected, while in full volume rendering, the post-convolution must be applied before each shading and compositing step. Using this technique, we are able to achieve speedups of two and more, without compromising rendering quality. We demonstrate our approach using an image-aligned sheet-buffered splatting algorithm, but our conclusions readily generalize to any volume rendering algorithm that advances across the volume in a slice-based fashion.
Using Graphs for Fast Error Term Approximation of Time-varying Datasets
(The Eurographics Association, 2003) Nuber, C.; LaMar, E. C.; Pascucci, V.; Hamann, B.; Joy, K. I.; G.-P. Bonneau and S. Hahmann and C. D. Hansen
We present a method for the efficient computation and storage of approximations of error tables used for error estimation of a region between different time steps in time-varying datasets. The error between two time steps is defined as the distance between the data of these time steps. Error tables are used to look up the error between different time steps of a time-varying dataset, especially when run time error computation is expensive. However, even the generation of error tables itself can be expensive. For n time steps, the exact error look-up table (which stores the error values for all pairs of time steps in a matrix) has a memory complexity and pre-processing time complexity of O(n2), and O(1) for error retrieval. Our approximate error look-up table approach uses trees, where the leaf nodes represent original time steps, and interior nodes contain an average (or best-representative) of the children nodes. The error computed on an edge of a tree describes the distance between the two nodes on that edge. Evaluating the error between two different time steps requires traversing a path between the two leaf nodes, and accumulating the errors on the traversed edges. For n time steps, this scheme has a memory complexity and pre-processing time complexity of O(nlog(n)), a significant improvement over the exact scheme; the error retrieval complexity is O(log(n)). As we do not need to calculate all possible n2 error terms, our approach is a fast way to generate the approximation.
Hierarchical Isosurface Segmentation Based on Discrete Curvature
(The Eurographics Association, 2003) Vivodtzev, Fabien; Linsen, Lars; Bonneau, Georges-Pierre; Hamann, Bernd; Joy, Kenneth I.; Olshausen, Bruno A.; G.-P. Bonneau and S. Hahmann and C. D. Hansen
A high-level approach to describe the characteristics of a surface is to segment it into regions of uniform curvature behavior and construct an abstract representation given by a (topology) graph. We propose a surface segmentation method based on discrete mean and Gaussian curvature estimates. The surfaces are obtained from three-dimensional imaging data sets by isosurface extraction after data presmoothing and postprocessing the isosurfaces by a surface-growing algorithm. We generate a hierarchical multiresolution representation of the isosurface. Segmentation and graph generation algorithms can be performed at various levels of detail. At a coarse level of detail, the algorithm detects the main features of the surface. This low-resolution description is used to determine constraints for the segmentation and graph generation at the higher resolutions. We have applied our methods to MRI data sets of human brains. The hierarchical segmentation framework can be used for brainmapping purposes.
Path Seeds and Flexible Isosurfaces Using Topology for Exploratory Visualization
(The Eurographics Association, 2003) Carr, Hamish; Snoeyink, Jack; G.-P. Bonneau and S. Hahmann and C. D. Hansen
Morse theory and the Reeb graph give topological summaries of the behaviour of continuous scalar functions. The contour tree augments the Reeb graph for the isosurfaces in a volume to store seed sets, which are starting points for extracting isosurfaces by the continuation method. We replace the minimal seed sets of van Kreveld et al. with path seeds, which generate paths that correspond directly to the individual components of an isosurface. From a path we get exactly one seed per component, which reduces storage and simplifies isosurface extraction. Moreover, the correspondence allows us to extend the contour spectrum of Bajaj et al. to an interface that we call flexible isosurfaces, in which individual contours with different isovalues can be displayed, manipulated and annotated. The largest contour segmentation, in which separate surfaces are generated for each local maximum of the field, is a special case of the flexible isosurface.
Interactive Feature Specification for Focus+Context Visualization of Complex Simulation Data
(The Eurographics Association, 2003) Doleisch, Helmut; Gasser, Martin; Hauser, Helwig; G.-P. Bonneau and S. Hahmann and C. D. Hansen
Visualization of high-dimensional, large data sets, resulting from computational simulation, is one of the most challenging fields in scientific viualization. When visualization aims at supporting the analysis of such data sets, feature-based approches are very useful to reduce the amount of data which is shown at each instance of time and guide the user to the most interesting areas of the data. When using feature-based visualization, one of the most difficult questions is how to extract or specify the features. This is mostly done (semi-)automatic up to now. Especially when interactive analysis of the data is the main goal of the visualization, tools supporting interactive specification of features are needed. In this paper we present a framework for flexible and interactive specification of high-dimensional and/or complex features in simulation data. The framework makes use of multiple, linked views from information as well as scientific visualization and is based on a simple and compact feature definition language (FDL). It allows the definition of one or several features, which can be complex and/or hierarchically described by brushing multiple dimensions (using non-binary and composite brushes). The result of the specification is linked to all views, thereby a focus+context style of visualization in 3D is realized. To demonstrate the usage of the specification, as well as the linked tools, applications from flow simulation in the automotive industry are presented.
Improving Topological Segmentation of Three-dimensional Vector Fields
(The Eurographics Association, 2003) Mahrous, Karim M.; Bennett, Janine C.; Hamann, Bernd; Joy, Kenneth I.; G.-P. Bonneau and S. Hahmann and C. D. Hansen
We present three enhancements to accelerate the extraction of separatrices of three-dimensional vector fields, using intelligently selected "sample" streamlines. These enhancements reduce the number of needed sample streamlines and their propagation length. Inflow/outflow matching supports the simultaneous extraction of topologically significant inflow and outflow separatrices in a single pass. An adaptive sampling approach is introduced and used to seed streamlines in a more meaningful and efficient manner. Cell-locking is a new concept that isolates regions of a data set that do not contain separatrices. This concept makes streamline propagation more efficient as streamlines are not propagated through cells that do not influence or contain separatrices. These enhancements enable us to perform separatrix construction for three-dimensional vector field data requiring less overall computation.
Interaction of Light and Tensor Fields
(The Eurographics Association, 2003) Zheng, Xiaoqiang; Pang, Alex; G.-P. Bonneau and S. Hahmann and C. D. Hansen
We present three new ways of looking at tensor volumes. All three methods are based on the interaction of simulated light and the tensor field. Conceptually, rays are shot from a certain direction into the tensor volume. These rays are influenced by the surrounding tensor field and bent as they traverse through the volume. The tensor is visualized by both the nature of the bent rays and by the collection of rays deposited on a receiving plate. The former is similar to streamlines, but shows paths of greatest influence by the tensor field. The latter is similar to caustic effects from photon maps, but shows the convergence or divergence of the rays through the tensor volume. We also use the concept of treating the tensor volume as a special lens that distorts an image. Using backward ray tracing through the tensor volume, we generate image distortions that also show internal properties of the tensor field. A key advantage of these techniques is that they can work directly with non-symmetric tensor fields without first decomposing them into components. Color images can also be found in www.soe.ucsc.edu/research/avis/tensorray.html.
Feature Flow Fields
(The Eurographics Association, 2003) Theisel, H.; Seidel, H.-P.; G.-P. Bonneau and S. Hahmann and C. D. Hansen
Feature tracking algorithms for instationary vector fields are usually based on a correspondence analysis of the features at different time steps. This paper introduces a method for feature tracking which is based on the integration of stream lines of a certain vector field called feature flow field. We analyze for which features the method of feature flow fields can be applied, we show how events in the flow can be detected using feature flow fields, and we show how to construct the feature flow fields for particular classes of features. Finally, we apply the technique to track critical points in a 2D instationary vector field.
Adaptive Smooth Scattered-data Approximation for Large-scale Terrain Visualization
(The Eurographics Association, 2003) Bertram, Martin; Tricoche, Xavier; Hagen, Hans; G.-P. Bonneau and S. Hahmann and C. D. Hansen
We present a fast method that adaptively approximates large-scale functional scattered data sets with hierarchical B-splines. The scheme is memory efficient, easy to implement and produces smooth surfaces. It combines adaptive clustering based on quadtrees with piecewise polynomial least squares approximations. The resulting surface components are locally approximated by a smooth B-spline surface obtained by knot removal. Residuals are computed with respect to this surface approximation, determining the clusters that need to be recursively refined, in order to satisfy a prescribed error bound. We provide numerical results for two terrain data sets, demonstrating that our algorithm works efficiently and accurate for large data sets with highly non-uniform sampling densities.
Rendering Vector Data over Global, Multi-resolution 3D Terrain
(The Eurographics Association, 2003) Wartell, Zachary; Kang, Eunjung; Wasilewski, Tony; Ribarsky, William; Faust, Nickolas; G.-P. Bonneau and S. Hahmann and C. D. Hansen
Modern desktop PCs are capable of taking 2D Geographic Information System (GIS) applications into the realm of interactive 3D virtual worlds. In prior work we developed and presented graphics algorithms and data management methods for interactive viewing of a 3D global terrain system for desktop and virtual reality systems. In this paper we present a key data structure and associated render-time algorithm for the combined display of multi-resolution 3D terrain and traditional GIS polyline vector data. Such vector data is traditionally used for representing geographic entities such as political borders, roads, rivers and cadastral information
ISOSLIDER: A System for Interactive Exploration of Isosurfaces
(The Eurographics Association, 2003) Chhugani, Jatin; Vishwanath, Sudhir; Cohen, Jonathan; Kumar, Subodh; G.-P. Bonneau and S. Hahmann and C. D. Hansen
We present ISOSLIDER, a system for interactive exploration of isosurfaces of a scalar field. Our algorithm focuses on fast update of isosurfaces for interactive display as a user makes small changes to the isovalue of the desired surface. We exploit the coherence of this update. Larger changes are supported as well. The update to the isosurface is made at a correct level of detail so that not too many operations need be performed nor too many triangles need be rendered. ISOSLIDER does not need to retain the entire volume in the main memory and stores most data out of core. The central idea of the ISOSLIDER algorithm is to determine salient isovalues where surface topology changes and pre-encode these changes so as to facilitate fast updates to the triangulation.
A Robust Level-Set Algorithm for Centerline Extraction
(The Eurographics Association, 2003) Telea, Alexandru; Vilanova, Anna; G.-P. Bonneau and S. Hahmann and C. D. Hansen
We present a robust method for extracting 3D centerlines from volumetric datasets. We start from a 2D skeletonization method to locate voxels centered with respect to three orthogonal slicing directions. Next, we introduce a new detection criterion to extract the centerline voxels from the above skeletons, followed by a thinning, reconnection, and a ranking step. Overall, the proposed method produces centerlines that are object-centered, connected, one voxel thick, robust with respect to object noisiness, handles arbitrary object topologies, comes with a simple pruning threshold, and is fast to compute. We compare our results with two other methods on a variety of real-world datasets
Smart Hardware-Accelerated Volume Rendering
(The Eurographics Association, 2003) Roettger1, Stefan; Guthe, Stefan; Weiskopf, Daniel; Ertl, Thomas; Strasser, Wolfgang; G.-P. Bonneau and S. Hahmann and C. D. Hansen
For volume rendering of regular grids the display of view-plane aligned slices has proven to yield both good quality and performance. In this paper we demonstrate how to merge the most important extensions of the original 3D slicing approach, namely the pre-integration technique, volumetric clipping, and advanced lighting. Our approach allows the suppression of clipping artifacts and achieves high quality while offering the flexibility to explore volume data sets interactively with arbitrary clip objects. We also outline how to utilize the proposed volumetric clipping approach for the display of segmented data sets. Moreover, we increase the rendering quality by implementing effi cient over-sampling with the pixel shader of consumer graphics accelerators. We give prove that at least 4- times over-sampling is needed to reconstruct the ray integral with sufficient accuracy even with pre-integration. As an alternative to this brute-force over-sampling approach we propose a hardware-accelerated ray caster which is able to perform over-sampling only where needed and which is able to gain additional speed by early ray termination and space leaping.
Visualizing Spatial Distribution Data Sets
(The Eurographics Association, 2003) Luo, Alison; Kao, David; Pang, Alex; G.-P. Bonneau and S. Hahmann and C. D. Hansen
In this paper, we define distributions as a new data type and address the challenges of visualizing spatial distribution data sets. Numerous visualization techniques exist today for dealing with scalar data. That is, there is a scalar value at each spatial location, which may also be changing over time. Likewise, techniques exist for dealing with vector, tensor and multivariate data sets. However, there is currently no systematic way of dealing with distribution data where there is a collection of values for the same variable at every location and time. Distribution data is increasingly becoming more common as computers and sensor technologies continue to improve. They have also been used in a number of fields ranging from agriculture, engineering design and manufacturing to weather forecasting. Rather than developing specialized visualization techniques for dealing with distribution data, the approach presented in this paper is to find a systematic way of extending existing visualization methods to handle this new data type. For example, we would like to be able to generate isosurfaces of 3D scalar distribution data sets, or generate streamlines of vector distribution data sets. In order to accomplish this goal, we propose the use of a set of mathematically and procedurally defined operators that allow us to work directly on distributions. Color images can also be found in www.cse.ucsc.edu/research/avis/operator.html.
MCMR: A Fluid View on Time Dependent Volume Data
(The Eurographics Association, 2003) Leeuw, Wim de; Liere, Robert van; G.-P. Bonneau and S. Hahmann and C. D. Hansen
Mass Conservative Motion Reconstruction is a new method for estimating motion in time dependent volume data. A time dependent vector field representing the movement of the data is computed from a sequence of scalar volume data sets. The principle of mass conservation in a continuum is used during the reconstruction. Standard flow visualization techniques are used for the visualization of the derived vector field. This paper presents the underlying concepts of MCMR, its implementation, its accuracy and applicability.
Case Study: Comparing Two Methods for Filtering External Motion in 4D Confocal Microscopy Data
(The Eurographics Association, 2003) Leeuw, Wim de; Liere, Robert van; G.-P. Bonneau and S. Hahmann and C. D. Hansen
In this case study, we compare two methods for filtering external motion in time dependent volume data sets acquired from confocal microscopy. The pros and cons of a landmark based and a voxel based method are discussed. We show that filtering external motion is an essential first step for the visualization of confocal data.

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