28-Issue 3

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https://diglib.eg.org/handle/10.2312/145

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Accelerating Volume Raycasting using Proxy Spheres
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Liu, Baoquan; Clapworthy, Gordon J.; Dong, Feng; H.-C. Hege, I. Hotz, and T. Munzner
In this paper, we propose an efficient solution that addresses the performance problems of current single-pass GPU raycasting algorithms. Our paper provides more control over the rendering process by introducing tighter ray segments for raycasting, while at the same time avoiding the introduction of any new rendering artefacts. We achieve this by dynamically generating, on the GPU, a coarsely fitted proxy geometry, composed of spheres, for the active blocks. The spheres are then rasterised into two z-buffers by a single rendering pass. The resulting two z-buffers are used as the first-hit and last-hit points for the subsequent raycaster. With this approach, only the valid ray segments between the two z-buffers need to be sampled during raycasting. This also provides more coherent parallelism on the GPU due to more consistent ray length and avoidance of the overheads and dynamic branching of performing checks on a per-sample basis during the raycasting pass. Our technique is ideal for dynamic data exploration in which both the transfer function and view parameters need to be changed frequently at runtime. The rendering results of our algorithm are identical to the general cube-based proxy geometry algorithm, but the performance can be up to 15.7 times faster. Furthermore, the approach can be adopted by any existing raycasting system in a straightforward way.
Bivariate Transfer Functions on Unstructured Grids
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Song, Yuyan; Chen, Wei; Maciejewski, Ross; Gaither, Kelly P.; Ebert, David S.; H.-C. Hege, I. Hotz, and T. Munzner
Multi-dimensional transfer functions are commonly used in rectilinear volume renderings to effectively portray materials, material boundaries and even subtle variations along boundaries. However, most unstructured grid rendering algorithms only employ one-dimensional transfer functions. This paper proposes a novel pre-integrated Projected Tetrahedra (PT) rendering technique that applies bivariate transfer functions on unstructured grids. For each type of bivariate transfer function, an analytical form that pre-integrates the contribution of a ray segment in one tetrahedron is derived, and can be precomputed as a lookup table to compute the color and opacity in a projected tetrahedron on-the-fly. Further, we show how to approximate the integral using the pre-integration method for faster unstructured grid rendering. We demonstrate the advantages of our approach with a variety of examples and comparisons with one-dimensional transfer functions.
The Chinese Room: Visualization and Interaction to Understand and Correct Ambiguous Machine Translation
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Albrecht, Joshua; Hwa, Rebecca; Marai, G. Elisabeta; H.-C. Hege, I. Hotz, and T. Munzner
We present The Chinese Room, a visualization interface that allows users to explore and interact with a multitude of linguistic resources in order to decode and correct poor machine translations. The target users of The Chinese Room are not bilingual and are not familiar with machine translation technologies. We investigate the ability of our system to assist such users in decoding and correcting faulty machine translations. We found that by collaborating with our application, end-users can overcome many difficult translation errors and disambiguate translated passages that were otherwise baffling. We also examine the utility of our system to machine translation researchers. Anecdotal evidence suggests that The Chinese Room can help such researchers develop better machine translation systems.
Collaborative Brushing and Linking for Co-located Visual Analytics of Document Collections
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Isenberg, Petra; Fisher, Danyel; H.-C. Hege, I. Hotz, and T. Munzner
Many real-world analysis tasks can benefit from the combined efforts of a group of people. Past research has shown that to design visualizations for collaborative visual analytics tasks, we need to support both individual as well as joint analysis activities. We present Cambiera, a tabletop visual analytics tool that supports individual and collaborative information foraging activities in large text document collections. We define collaborative brushing and linking as an awareness mechanism that enables analysts to follow their own hypotheses during collaborative sessions while still remaining aware of the group's activities. With Cambiera, users are able to collaboratively search through documents, maintaining awareness of each others' work and building on each others' findings.
Combining Map Displays and 3D Visualizations for the Analysis of Scalar Data on Cerebral Aneurysm Surfaces
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Neugebauer, Mathias; Gasteiger, Rocco; Beuing, Oliver; Diehl, Volker; Skalej, Martin; Preim, Bernhard; H.-C. Hege, I. Hotz, and T. Munzner
Cerebral aneurysms result from a congenital or evolved weakness of stabilizing parts of the vessel wall and potentially lead to rupture and a life-threatening bleeding. Current medical research concentrates on the integration of blood flow simulation results for risk assessment of cerebral aneurysms. Scalar flow characteristics close to the aneurysm surface, such as wall shear stress, form an important part of the simulation results. Aneurysms exhibit variable surface shapes with only few landmarks. Therefore, the exploration and mental correlation of different surface regions is a difficult task. In this paper, we present an approach for the intuitive and interactive overview visualization of near wall flow data that is mapped onto the surface of a 3D model of a cerebral aneurysm. We combine a multi-perspective 2D projection map with a standard 3D visualization and present techniques to facilitate the correlation between a 3D model and a related 2D map. An informal evaluation with 4 experienced radiologists has shown that the map-based overview actually improves the surface exploration. Furthermore, different color schemes were discussed and, as a result, an appropriate color scheme for the visual analysis of the wall shear stress is presented.
Comparing Parameter Manipulation with Mouse, Pen, and Slider User Interfaces
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Swindells, Colin; Tory, Melanie; Dreezer, Rebecca; H.-C. Hege, I. Hotz, and T. Munzner
Visual fixation on one s tool(s) takes much attention away from one s primary task. Following the belief that the best tools disappear and become invisible to the user, we present a study comparing visual fixations (eye gaze within locations on a graphical display) and performance for mouse, pen, and physical slider user interfaces. Participants conducted a controlled, yet representative, color matching task that required user interaction representative of many data exploration tasks such as parameter exploration of medical or fuel cell data. We demonstrate that users may spend up to 95% fewer visual fixations on physical sliders versus standard mouse and pen tools without any loss in performance for a generalized visual performance task.
Context-aware Volume Modeling of Skeletal Muscles
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Yan, Zhicheng; Chen, Wei; Lu, Aidong; Ebert, David S.; H.-C. Hege, I. Hotz, and T. Munzner
This paper presents an interactive volume modeling method that constructs skeletal muscles from an existing volumetric dataset. Our approach provides users with an intuitive modeling interface and produces compelling results that conform to the characteristic anatomy in the input volume. The algorithmic core of our method is an intuitive anatomy classification approach, suited to accommodate spatial constraints on the muscle volume. The presented work is useful in illustrative visualization, volumetric information fusion and volume illustration that involve muscle modeling, where the spatial context should be faithfully preserved.
Direct Visualization of Deformation in Volumes
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Busking, Stef; Botha, Charl P.; Post, Frits H.; H.-C. Hege, I. Hotz, and T. Munzner
Deformation is a topic of interest in many disciplines. In particular in medical research, deformations of surfaces and even entire volumetric structures are of interest. Clear visualization of such deformations can lead to important insight into growth processes and progression of disease. We present new techniques for direct focus+context visualization of deformation fields representing transformations between pairs of volumetric datasets. Typically, such fields are computed by performing a non-rigid registration between two data volumes. Our visualization is based on direct volume rendering and uses the GPU to compute and interactively visualize features of these deformation fields in real-time. We integrate visualization of the deformation field with visualization of the scalar volume affected by the deformations. Furthermore, we present a novel use of texturing in volume rendered visualizations to show additional properties of the vector field on surfaces in the volume.
A Directional Occlusion Shading Model for Interactive Direct Volume Rendering
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Schott, Mathias; Pegoraro, Vincent; Hansen, Charles; Boulanger, Kévin; Bouatouch, Kadi; H.-C. Hege, I. Hotz, and T. Munzner
Volumetric rendering is widely used to examine 3D scalar fields from CT/MRI scanners and numerical simulation datasets. One key aspect of volumetric rendering is the ability to provide perceptual cues to aid in understanding structure contained in the data. While shading models that reproduce natural lighting conditions have been shown to better convey depth information and spatial relationships, they traditionally require considerable (pre)computation. In this paper, a shading model for interactive direct volume rendering is proposed that provides perceptual cues similar to those of ambient occlusion, for both solid and transparent surface-like features. An image space occlusion factor is derived from the radiative transport equation based on a specialized phase function. The method does not rely on any precomputation and thus allows for interactive explorations of volumetric data sets via on-the-fly editing of the shading model parameters or (multi-dimensional) transfer functions while modifications to the volume via clipping planes are incorporated into the resulting occlusion-based shading.
DocuBurst: Visualizing Document Content using Language Structure
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Collins, Christopher; Carpendale, Sheelagh; Penn, Gerald; H.-C. Hege, I. Hotz, and T. Munzner
Textual data is at the forefront of information management problems today. One response has been the development of visualizations of text data. These visualizations, commonly based on simple attributes such as relative word frequency, have become increasingly popular tools. We extend this direction, presenting the first visualization of document content which combines word frequency with the human-created structure in lexical databases to create a visualization that also reflects semantic content. DocuBurst is a radial, space-filling layout of hyponymy (the IS-A relation), overlaid with occurrence counts of words in a document of interest to provide visual summaries at varying levels of granularity. Interactive document analysis is supported with geometric and semantic zoom, selectable focus on individual words, and linked access to source text.
Efficient and Adaptive Rendering of 2-D Continuous Scatterplots
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Bachthaler, Sven; Weiskopf, Daniel; H.-C. Hege, I. Hotz, and T. Munzner
We extend the rendering technique for continuous scatterplots to allow for a broad class of interpolation methods within the spatial grid instead of only linear interpolation. To do this, we propose an approach that projects the image of a cell from the spatial domain to the scatterplot domain. We approximate this image using either the convex hull or an axis-aligned rectangle that forms a tight fit of the projected points. In both cases, the approach relies on subdivision in the spatial domain to control the approximation error introduced in the scatterplot domain. Acceleration of this algorithm in homogeneous regions of the spatial domain is achieved using an octree hierarchy. The algorithm is scalable and adaptive since it allows us to balance computation time and scatterplot quality. We evaluate and discuss the results with respect to accuracy and computational speed. Our methods are applied to examples of 2-D transfer function design.
Enclosing Surfaces for Point Clusters Using 3D Discrete Voronoi Diagrams
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Rosenthal, Paul; Linsen, Lars; H.-C. Hege, I. Hotz, and T. Munzner
Point clusters occur in both spatial and non-spatial data. In the former context they may represent segmented particle data, in the latter context they may represent clusters in scatterplots. In order to visualize such point clusters, enclosing surfaces lead to much better comprehension than pure point renderings. We propose a flexible system for the generation of enclosing surfaces for 3D point clusters. We developed a GPUbased 3D discrete Voronoi diagram computation that supports all surface extractions. Our system provides three different types of enclosing surfaces. By generating a discrete distance field to the point cluster and extracting an isosurface from the field, an enclosing surface with any distance to the point cluster can be generated. As a second type of enclosing surfaces, a hull of the point cluster is extracted. The generation of the hull uses a projection of the discrete Voronoi diagram of the point cluster to an isosurface to generate a polygonal surface. Generated hulls of non-convex clusters are also non-convex. The third type of enclosing surfaces can be created by computing a distance field to the hull and extracting an isosurface from the distance field. This method exhibits reduced bumpiness and can extract surfaces arbitrarily close to the point cluster without losing connectedness. We apply our methods to the visualization of multidimensional spatial and non-spatial data. Multidimensional clusters are extracted and projected into a 3D visual space, where the point clusters are visualized. The respective clusters can also be visualized in object space when dealing with multidimensional particle data.
Extended Excentric Labeling
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Bertini, Enrico; Rigamonti, Maurizio; Lalanne, Denis; H.-C. Hege, I. Hotz, and T. Munzner
The paper presents an extension to the Excentric Labeling, a labeling technique to dynamically show labels around a movable lens. Each labels refers to one object within the lens and is connected to it through a line. The original implementation has several known limitations and potential improvements that we address in this work, like: high density areas, uneven density distributions, and summary statistics. We describe the implemented extensions and present a think-aloud user study. The study shows that users can naturally understand and easily operate the majority of the implemented function but label scrolling, which requires additional research. From the study we also gained unanticipated requirements and interesting directions for further research.
Force-Directed Edge Bundling for Graph Visualization
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Holten, Danny; Wijk, Jarke J. van; H.-C. Hege, I. Hotz, and T. Munzner
Abstract Graphs depicted as node-link diagrams are widely used to show relationships between entities. However, nodelink diagrams comprised of a large number of nodes and edges often suffer from visual clutter. The use of edge bundling remedies this and reveals high-level edge patterns. Previous methods require the graph to contain a hierarchy for this, or they construct a control mesh to guide the edge bundling process, which often results in bundles that show considerable variation in curvature along the overall bundle direction. We present a new edge bundling method that uses a self-organizing approach to bundling in which edges are modeled as flexible springs that can attract each other. In contrast to previous methods, no hierarchy is used and no control mesh. The resulting bundled graphs show significant clutter reduction and clearly visible high-level edge patterns. Curvature variation is furthermore minimized, resulting in smooth bundles that are easy to follow. Finally, we present a rendering technique that can be used to emphasize the bundling.
Fully Automatic Visualisation of Overlapping Sets
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Simonetto, Paolo; Auber, David; Archambault, Daniel; H.-C. Hege, I. Hotz, and T. Munzner
Visualisation of taxonomies and sets has recently become an active area of research. Many application fields now require more than a strict classification of elements into a hierarchy tree. Euler diagrams, one of the most natural ways of depicting intersecting sets, may provide a solution to these problems. In this paper, we present an approach for the automatic generation of Euler-like diagrams. This algorithm differs from previous approaches in that it has no undrawable instances of input, allowing it to be used in systems where the output is always required. We also improve the readability of Euler diagrams through the use of Bézier curves and transparent coloured textures. Our approach has been implemented using the Tulip platform. Both the source and executable program used to generate the results are freely available.
Fused Multi-Volume DVR using Binary Space Partitioning
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Lindholm, Stefan; Ljung, Patric; Hadwiger, Markus; Ynnerman, Anders; H.-C. Hege, I. Hotz, and T. Munzner
Multiple-volume visualization is a growing field in medical imaging providing simultaneous exploration of volumes acquired from varying modalities. However, high complexity results in an increased strain on performance compared to single volume rendering as scenes may consist of volumes with arbitrary orientations and rendering is performed with varying sample densities. Expensive image order techniques such as depth peeling have previously been used to perform the necessary calculations. In this work we present a view-independent region based scene description for multi-volume pipelines. Using Binary Space Partitioning we are able to create a simple interface providing all required information for advanced multi-volume renderings while introducing a minimal overhead for scenes with few volumes. The modularity of our solution is demonstrated by the use of visual development and performance is documented with benchmarks and real-time simulations.
The Gödel Engine - An Interactive Approach to Visualization in General Relativity
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Grave, Frank; Müller, Thomas; Dachsbacher, Carsten; Wunner, Günter; H.-C. Hege, I. Hotz, and T. Munzner
We present a methodical new approach to visualize the aspects of general relativity from a self-centered perspective. We focus on the visualization of the Gödel universe, which is an exact solution to Einstein s field equations of general relativity. This model provides astounding features such as the existence of an optical horizon and the possibility of time travel. Although we know that our universe is not of Gödel type, we can using this solution to Einstein s equations visualize and understand the effects resulting from the theory of relativity, which itself has been verified on the large scale in numerous experiments over the last century. We derive the analytical solution to the geodesic equations of Gödel s universe for special initial conditions. Along with programmable graphics hardware we achieve a tremendous speedup for the visualization of general relativity. This enables us to interactively explore the physical aspects and optical effects of Gödel s universe. We also demonstrate how the analytical solution enables dynamic lighting with local illumination models. Our implementation is tailored for Gödel s universe and five orders of magnitude faster than previous approaches. It can be adapted to manifolds for which an analytical expression of the propagation of light is available.
Hierarchical Vortex Regions in Swirling Flow
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Petz, Christoph; Kasten, Jens; Prohaska, Steffen; Hege, Hans-Christian; H.-C. Hege, I. Hotz, and T. Munzner
We propose a new criterion to characterize hierarchical two-dimensional vortex regions induced by swirling motion. Central to the definition are closed loops that intersect the flow field at a constant angle. The union of loops belonging to the same area of swirling motion defines a vortex region. These regions are disjunct but may be nested, thus introducing a spatial hierarchy of vortex regions. We present a parameter free algorithm for the identification of these regions. Since they are not restricted to star- or convex-shaped geometries, we are able to identify also intricate regions, e.g., of elongated vortices. Computing an integrated value for each loop and mapping these values to a vortex region, introduces new ways for visualizing or filtering the vortex regions. Exemplary, an application based on the Rankine vortex model is presented. We apply our method to several CFD datasets and compare our results to existing approaches.
High-Quality Volumetric Reconstruction on Optimal Lattices for Computed Tomography
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Finkbeiner, Bernhard; Alim, Usman R.; Ville, Dimitri Van De; Möller, Torsten; H.-C. Hege, I. Hotz, and T. Munzner
Within the context of emission tomography, we study volumetric reconstruction methods based on the Expectation Maximization (EM) algorithm. We show, for the first time, the equivalence of the standard implementation of the EM-based reconstruction with an implementation based on hardware-accelerated volume rendering for nearest- neighbor (NN) interpolation. This equivalence suggests that higher-order kernels should be used with caution and do not necessarily lead to better performance. We also show that the EM algorithm can easily be adapted for different lattices, the body-centered cubic (BCC) one in particular. For validation purposes, we use the 3D version of the Shepp-Logan synthetic phantom, for which we derive closed-form analytical expressions of the projection data. The experimental results show the theoretically-predicted optimality of NN interpolation in combination with the EM algorithm, for both the noiseless and the noisy case. Moreover, reconstruction on the BCC lattice leads to superior accuracy, more compact data representation, and better noise reduction compared to the Cartesian one. Finally, we show the usefulness of the proposed method for optical projection tomography of a mouse embryo.
Illuminated 3D Scatterplots
(The Eurographics Association and Blackwell Publishing Ltd., 2009) Sanftmann, Harald; Weiskopf, Daniel; H.-C. Hege, I. Hotz, and T. Munzner
In contrast to 2D scatterplots, the existing 3D variants have the advantage of showing one additional data dimension, but suffer from inadequate spatial and shape perception and therefore are not well suited to display structures of the underlying data. We improve shape perception by applying a new illumination technique to the pointcloud representation of 3D scatterplots. Points are classified as locally linear, planar, and volumetric structures according to the eigenvalues of the inverse distance-weighted covariance matrix at each data element. Based on this classification, different lighting models are applied: codimension-2 illumination, surface illumination, and emissive volumetric illumination. Our technique lends itself to efficient GPU point rendering and can be combined with existing methods like semi-transparent rendering, halos, and depth or attribute based color coding. The user can interactively navigate in the dataset and manipulate the classification and other visualization parameters. We demonstrate our visualization technique by showing examples of multi-dimensional data and of generic pointcloud data.

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