Volume 32 (2013)

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https://diglib.eg.org/handle/10.2312/178

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Comparative Visualization of Tracer Uptake in In Vivo Small Animal PET/CT Imaging of the Carotid Arteries
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Diepenbrock, Stefan; Hermann, Sven; Schäfers, Michael; Kuhlmann, Michael; Hinrichs, Klaus; B. Preim, P. Rheingans, and H. Theisel
Cardiovascular diseases are the main cause of death in the western world. Medical research on atherosclerosis is therefore of great interest and a very active research topic. We present a visualization system that supports scientists in exploring plaque development and evaluating the applicability of PET tracers for early diagnosis of cardiovascular diseases. In our application case a cone shaped cuff has been implanted around the carotid artery of ApoE knockout mice, fed with a high cholesterol western type diet. As a result, vascular lesions develop upstream and downstream from the cuff. Tracer uptake induced by these lesions needs to be analyzed in order to evaluate the effectiveness of different PET tracers. We discuss the approach previously utilized to perform this kind of analysis, the problems arising from in vivo image acquisition (in contrast to ex vivo) and the design process of our application. In close cooperation with domain experts we have developed new visualization techniques that display PET activity in the vessel wall and surrounding tissue in a single image. We use the vessel wall detected in the CT image to perform a normalized circular projection which allows the user to judge PET signal distribution in relation to the deformed vessel. Based on this projection a quantitative analysis of a defined region adjacent to the vessel wall can be performed and compared to the artery without the cuff.
Patch-Collaborative Spectral Point-Cloud Denoising
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Rosman, G.; Dubrovina, A.; Kimmel, R.; Holly Rushmeier and Oliver Deussen
We present a new framework for point cloud denoising by patch-collaborative spectral analysis. A collaborative generalization of each surface patch is defined, combining similar patches from the denoised surface. The Laplace–Beltrami operator of the collaborative patch is then used to selectively smooth the surface in a robust manner that can gracefully handle high levels of noise, yet preserves sharp surface features. The resulting denoising algorithm competes favourably with state‐of‐the‐art approaches, and extends patch‐based algorithms from the image processing domain to point clouds of arbitrary sampling. We demonstrate the accuracy and noise‐robustness of the proposed algorithm on standard benchmark models as well as range scans, and compare it to existing methods for point cloud denoising.We present a new framework for point cloud denoising by patch‐collaborative spectral analysis. A collaborative generalization of each surface patch is defined, combining similar patches from the denoised surface. The Laplace‐Beltrami operator of the collaborative patch is then used to selectively smooth the surface in a robust manner that can gracefully handle high levels of noise, yet preserves sharp surface features.
Example-based Interpolation and Synthesis of Bidirectional Texture Functions
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Ruiters, Roland; Schwartz, Christopher; Klein, Reinhard; I. Navazo, P. Poulin
Bidirectional Texture Functions (BTF) have proven to be a well-suited representation for the reproduction of measured real-world surface appearance and provide a high degree of realism. We present an approach for designing novel materials by interpolating between several measured BTFs. For this purpose, we transfer concepts from existing texture interpolation methods to the much more complex case of material interpolation. We employ a separation of the BTF into a heightmap and a parallax compensated BTF to cope with problems induced by parallax, masking and shadowing within the material. By working only on the factorized representation of the parallax compensated BTF and the heightmap, it is possible to efficiently perform the material interpolation. By this novel method to mix existing BTFs, we are able to design plausible and realistic intermediate materials for a large range of different opaque material classes. Furthermore, it allows for the synthesis of tileable and seamless BTFs and finally even the generation of gradually changing materials following user specified material distribution maps.
Vega: Non-Linear FEM Deformable Object Simulator
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Sin, F. S.; Schroeder, D.; Barbic, J.; Holly Rushmeier and Oliver Deussen
This practice and experience paper describes a robust C++ implementation of several non-linear solid three-dimensional deformable object strategies commonly employed in computer graphics, named the Vega finite element method (FEM) simulation library. Deformable models supported include co-rotational linear FEM elasticity, Saint-Venant Kirchhoff FEM model, mass-spring system and invertible FEM models: neo-Hookean, Saint-Venant Kirchhoff and Mooney-Rivlin. We provide several timestepping schemes, including implicit Newmark and backward Euler integrators, and explicit central differences. The implementation of material models is separated from integration, which makes it possible to employ our code not only for simulation, but also for deformable object control and shape modelling. We extensively compare the different material models and timestepping schemes. We provide practical experience and insight gained while using our code in several computer animation and simulation research projects.This practice and experience paper describes a robust C++ implementation of several nonlinear solid 3D deformable object strategies commonly employed in computer graphics, named the Vega FEM simulation library. Deformable models supported include co-rotational linear FEM elasticity, Saint-Venant Kirchhoff FEM model, mass-spring system, and invertible FEM models: neo-Hookean, Saint-Venant Kirchhoff, and Mooney-Rivlin. We provide several timestepping schemes, including implicit Newmark and backward Euler integrators, and explicit central differences. The implementation of material models is separated from integration, which makes it possible to employ our code not only for simulation, but also for deformable object control and shape modeling. We extensively compare the different material models and timestepping schemes. We provide practical experience and insight gained while using our code in several computer animation and simulation research projects.
Temporally Coherent Adaptive Sampling for Imperfect Shadow Maps
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Barák, Tomas; Bittner, Jiri; Havran, Vlastimil; Nicolas Holzschuch and Szymon Rusinkiewicz
We propose a new adaptive algorithm for determining virtual point lights (VPL) in the scope of real-time instant radiosity methods, which use a limited number of VPLs. The proposed method is based on Metropolis-Hastings sampling and exhibits better temporal coherence of VPLs, which is particularly important for real-time applications dealing with dynamic scenes. We evaluate the properties of the proposed method in the context of the algorithm based on imperfect shadow maps and compare it with the commonly used inverse transform method. The results indicate that the proposed technique can significantly reduce the temporal flickering artifacts even for scenes with complex materials and textures. Further, we propose a novel splatting scheme for imperfect shadow maps using hardware tessellation. This scheme significantly improves the rendering performance particularly for complex and deformable scenes. We thoroughly analyze the performance of the proposed techniques on test scenes with detailed materials, moving camera, and deforming geometry.
Sparse Iterative Closest Point
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Bouaziz, Sofien; Tagliasacchi, Andrea; Pauly, Mark; Yaron Lipman and Hao Zhang
Rigid registration of two geometric data sets is essential in many applications, including robot navigation, surface reconstruction, and shape matching. Most commonly, variants of the Iterative Closest Point (ICP) algorithm are employed for this task. These methods alternate between closest point computations to establish correspondences between two data sets, and solving for the optimal transformation that brings these correspondences into alignment. A major difficulty for this approach is the sensitivity to outliers and missing data often observed in 3D scans. Most practical implementations of the ICP algorithm address this issue with a number of heuristics to prune or reweight correspondences. However, these heuristics can be unreliable and difficult to tune, which often requires substantial manual assistance. We propose a new formulation of the ICP algorithm that avoids these difficulties by formulating the registration optimization using sparsity inducing norms. Our new algorithm retains the simple structure of the ICP algorithm, while achieving superior registration results when dealing with outliers and incomplete data. The complete source code of our implementation is provided at http://lgg.epfl.ch/sparseicp.
Computational Simulation of Alternative Photographic Processes
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Echevarria, Jose I.; Wilensky, Gregg; Krishnaswamy, Aravind; Kim, Byungmoon; Gutierrez, Diego; Nicolas Holzschuch and Szymon Rusinkiewicz
We present a novel computational framework for physically and chemically-based simulations of analog alternative photographic processes. In the real world, these processes allow the creation of very personal and unique depictions due to the combination of the chemicals used, the physical interaction with liquid solutions, and the individual craftsmanship of the artist. Our work focuses not only on achieving similar compelling results, but on the manual process as well, introducing a novel exploratory approach for interactive digital image creation and manipulation. With such an emphasis on the user interaction, our simulations are devised to run on tablet devices; thus we propose the combination of a lightweight data-driven model to simulate the chemical reactions involved, with efficient fluids simulations that modulate them. This combination allows realistic gestures-based user interaction with constant visual feedback in real-time. Using the proposed framework, we have built two prototypes with different tradeoffs between realism and flexibility, showing its potential to build novel image editing tools.
Interactive Facades - Analysis and Synthesis of Semi-Regular Facades
(The Eurographics Association and Blackwell Publishing Ltd., 2013) AlHalawani, Sawsan; Yang, Yong-Liang; Liu, Han; Mitra, Niloy J.; I. Navazo, P. Poulin
Urban facades regularly contain interesting variations due to allowed deformations of repeated elements (e.g., windows in different open or close positions) posing challenges to state-of-the-art facade analysis algorithms. We propose a semi-automatic framework to recover both repetition patterns of the elements and their individual deformation parameters to produce a factored facade representation. Such a representation enables a range of applications including interactive facade images, improved multi-view stereo reconstruction, facade-level change detection, and novel image editing possibilities.
Optimizing Disparity for Motion in Depth
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Kellnhofer, Petr; Ritschel, Tobias; Myszkowski, Karol; Seidel, Hans-Peter; Nicolas Holzschuch and Szymon Rusinkiewicz
Beyond the careful design of stereo acquisition equipment and rendering algorithms, disparity post-processing has recently received much attention, where one of the key tasks is to compress the originally large disparity range to avoid viewing discomfort. The perception of dynamic stereo content however, relies on reproducing the full disparity-time volume that a scene point undergoes in motion. This volume can be strongly distorted in manipulation, which is only concerned with changing disparity at one instant in time, even if the temporal coherence of that change is maintained. We propose an optimization to preserve stereo motion of content that was subject to an arbitrary disparity manipulation, based on a perceptual model of temporal disparity changes. Furthermore, we introduce a novel 3D warping technique to create stereo image pairs that conform to this optimized disparity map. The paper concludes with perceptual studies of motion reproduction quality and task performance in a simple game, showing how our optimization can achieve both viewing comfort and faithful stereo motion.
Nonparametric Models for Uncertainty Visualization
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Pöthkow, Kai; Hege, Hans-Christian; B. Preim, P. Rheingans, and H. Theisel
An uncertain (scalar, vector, tensor) field is usually perceived as a discrete random field with a priori unknown probability distributions. To compute derived probabilities, e.g. for the occurrence of certain features, an appropriate probabilistic model has to be selected. The majority of previous approaches in uncertainty visualization were restricted to Gaussian fields. In this paper we extend these approaches to nonparametric models, which are much more flexible, as they can represent various types of distributions, including multimodal and skewed ones. We present three examples of nonparametric representations: (a) empirical distributions, (b) histograms and (c) kernel density estimates (KDE). While the first is a direct representation of the ensemble data, the latter two use reconstructed probability density functions of continuous random variables. For KDE we propose an approach to compute valid consistent marginal distributions and to efficiently capture correlations using a principal component transformation. Furthermore, we use automatic bandwidth selection, obtaining a model for probabilistic local feature extraction. The methods are demonstrated by computing probabilities of level crossings, critical points and vortex cores in simulated biofluid dynamics and climate data.
A Data-Driven Approach to Realistic Shape Morphing
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Gao, Lin; Lai, Yu-Kun; Huang, Qi-Xing; Hu, Shi-Min; I. Navazo, P. Poulin
Morphing between 3D objects is a fundamental technique in computer graphics. Traditional methods of shape morphing focus on establishing meaningful correspondences and finding smooth interpolation between shapes. Such methods however only take geometric information as input and thus cannot in general avoid producing unnatural interpolation, in particular for large-scale deformations. This paper proposes a novel data-driven approach for shape morphing. Given a database with various models belonging to the same category, we treat them as data samples in the plausible deformation space. These models are then clustered to form local shape spaces of plausible deformations. We use a simple metric to reasonably represent the closeness between pairs of models. Given source and target models, the morphing problem is casted as a global optimization problem of finding a minimal distance path within the local shape spaces connecting these models. Under the guidance of intermediate models in the path, an extended as-rigid-as-possible interpolation is used to produce the final morphing. By exploiting the knowledge of plausible models, our approach produces realistic morphing for challenging cases as demonstrated by various examples in the paper.
An Area-Preserving Parametrization for Spherical Rectangles
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Ureña, Carlos; Fajardo, Marcos; King, Alan; Nicolas Holzschuch and Szymon Rusinkiewicz
We present an area-preserving parametrization for spherical rectangles which is an analytical function with domain in the unit rectangle [0;1]2 and range in a region included in the unit-radius sphere. The parametrization preserves areas up to a constant factor and is thus very useful in the context of rendering as it allows to map random sample point sets in [0;1]2 onto the spherical rectangle. This allows for easily incorporating stratified, quasi-Monte Carlo or other sampling strategies in algorithms that compute scattering from planar rectangular emitters.
An Interactive Analysis and Exploration Tool for Epigenomic Data
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Younesy, Hamidreza; Nielsen, Cydney B.; Möller, Torsten; Alder, Olivia; Cullum, Rebecca; Lorincz, Matthew C.; Karimi, Mohammad M.; Jones, Steven J. M.; B. Preim, P. Rheingans, and H. Theisel
In this design study, we present an analysis and abstraction of the data and tasks related to the domain of epigenomics, and the design and implementation of an interactive tool to facilitate data analysis and visualization in this domain. Epigenomic data can be grouped into subsets either by k-means clustering or by querying for combinations of presence or absence of signal (on/off) in different epigenomic experiments. These steps can easily be interleaved and the comparison of different workflows is explicitly supported. We took special care to contain the exponential expansion of possible on/off combinations by creating a novel querying interface. An interactive heat map facilitates the exploration and comparison of different clusters. We validated our iterative design by working closely with two groups of biologists on different biological problems. Both groups quickly found new insight into their data as well as claimed that our tool would save them several hours or days of work over using existing tools.
Freeform Shadow Boundary Editing
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Mattausch, Oliver; Igarashi, Takeo; Wimmer, Michael; I. Navazo, P. Poulin
We present an algorithm for artistically modifying physically based shadows. With our tool, an artist can directly edit the shadow boundaries in the scene in an intuitive fashion similar to freeform curve editing. Our algorithm then makes these shadow edits consistent with respect to varying light directions and scene configurations, by creating a shadow mesh from the new silhouettes. The shadow mesh helps a modified shadow volume algorithm cast shadows that conform to the artistic shadow boundary edits, while providing plausible interaction with dynamic environments, including animation of both characters and light sources. Our algorithm provides significantly more fine-grained local and direct control than previous artistic light editing methods, which makes it simple to adjust the shadows in a scene to reach a particular effect, or to create interesting shadow shapes and shadow animations. All cases are handled with a single intuitive interface, be it soft shadows, or (self-)shadows on arbitrary receivers.
Generating Pointillism Paintings Based on Seurat's Color Composition
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Wu, Yi-Chian; Tsai, Yu-Ting; Lin, Wen-Chieh; Li, Wen-Hsin; Nicolas Holzschuch and Szymon Rusinkiewicz
This paper presents a novel example-based stippling technique that employs a simple and intuitive concept to convert a color image into a pointillism painting. Our method relies on analyzing and imitating the color distributions of Seurat's paintings to obtain a statistical color model. Then, this model can be easily combined with the modified multi-class blue noise sampling to stylize an input image with characteristics of color composition in Seurat's paintings. The blue noise property of the output image also ensures that the color points are randomly located but remain spatially uniform. In our experiments, the multivariate goodness-of-fit tests were adopted to quantitatively analyze the results of the proposed and previous methods, further confirming that the color composition of our results are more similar to Seurat's painting style than that of previous approaches. Additionally, we also conducted a user study participated by artists to qualitatively evaluate the synthesized images of the proposed method.
Physics Storyboards
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Ha, Sehoon; McCann, Jim; Liu, C. Karen; Popovic, Jovan; I. Navazo, P. Poulin
Physical simulation and other procedural methods are increasingly popular tools in interactive applications because they generate complex and reactive behaviors given only a few parameter settings. This automation accelerates initial implementation, but also introduces a need to tune the available parameters until the desired behaviors emerge. These adjustments are typically performed iteratively, with the designer repeatedly running- and interacting with-the procedural animation with different parameter settings. Such a process is inaccurate, time consuming, and requires deep understanding and intuition, as parameters often have complex, nonlinear effects. Instead, we propose that designers construct physics storyboards to accelerate the process of tuning interactive, procedural animations. Physics storyboards are collections of space-time snapshots that highlight critical events and outcomes. They can be used to summarize the effects of parameter changes (without requiring the designer to perform extensive play-testing); and-when augmented with designer-provided evaluation functions-allow automatic parameter selection. We describe our implementation of this method, including how we use sampling to ensure that our automatically-selected parameters generalize, and how we time-warp user input to adapt it to changing parameters. We validate our implementation by using it to perform various design tasks in three example games.
Synthetic Controllable Turbulence Using Robust Second Vorticity Confinement
(The Eurographics Association and Blackwell Publishing Ltd., 2013) He, S.; Lau, R. W. H.; Holly Rushmeier and Oliver Deussen
Capturing fine details of turbulence on a coarse grid is one of the main tasks in real-time fluid simulation. Existing methods for doing this have various limitations. In this paper, we propose a new turbulence method that uses a refined second vorticity confinement method, referred to as robust second vorticity confinement, and a synthesis scheme to create highly turbulent effects from coarse grid. The new technique is sufficiently stable to efficiently produce highly turbulent flows, while allowing intuitive control of vortical structures. Second vorticity confinement captures and defines the vortical features of turbulence on a coarse grid. However, due to the stability problem, it cannot be used to produce highly turbulent flows. In this work, we propose a robust formulation to improve the stability problem by making the positive diffusion term to vary with helicity adaptively. In addition, we also employ our new method to procedurally synthesize the high-resolution flow fields. As shown in our results, this approach produces stable high-resolution turbulence very efficiently.Capturing fine details of turbulence on a coarse grid is one of the main tasks in real-time fluid simulation. Existing methods for doing this have various limitations. In this paper, we propose a new turbulence method that uses a refined Second Vorticity Confinement method, referred to as Robust Second Vorticity Confinement, and a synthesis scheme to create highly turbulent effects from coarse grid. The new technique is sufficiently stable to efficiently produce highly turbulent flows, while allowing intuitive control of vortical structures. Second Vorticity Confinement captures and defines the vortical features of turbulence on a coarse grid. However, due to the stability problem, it cannot be used to produce highly turbulent flows. In this work, we propose a robust formulation to improve the stability problem by making the positive diffusion term to vary with helicity adaptively. In addition, we also employ our new method to procedurally synthesize the high resolution flow fields. As shown in our results, this approach produces stable high resolution turbulence very efficiently.
PHOG: Photometric and Geometric Functions for Textured Shape Retrieval
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Biasotti, Silvia; Cerri, Andrea; Giorgi, Daniela; Spagnuolo, Michaela; Yaron Lipman and Hao Zhang
In this paper we target the problem of textured 3D object retrieval. As a first contribution, we show how to include photometric information in the persistence homology setting, also proposing a novel theoretical result about multidimensional persistence spaces. As a second contribution, we introduce a generalization of the integral geodesic distance which fuses shape and color properties. Finally, we adopt a purely geometric description based on the selection of geometric functions that are mutually independent. The photometric, hybrid and geometric descriptions are combined into a signature, whose performance is tested on a benchmark dataset.
Watertight Scenes from Urban LiDAR and Planar Surfaces
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Kreveld, Marc van; Lankveld, Thijs van; Veltkamp, Remco C.; Yaron Lipman and Hao Zhang
The demand for large geometric models is increasing, especially of urban environments. This has resulted in production of massive point cloud data from images or LiDAR. Visualization and further processing generally require a detailed, yet concise representation of the scene's surfaces. Related work generally either approximates the data with the risk of over-smoothing, or interpolates the data with excessive detail. Many surfaces in urban scenes can be modeled more concisely by planar approximations. We present a method that combines these polygons into a watertight model. The polygon-based shape is closed with free-form meshes based on visibility information. To achieve this, we divide 3-space into inside and outside volumes by combining a constrained Delaunay tetrahedralization with a graph-cut. We compare our method with related work on several large urban LiDAR data sets. We construct similar shapes with a third fewer triangles to model the scenes. Additionally, our results are more visually pleasing and closer to a human modeler's description of urban scenes using simple boxes.
ArtiSketch: A System for Articulated Sketch Modeling
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Levi, Zohar; Gotsman, Craig; I. Navazo, P. Poulin
We present ArtiSketch - a system which allows the conversion of a wealth of existing 2D content into 3D content by users who do not necessarily possess artistic skills. Using ArtiSketch, a novice user may describe a 3D model as a set of articulated 2D sketches of a shape from different viewpoints. ArtiSketch then automatically converts the sketches to an articulated 3D object. Using common interactive tools, the user provides an initial estimate of the 3D skeleton pose for each frame, which ArtiSketch refines to be consistent between frames. This skeleton may then be manipulated independently to generate novel poses of the 3D model.

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