Volume 30 (2011)

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11th International Symposium on Virtual Reality, Archaology and Cultural Heritge: 8th Eurographics Workshop on Graphics and Cultural Heritage
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Artusi, Alessandro; Joly, Morwena; Lucet, Geneviève; Ribes, Alejandro; Pitzalis, Denis; Eduard Groeller and Holly Rushmeier
32nd EUROGRAPHICS General Assembly
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Eduard Groeller and Holly Rushmeier
Adaptive Records for Irradiance Caching
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Ribardière, Mickaël; Carré, Samuel; Bouatouch, Kadi; Eduard Groeller and Holly Rushmeier
Irradiance Caching is one of the most widely used algorithms to speed up global illumination. In this paper, we propose an algorithm based on the Irradiance Caching scheme that allows us (1) to adjust the density of cached records according to illumination changes and (2) to efficiently render the high‐frequency illumination changes. To achieve this, a new record footprint is presented. Although the original method uses records having circular footprints depending only on geometrical features, our record footprints have a more complex shape which accounts for both geometry and irradiance variations. Irradiance values are computed using a classical Monte Carlo ray tracing method that simplifies the determination of nearby objects and the pre‐computation of the shape of the influence zone of the current record. By gathering irradiance due to all the incident rays, illumination changes are evaluated to adjust the footprint’s records. As a consequence, the record footprints are smaller where illumination gradients are high. With this technique, the record density depends on the irradiance variations. Strong variations of irradiance (due to direct contributions for example) can be handled and evaluated accurately. Caching direct illumination is of high importance, especially in the case of scenes having many light sources with complex geometry as well as surfaces exposed to daylight. Recomputing direct illumination for the whole image can be very time‐consuming, especially for walkthrough animation rendering or for high‐resolution pictures. Storing such contributions in the irradiance cache seems to be an appropriate solution to accelerate the final rendering pass.
All-Hex Mesh Generation via Volumetric PolyCube Deformation
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Gregson, James; Sheffer, Alla; Zhang, Eugene; Mario Botsch and Scott Schaefer
While hexahedral mesh elements are preferred by a variety of simulation techniques, constructing quality all-hex meshes of general shapes remains a challenge. An attractive hex-meshing approach, often referred to as submapping, uses a low distortion mapping between the input model and a PolyCube (a solid formed from a union of cubes), to transfer a regular hex grid from the PolyCube to the input model. Unfortunately, the construction of suitable PolyCubes and corresponding volumetric maps for arbitrary shapes remains an open problem. Our work introduces a new method for computing low-distortion volumetric PolyCube deformations of general shapes and for subsequent all-hex remeshing. For a given input model, our method simultaneously generates an appropriate PolyCube structure and mapping between the input model and the PolyCube. From these we automatically generate good quality all-hex meshes of complex natural and man-made shapes.
Allometric Scaling for Character Design
(The Eurographics Association and Blackwell Publishing Ltd., 2011) McGraw, T.; Kawai, T.; Richards, J.; Eduard Groeller and Holly Rushmeier
Geometric scaling transformations do not respect the biological processes which govern the size and shape of living creatures. In this paper, we describe an approach to scaling which can be related to biological function. We use known biological laws of allometry which are expressed as power laws to control the mesh deformation in the frequency domain. This approach is motivated by the relation between fractal biological systems and their underlying power-law spectra. We demonstrate our approach to biology-aware character scaling on triangle meshes representing quadrupedal mammals.
Anaglyph Stereo Without Ghosting
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Sanftmann, Harald; Weiskopf, Daniel; Ravi Ramamoorthi and Erik Reinhard
Anaglyph stereo provides a low-budget solution to viewing stereoscopic images. However, it may suffer from ghosting and bad color reproduction. Here we address the first issue. We present a novel technique to perceptually calibrate an anaglyph stereoscopic system and to use the calibration to eliminate ghosting from the anaglyph image. We build a model based on luminance perception by the left and right eyes through the anaglyph glasses. We do not rely on power spectra of a monitor or on transmission spectra of anaglyph glasses, but show how the five parameters of our model can be captured with just a few measurements within a minute. We present how full color, half color, and gray anaglyphs can be rendered with our technique and compare them to the traditional method.
Anatomy-Guided Multi-Level Exploration of Blood Flow in Cerebral Aneurysms
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Neugebauer, Mathias; Janiga, Gabor; Beuing, Oliver; Skalej, Martin; Preim, Bernhard; H. Hauser, H. Pfister, and J. J. van Wijk
For cerebral aneurysms, the ostium, the area of inflow, is an important anatomic landmark, since it separates the pathological vessel deformation from the healthy parent vessel. A better understanding of the inflow characteristics, the flow inside the aneurysm and the overall change of pre- and post-aneurysm flow in the parent vessel provide insights for medical research and the development of new risk-reduced treatment options. We present an approach for a qualitative, visual flow exploration that incorporates the ostium and derived anatomical landmarks. It is divided into three scopes: a global scope for exploration of the in- and outflow, an ostium scope that provides characteristics of the flow profile close to the ostium and a local scope for a detailed exploration of the flow in the parent vessel and the aneurysm. The approach was applied to five representative datasets, including measured and simulated blood flow. Informal interviews with two board-certified radiologists confirmed the usefulness of the provided exploration tools and delivered input for the integration of the ostium-based flow analysis into the overall exploration workflow.
As-Killing-As-Possible Vector Fields for Planar Deformation
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Solomon, Justin; Ben-Chen, Mirela; Butscher, Adrian; Guibas, Leonidas; Mario Botsch and Scott Schaefer
Cartoon animation, image warping, and several other tasks in two-dimensional computer graphics reduce to the formulation of a reasonable model for planar deformation. A deformation is a map from a given shape to a new one, and its quality is determined by the type of distortion it introduces. In many applications, a desirable map is as isometric as possible. Finding such deformations, however, is a nonlinear problem, and most of the existing solutions approach it by minimizing a nonlinear energy. Such methods are not guaranteed to converge to a global optimum and often suffer from robustness issues. We propose a new approach based on approximate Killing vector fields (AKVFs), first introduced in shape processing. AKVFs generate near-isometric deformations, which can be motivated as direction fields minimizing an as-rigid-as-possible (ARAP) energy to first order. We first solve for an AKVF on the domain given user constraints via a linear optimization problem and then use this AKVF as the initial velocity field of the deformation. In this way, we transfer the inherent nonlinearity of the deformation problem to finding trajectories for each point of the domain having the given initial velocities. We show that a specific class of trajectories - the set of logarithmic spirals - is especially suited for this task both in practice and through its relationship to linear holomorphic vector fields. We demonstrate the effectiveness of our method for planar deformation by comparing it with existing state-of-the-art deformation methods.
Assisted Descriptor Selection Based on Visual Comparative Data Analysis
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Bremm, Sebastian; Landesberger, Tatiana von; Bernard, Jürgen; Schreck, Tobias; H. Hauser, H. Pfister, and J. J. van Wijk
Exploration and selection of data descriptors representing objects using a set of features are important components in many data analysis tasks. Usually, for a given dataset, an optimal data description does not exist, as the suitable data representation is strongly use case dependent. Many solutions for selecting a suitable data description have been proposed. In most instances, they require data labels and often are black box approaches. Non-expert users have difficulties to comprehend the coherency of input, parameters, and output of these algorithms. Alternative approaches, interactive systems for visual feature selection, overburden the user with an overwhelming set of options and data views. Therefore, it is essential to offer the users a guidance in this analytical process. In this paper, we present a novel system for data description selection, which facilitates the user's access to the data analysis process. As finding of suitable data description consists of several steps, we support the user with guidance. Our system combines automatic data analysis with interactive visualizations. By this, the system provides a recommendation for suitable data descriptor selections. It supports the comparison of data descriptors with differing dimensionality for unlabeled data. We propose specialized scores and interactive views for descriptor comparison. The visualization techniques are scatterplot-based and grid-based. For the latter case, we apply Self-Organizing Maps as adaptive grids which are well suited for large multi-dimensional data sets. As an example, we demonstrate the usability of our system on a real-world biochemical application.
Asynchronous Evolution for Fully-Implicit and Semi-Implicit Time Integration
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Schroeder, Craig; Kwatra, Nipun; Zheng, Wen; Fedkiw, Ron; Bing-Yu Chen, Jan Kautz, Tong-Yee Lee, and Ming C. Lin
We propose a series of techniques for hybridizing implicit and semi-implicit time integration methods in a manner that retains much of the speed of the implicit method without sacrificing all of the higher quality vibrations one obtains with methods that handle elastic forces explicitly. We propose our scheme in the context of asynchronous methods, where different parts of the mesh are evolved at different time steps. Whereas traditional asynchronous methods evolve each element independently, we partition all of our elements into two groups: one group evolved at the frame rate using a fully implicit scheme, and another group which takes a number of substeps per frame using a scheme that is implicit on damping forces and explicit on the elastic forces. This allows for a straightforward coupling between the implicit and semi-implicit methods at frame boundaries for added stability. As has been stressed by various authors, asynchronous schemes take some of the pressure off of mesh generation, allowing time evolution to remain efficient even in the face of sliver elements. Finally, we propose a force distributing projection method which allows one to redistribute the forces felt on boundaries between implicit and semi-implicit regions of the mesh in a manner that yields improved visual quality.
Authoring Hierarchical Road Networks
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Galin, Eric; Peytavie, Adrien; Guérin, Eric; Benes, Bedrich; Bing-Yu Chen, Jan Kautz, Tong-Yee Lee, and Ming C. Lin
We present a procedural method for generating hierarchical road networks connecting cities, towns and villages over large terrains. Our approach relies on an original geometric graph generation algorithm based on a non- Euclidean metric combined with a path merging algorithm that creates junctions between the different types of roads. Unlike previous work, our method allows high level user control by manipulating the density and the pattern of the network. The geometry of the highways, primary and secondary roads as well as the interchanges and intersections are automatically created from the graph structure by instantiating generic parameterized models.
Automatic Registration of Multi-Projector Domes Using a Single Uncalibrated Camera
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Sajadi, Behzad; Majumder, Aditi; H. Hauser, H. Pfister, and J. J. van Wijk
In this paper we present a novel technique for easily calibrating multiple casually aligned projectors on spherical domes using a single uncalibrated camera. Using the prior knowledge of the display surface being a dome, we can estimate the camera intrinsic and extrinsic parameters and the projector to display surface correspondences automatically using a set of images. These images include the image of the dome itself and a projected pattern from each projector. Using these correspondences we can register images from the multiple projectors on the dome. Further, we can register displays which are not entirely visible in a single camera view using multiple pan and tilted views of an uncalibrated camera making our method suitable for displays of different size and resolution. We can register images from any arbitrary viewpoint making it appropriate for a single head-tracked user in a 3D visualization system. Also, we can use several cartographic mapping techniques to register images in a manner that is appropriate for multi-user visualization. Domes are known to produce a tremendous sense of immersion and presence in visualization systems. Yet, till date, there exists no easy way to register multiple projectors on a dome to create a high-resolution realistic visualizations. To the best of our knowledge, this is the first method that can achieve accurate geometric registration of multiple projectors on a dome simply and automatically using a single uncalibrated camera.
Bertin was Right: An Empirical Evaluation of Indexing to Compare Multivariate Time-Series Data Using Line Plots
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Aigner, W.; Kainz, C.; Ma, R.; Miksch, S.; Eduard Groeller and Holly Rushmeier
Line plots are very well suited for visually representing time-series. However, several difficulties arise when multivariate heterogeneous time-series data is displayed and compared visually. Especially, if the developments and trends of time-series of different units or value ranges need to be compared, a straightforward overlay could be visually misleading. To mitigate this, visualization pioneer Jacques Bertin presented a method called indexing that transforms data into comparable units for visual representation. In this paper, we want to provide empirical evidence for this method and present a comparative study of the three visual comparison methods linear scale with juxtaposition, log scale with superimposition and indexing. Although for task completion times, indexing only shows slight advantages, the results support the assumption that the indexing method enables the user to perform comparison tasks with a significantly lower error rate. Furthermore, a post-test questionnaire showed that the majority of the participants favour the indexing method over the two other comparison methods.
Bipartite Polar Classification for Surface Reconstruction
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Chen, Yi-Ling; Lee, Tung-Ying; Chen, Bing-Yu; Lai, Shang-Hong; Bing-Yu Chen, Jan Kautz, Tong-Yee Lee, and Ming C. Lin
In this paper, we propose bipartite polar classification to augment an input unorganized point set P with two disjoint groups of points distributed around the ambient space of P to assist the task of surface reconstruction. The goal of bipartite polar classification is to obtain a space partitioning of P by assigning pairs of Voronoi poles into two mutually invisible sets lying in the opposite sides of P through direct point set visibility examination. Based on the observation that a pair of Voronoi poles are mutually invisible, spatial classification is accomplished by carving away visible exterior poles with their counterparts simultaneously determined as interior ones. By examining the conflicts of mutual invisibility, holes or boundaries can also be effectively detected, resulting in a hole-aware space carving technique. With the classified poles, the task of surface reconstruction can be facilitated by more robust surface normal estimation with global consistent orientation and off-surface point specification for variational implicit surface reconstruction. We demonstrate the ability of the bipartite polar classification to achieve robust and efficient space carving on unorganized point clouds with holes and complex topology and show its application to surface reconstruction.
Blur-Aware Image Downsampling
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Trentacoste, Matthew; Mantiuk, Rafal; Heidrich, Wolfgang; M. Chen and O. Deussen
Resizing to a lower resolution can alter the appearance of an image. In particular, downsampling an image causes blurred regions to appear sharper. It is useful at times to create a downsampled version of the image that gives the same impression as the original, such as for digital camera viewfinders. To understand the effect of blur on image appearance at different image sizes, we conduct a perceptual study examining how much blur must be present in a downsampled image to be perceived the same as the original. We find a complex, but mostly image-independent relationship between matching blur levels in images at different resolutions. The relationship can be explained by a model of the blur magnitude analyzed as a function of spatial frequency. We incorporate this model in a new appearance-preserving downsampling algorithm, which alters blur magnitude locally to create a smaller image that gives the best reproduction of the original image appearance.
BSSRDF Estimation from Single Images
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Munoz, Adolfo; Echevarria, Jose I.; Seron, Francisco J.; Lopez-Moreno, Jorge; Glencross, Mashhuda; Gutierrez, Diego; M. Chen and O. Deussen
We present a novel method to estimate an approximation of the reflectance characteristics of optically thick, homogeneous translucent materials using only a single photograph as input. First, we approximate the diffusion profile as a linear combination of piecewise constant functions, an approach that enables a linear system minimization and maximizes robustness in the presence of suboptimal input data inferred from the image. We then fit to a smoother monotonically decreasing model, ensuring continuity on its first derivative. We show the feasibility of our approach and validate it in controlled environments, comparing well against physical measurements from previous works. Next, we explore the performance of our method in uncontrolled scenarios, where neither lighting nor geometry are known. We show that these can be roughly approximated from the corresponding image by making two simple assumptions: that the object is lit by a distant light source and that it is globally convex, allowing us to capture the visual appearance of the photographed material. Compared with previous works, our technique offers an attractive balance between visual accuracy and ease of use, allowing its use in a wide range of scenarios including off-the-shelf, single images, thus extending the current repertoire of real-world data acquisition techniques.
CheckViz: Sanity Check and Topological Clues for Linear and Non-Linear Mappings
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Lespinats, Sylvain; Aupetit, Michaël; Eduard Groeller and Holly Rushmeier
Multidimensional scaling is a must-have tool for visual data miners, projecting multidimensional data onto a two-dimensional plane. However, what we see is not necessarily what we think about. In many cases, end-users do not take care of scaling the projection space with respect to the multidimensional space. Anyway, when using non-linear mappings, scaling is not even possible. Yet, without scaling geometrical structures which might appear do not make more sense than considering a random map. Without scaling, we shall not make inference from the display back to the multidimensional space. No clusters, no trends, no outliers, there is nothing to infer without first quantifying the mapping quality. Several methods to qualify mappings have been devised. Here, we propose CheckViz, a new method belonging to the framework of Verity Visualization. We define a two-dimensional perceptually uniform colour coding which allows visualizing tears and false neighbourhoods, the two elementary and complementary types of geometrical mapping distortions, straight onto the map at the location where they occur. As examples shall demonstrate, this visualization method is essential to help users make sense out of the mappings and to prevent them from over interpretations. It could be applied to check other mappings as well.
Coarse-to-Fine Combinatorial Matching for Dense Isometric Shape Correspondence
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Sahillioglu, Yusuf; Yemez, Yucel; Mario Botsch and Scott Schaefer
We present a dense correspondence method for isometric shapes, which is accurate yet computationally efficient. We minimize the isometric distortion directly in the 3D Euclidean space, i.e., in the domain where isometry is originally defined, by using a coarse-to-fine sampling and combinatorial matching algorithm. Our method does not require any initialization and aims to find an accurate solution in the minimum-distortion sense for perfectly isometric shapes. We demonstrate the performance of our method on various isometric (or nearly isometric) pairs of shapes.
Coherent Out-of-Core Point-Based Global Illumination
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Kontkanen, Janne; Tabellion, Eric; Overbeck, Ryan S.; Ravi Ramamoorthi and Erik Reinhard
We describe a new technique for coherent out-of-core point-based global illumination and ambient occlusion. Point-based global illumination (PBGI) is used in production to render tremendously complex scenes, so in-core storage of point and octree data structures quickly becomes a problem. However, a simple out-of-core extension of a classical top-down octree building algorithm would be extremely inefficient due to large amount of I/O required. Our method extends previous PBGI algorithms with an out-of-core technique that uses minimal I/O and stores data on disk compactly and in coherent chunks for later access during shading. Using properties of a space-filling Z-curve, we are able to preprocess the data in two passes: an external 1D-sort and an octree construction pass.
Combinatorial Bidirectional Path-Tracing for Efficient Hybrid CPU/GPU Rendering
(The Eurographics Association and Blackwell Publishing Ltd., 2011) Pajot, Anthony; Barthe, Loïc; Paulin, Mathias; Poulin, Pierre; M. Chen and O. Deussen
This paper presents a reformulation of bidirectional path-tracing that adequately divides the algorithm into processes efficiently executed in parallel on both the CPU and the GPU. We thus benefit from high-level optimization techniques such as double buffering, batch processing, and asyncronous execution, as well as from the exploitation of most of the CPU, GPU, and memory bus capabilities. Our approach, while avoiding pure GPU implementation limitations (such as limited complexity of shaders, light or camera models, and processed scene data sets), is more than ten times faster than standard bidirectional path-tracing implementations, leading to performance suitable for production-oriented rendering engines.

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