Volume 31 (2012)

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Dihedral Angle Mesh Error: a Fast Perception Correlated Distortion Measure for Fixed Connectivity Triangle Meshes
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Vása, Libor; Rus, Jan; Eitan Grinspun and Niloy Mitra
In computer graphics, triangle meshes are ubiquitous as a representation of surface models. Processing of this kind of data, such as compression or watermarking, often involves an unwanted distortion of the surface geometry. Advanced processing algorithms are continuously being proposed, aiming at improving performance (compression ratio, watermark robustness and capacity), while minimizing the introduced distortion. In most cases, the final resulting mesh is intended to be viewed by a human observer, and it is therefore necessary to minimise the amount of distortion perceived by the human visual system. However, only recently there have been studies published on subjective experiments in this field, showing that previously used objective error measures exhibit rather poor correlation with the results of subjective experiments. In this paper, we present results of our own large subjective testing aimed at human perception of triangle mesh distortion. We provide an independent confirmation of the previous result by Lavoué et al. that most current metrics perform poorly, with the exception of the MSDM/MSDM2 metrics. We propose a novel metric based on measuring the distortion of dihedral angles, which provides even higher correlation with the results of our experiments and experiments performed by other researchers. Our metric is about two orders of magnitude faster than MSDM/MSDM2, which makes it much more suitable for usage in iterative optimisation algorithms.
Feature-Preserving Displacement Mapping With Graphics Processing Unit (GPU) Tessellation
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Jang, Hanyoung; Han, JungHyun; Holly Rushmeier and Oliver Deussen
Displacement mapping reconstructs a high‐frequency surface by adding geometric details encoded in the displacement map to the coarse base surface. In the context of hardware tessellation supported by GPUs, this paper aims at feature‐preserving surface reconstruction, and proposes the generation of a displacement map that displaces more vertices towards the higher‐frequency feature parts of the target mesh. In order to generate the feature‐preserving displacement map, surface features of the target mesh are estimated, and then the target mesh is parametrized and sampled using the features. At run time, the base surface is semi‐uniformly tessellated by hardware, and then the vertices of the tessellated mesh are displaced non‐uniformly along the 3‐D vectors stored in the displacement map. The experimental results show that the surfaces reconstructed by the proposed method are of a higher quality than those reconstructed by other methods.Displacement mapping reconstructs a high‐frequency surface by adding geometric details encoded in the displacement map to the coarse base surface. In the context of hardware tessellation supported by GPUs, this paper aims at feature‐preserving surface reconstruction, and proposes the generation of a displacement map that displaces more vertices towards the higher‐frequency feature parts of the target mesh. In order to generate the feature‐preserving displacement map, surface features of the target mesh are estimated, and then the target mesh is parametrized and sampled using the features. At run time, the base surface is semi‐uniformly tessellated by hardware, and then the vertices of the tessellated mesh are displaced non‐uniformly along the 3‐D vectors stored in the displacement map.
Fast Grasp Synthesis for Various Shaped Objects
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Kyota, Fumihito; Saito, Suguru; P. Cignoni and T. Ertl
Human-like grasp planning is difficult because a human hand has a high number of degrees of freedom, and there are many grasping styles depending on the shape of an object and purpose. We propose a fast grasp synthesis system which enables a user to choose the desired grasping styles from a set of grasp types in a human grasp taxonomy. Given a 3D model of an object, our system detects graspable positions and generates grasping hand postures in every applicable grasp types in the grasp taxonomy for each grasping position. Hand postures are generated separately for each digit, and hand alignment is then refined iteratively. A user can also specify the grasping position by moving the cursor onto object surface, as well as grasp type. The generated hand postures are shown as a table of thumbnail images, and the user can select the grasping hand posture by clicking on one of those postures. Our system enables interactive generation of various grasping hand postures in real time.
Smart Scribbles for Sketch Segmentation
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Noris, G.; Sýkora, D.; Shamir, A.; Coros, S.; Whited, B.; Simmons, M.; Hornung, A.; Gross, M.; Sumner, R.; Holly Rushmeier and Oliver Deussen
We present ‘Smart Scribbles’—a new scribble‐based interface for user‐guided segmentation of digital sketchy drawings. In contrast to previous approaches based on simple selection strategies, Smart Scribbles exploits richer geometric and temporal information, resulting in a more intuitive segmentation interface. We introduce a novel energy minimization formulation in which both geometric and temporal information from digital input devices is used to define stroke‐to‐stroke and scribble‐to‐stroke relationships. Although the minimization of this energy is, in general, an NP‐hard problem, we use a simple heuristic that leads to a good approximation and permits an interactive system able to produce accurate labellings even for cluttered sketchy drawings. We demonstrate the power of our technique in several practical scenarios such as sketch editing, as‐rigid‐as‐possible deformation and registration, and on‐the‐fly labelling based on pre‐classified guidelines.We present Smart Scribbles, a new scribble‐based interface for user‐guided segmentation of digital sketchy drawings. In contrast to previous approaches based on simple selection strategies, Smart Scribbles exploits richer geometric and temporal information, resulting in a more intuitive segmentation interface. We introduce a novel energy minimization formulation in which both geometric and temporal information from digital input devices is used to define stroke‐to‐stroke and scribble‐to‐stroke relationships. Although the minimization of this energy is, in general, a NP‐hard problem, we use a simple heuristic that leads to a good approximation and permits an interactive system able to produce accurate labelings even for cluttered sketchy drawings. We demonstrate the power of our technique in several practical scenarios such as sketch editing, as‐rigid‐as‐possible deformation and registration, and on‐the‐fly labeling based on pre‐classified guidelines.
Parallel Computation of 3D Morse-Smale Complexes
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Shivashankar, Nithin; Natarajan, Vijay; S. Bruckner, S. Miksch, and H. Pfister
The Morse-Smale complex is a topological structure that captures the behavior of the gradient of a scalar function on a manifold. This paper discusses scalable techniques to compute the Morse-Smale complex of scalar functions defined on large three-dimensional structured grids. Computing the Morse-Smale complex of three-dimensional domains is challenging as compared to two-dimensional domains because of the non-trivial structure introduced by the two types of saddle criticalities. We present a parallel shared-memory algorithm to compute the Morse- Smale complex based on Forman's discrete Morse theory. The algorithm achieves scalability via synergistic use of the CPU and the GPU. We first prove that the discrete gradient on the domain can be computed independently for each cell and hence can be implemented on the GPU. Second, we describe a two-step graph traversal algorithm to compute the 1-saddle-2-saddle connections efficiently and in parallel on the CPU. Simultaneously, the extremasaddle connections are computed using a tree traversal algorithm on the GPU.
Temporal Coherence Methods in Real‐Time Rendering
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Scherzer, Daniel; Yang, Lei; Mattausch, Oliver; Nehab, Diego; Sander, Pedro V.; Wimmer, Michael; Eisemann, Elmar; Holly Rushmeier and Oliver Deussen
Nowadays, there is a strong trend towards rendering to higher‐resolution displays and at high frame rates. This development aims at delivering more detail and better accuracy, but it also comes at a significant cost. Although graphics cards continue to evolve with an ever‐increasing amount of computational power, the speed gain is easily counteracted by increasingly complex and sophisticated shading computations. For real‐time applications, the direct consequence is that image resolution and temporal resolution are often the first candidates to bow to the performance constraints (e.g. although full HD is possible, PS3 and XBox often render at lower resolutions).In order to achieve high‐quality rendering at a lower cost, one can exploit temporal coherence (TC). The underlying observation is that a higher resolution and frame rate do not necessarily imply a much higher workload, but a larger amount of redundancy and a higher potential for amortizing rendering over several frames. In this survey, we investigate methods that make use of this principle and provide practical and theoretical advice on how to exploit TC for performance optimization. These methods not only allow incorporating more computationally intensive shading effects into many existing applications, but also offer exciting opportunities for extending high‐end graphics applications to lower‐spec consumer‐level hardware. To this end, we first introduce the notion and main concepts of TC, including an overview of historical methods. We then describe a general approach, image‐space reprojection, with several implementation algorithms that facilitate reusing shading information across adjacent frames. We also discuss data‐reuse quality and performance related to reprojection techniques. Finally, in the second half of this survey, we demonstrate various applications that exploit TC in real‐time rendering.In order to achieve high‐quality rendering at a lower cost, one can exploit temporal coherence (TC). The underlying observation is that a higher resolution and frame rate do not necessarily imply a much higher workload, but a larger amount of redundancy and a higher potential for amortizing rendering over several frames. In this survey, we investigate methods that make use of this principle and provide practical and theoretical advice on how to exploit TC for performance optimization. These methods not only allow incorporating more computationally intensive shading effects intomany existing applications, but also offer exciting opportunities for extending high‐end graphics applications to lower‐spec consumer‐level hardware.
Comparison of Four Subjective Methods for Image Quality Assessment
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Mantiuk, Rafał K.; Tomaszewska, Anna; Mantiuk, Radosław; Holly Rushmeier and Oliver Deussen
To provide a convincing proof that a new method is better than the state of the art, computer graphics projects are often accompanied by user studies, in which a group of observers rank or rate results of several algorithms. Such user studies, known as subjective image quality assessment experiments, can be very time‐consuming and do not guarantee to produce conclusive results. This paper is intended to help design efficient and rigorous quality assessment experiments and emphasise the key aspects of the results analysis. To promote good standards of data analysis, we review the major methods for data analysis, such as establishing confidence intervals, statistical testing and retrospective power analysis. Two methods of visualising ranking results together with the meaningful information about the statistical and practical significance are explored. Finally, we compare four most prominent subjective quality assessment methods: single‐stimulus, double‐stimulus, forced‐choice pairwise comparison and similarity judgements. We conclude that the forced‐choice pairwise comparison method results in the smallest measurement variance and thus produces the most accurate results. This method is also the most time‐efficient, assuming a moderate number of compared conditions.To provide a convincing proof that a new method is better than the state‐of‐the‐art, computer graphics projects are often accompanied by user studies, in which a group of observers rank or rate results of several algorithms. Such user studies, known as subjective image quality assessment experiments, can be very time consuming and do not guarantee to produce conclusive results. This paper is intended to help design efficient and rigorous quality assessment experiments and emphasise the key aspects of the results analysis.
SimpleFlow: A Non-iterative, Sublinear Optical Flow Algorithm
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Tao, Michael; Bai, Jiamin; Kohli, Pushmeet; Paris, Sylvain; P. Cignoni and T. Ertl
Optical flow is a critical component of video editing applications, e.g. for tasks such as object tracking, segmentation, and selection. In this paper, we propose an optical flow algorithm called SimpleFlow whose running times increase sublinearly in the number of pixels. Central to our approach is a probabilistic representation of the motion flow that is computed using only local evidence and without resorting to global optimization. To estimate the flow in image regions where the motion is smooth, we use a sparse set of samples only, thereby avoiding the expensive computation inherent in traditional dense algorithms. We show that our results can be used as is for a variety of video editing tasks. For applications where accuracy is paramount, we use our result to bootstrap a global optimization. This significantly reduces the running times of such methods without sacrificing accuracy. We also demonstrate that the SimpleFlow algorithm can process HD and 4K footage in reasonable times.
Nearly Recurrent Components in 3D Piecewise Constant Vector Fields
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Szymczak, Andrzej; Brunhart-Lupo, Nicholas; S. Bruckner, S. Miksch, and H. Pfister
We present an algorithm for computing nearly recurrent components, that represent areas of close to circulating or stagnant flow, for 3D piecewise constant (PC) vector fields defined on regular grids. Using a number of analytical and simulated data sets, we demonstrate that nearly recurrent components can provide interesting insight into the topological structure of 3D vector fields. Our approach is based on prior work on Morse decompositions for PC vector fields on surfaces and extends concepts previously developed with this goal in mind to the case of 3D vector fields defined on regular grids. Our contributions include a description of trajectories of 3D piecewise constant vector fields and an extension of the transition graph, a finite directed graph that represents all trajectories, to the 3D case. Nearly recurrent components are defined by strongly connected components of the transition graph.
Iterative Image Warping
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Bowles, Huw; Mitchell, Kenny; Sumner, Robert W.; Moore, Jeremy; Gross, Markus; P. Cignoni and T. Ertl
Animated image sequences often exhibit a large amount of inter-frame coherence which standard rendering algorithms and pipelines are ill-equipped to exploit, limiting their efficiency. To address this inefficiency we transfer rendering results across frames using a novel image warping algorithm based on fixed point iteration. We analyze the behavior of the iteration and describe two alternative algorithms designed to suit different performance requirements. Further, to demonstrate the versatility of our approach we apply it to a number of spatio-temporal rendering problems including 30-to-60Hz frame upsampling, stereoscopic 3D conversion, defocus and motion blur. Finally we compare our approach against existing image warping methods and demonstrate a significant performance improvement.
Repetition Maximization based Texture Rectification
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Aiger, Dror; Cohen-Or, Daniel; Mitra, Niloy J.; P. Cignoni and T. Ertl
Many photographs are taken in perspective. Techniques for rectifying resulting perspective distortions typically rely on the existence of parallel lines in the scene. In scenarios where such parallel lines are hard to automatically extract or manually annotate, the unwarping process remains a challenge. In this paper, we introduce an automatic algorithm to rectifying images containing textures of repeated elements lying on an unknown plane. We unwrap the input by maximizing for image self-similarity over the space of homography transformations. We map a set of detected regional descriptors to surfaces in a transformation space, compute the intersection points among triplets of such surfaces, and then use consensus among the projected intersection points to extract the correcting transform. Our algorithm is global, robust, and does not require explicit or accurate detection of similar elements. We evaluate our method on a variety of challenging textures and images. The rectified outputs are directly useful for various tasks including texture synthesis, image completion, etc.
Interface Exchange as an Indicator for Eddy Heat Transport
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Williams, Sean; Petersen, Mark; Hecht, Matthew; Maltrud, Mathew; Patchett, John; Ahrens, James; Hamann, Bernd; S. Bruckner, S. Miksch, and H. Pfister
The ocean contains many large-scale, long-lived vortices, called mesoscale eddies, that are believed to have a role in the transport and redistribution of salt, heat, and nutrients throughout the ocean. Determining this role, however, has proven to be a challenge, since the mechanics of eddies are only partly understood; a standard definition for these ocean eddies does not exist and, therefore, scientifically meaningful, robust methods for eddy extraction, characterization, tracking and visualization remain a challenge. To shed light on the nature and potential roles of eddies, we extend our previous work on eddy identification and tracking to construct a new metric to characterize the transfer of water into and out of eddies across their boundary, and produce several visualizations of this new metric to provide clues about the role eddies play in the global ocean.
Registration Based Non-uniform Motion Deblurring
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Cho, Sunghyun; Cho, Hojin; Tai, Yu-Wing; Lee, Seungyong; C. Bregler, P. Sander, and M. Wimmer
This paper proposes an algorithm which uses image registration to estimate a non-uniform motion blur point spread function (PSF) caused by camera shake. Our study is based on a motion blur model which models blur effects of camera shakes using a set of planar perspective projections (i.e., homographies). This representation can fully describe motions of camera shakes in 3D which cause non-uniform motion blurs. We transform the non-uniform PSF estimation problem into a set of image registration problems which estimate homographies of the motion blur model one-by-one through the Lucas-Kanade algorithm. We demonstrate the performance of our algorithm using both synthetic and real world examples. We also discuss the effectiveness and limitations of our algorithm for non-uniform deblurring.
Wake Synthesis For Shallow Water Equation
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Pan, Zherong; Huang, Jin; Tong, Yiying; Bao, Hujun; C. Bregler, P. Sander, and M. Wimmer
In fluid animation, wake is one of the most important phenomena usually seen when an object is moving relative to the flow. However, in current shallow water simulation for interactive applications, this effect is greatly smeared out. In this paper, we present a method to efficiently synthesize these wakes. We adopt a generalized SPH method for shallow water simulation and two way solid fluid coupling. In addition, a 2D discrete vortex method is used to capture the detailed wake motions behind an obstacle, enriching the motion of SWE simulation. Our method is highly efficient since only 2D simulation is required. Moreover, by using a physically inspired procedural approach for particle seeding, DVM particles are only created in the wake region. Therefore, very few particles are required while still generating realistic wake patterns. When coupled with SWE, we show that these patterns can be seen using our method with marginal overhead.
Interactive Self-Organizing Windows
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Steinberger, Markus; Waldner, Manuela; Schmalstieg, Dieter; P. Cignoni and T. Ertl
In this paper, we present the design and implementation of a dynamic window management technique that changes the perception of windows as fixed-sized rectangles. The primary goal of self-organizing windows is to automatically display the most relevant information for a user's current activity, which removes the burden of organizing and arranging windows from the user. We analyze the image-based representation of each window and identify coherent pieces of information. The windows are then automatically moved, scaled and composed in a contentaware manner to fit the most relevant information into the limited area of the screen. During the design process, we consider findings from previous experiments and show how users can benefit from our system. We also describe how the immense processing power of current graphics processing units can be exploited to build an interactive system that finds an optimal solution within the complex design space of all possible window transformations in real time.
Acoustic Rendering and Auditory–Visual Cross-Modal Perception and Interaction
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Hulusic, Vedad; Harvey, Carlo; Debattista, Kurt; Tsingos, Nicolas; Walker, Steve; Howard, David; Chalmers, Alan; Holly Rushmeier and Oliver Deussen
In recent years research in the three-dimensional sound generation field has been primarily focussed upon new applications of spatialized sound. In the computer graphics community the use of such techniques is most commonly found being applied to virtual, immersive environments. However, the field is more varied and diverse than this and other research tackles the problem in a more complete, and computationally expensive manner. Furthermore, the simulation of light and sound wave propagation is still unachievable at a physically accurate spatio-temporal quality in real time. Although the Human Visual System (HVS) and the Human Auditory System (HAS) are exceptionally sophisticated, they also contain certain perceptional and attentional limitations. Researchers, in fields such as psychology, have been investigating these limitations for several years and have come up with findings which may be exploited in other fields. This paper provides a comprehensive overview of the major techniques for generating spatialized sound and, in addition, discusses perceptual and cross-modal influences to consider. We also describe current limitations and provide an in-depth look at the emerging topics in the field.
Realistic Following Behaviors for Crowd Simulation
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Lemercier, Samuel; Jelic, Asja; Kulpa, Richard; Hua, Jiale; Fehrenbach, Jérôme; Degond, Pierre; Appert-Rolland, Cécile; Donikian, Stéphane; Pettré, Julien; P. Cignoni and T. Ertl
While walking through a crowd, a pedestrian experiences a large number of interactions with his neighbors. The nature of these interactions is varied, and it has been observed that macroscopic phenomena emerge from the combination of these local interactions. Crowd models have hitherto considered collision avoidance as the unique type of interactions between individuals, few have considered walking in groups. By contrast, our paper focuses on interactions due to the following behaviors of pedestrians. Following is frequently observed when people walk in corridors or when they queue. Typical macroscopic stop-and-go waves emerge under such traffic conditions. Our contributions are, first, an experimental study on following behaviors, second, a numerical model for simulating such interactions, and third, its calibration, evaluation and applications. Through an experimental approach, we elaborate and calibrate a model from microscopic analysis of real kinematics data collected during experiments. We carefully evaluate our model both at the microscopic and the macroscopic levels. We also demonstrate our approach on applications where following interactions are prominent.
Stochastic Progressive Photon Mapping for Dynamic Scenes
(The Eurographics Association and John Wiley and Sons Ltd., 2012) Weiss, Maayan; Grosch, Thorsten; P. Cignoni and T. Ertl
Stochastic Progressive Photon Mapping (SPPM) is a method to simulate consistent global illumination. It is especially useful for complicated light paths like caustics seen through a glass surface. Up to now, SPPM can only be applied to a static scene and noise-free images require hours to compute. Our approach is to extend this method to dynamic scenes (DSPPM) for an efficient simulation of animated objects and materials. We identify both hit point and photon information that can be re-used for the pixel statistics of multiple frames. In comparison to an SPPM simulation performed for each frame, we achieve a 1.96 - 9.53 speedup in our test scenes without changing correctness or simulation quality.
Analytic Curve Skeletons for 3D Surface Modeling and Processing
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Thiery, Jean-Marc; Buchholz, Bert; Tierny, Julien; Boubekeur, Tamy; C. Bregler, P. Sander, and M. Wimmer
We present a new curve skeleton model designed for surface modeling and processing. This skeleton is defined as the geometrical integration of a piecewise harmonic parameterization defined over a disk-cylinder surface decomposition. This decomposition is computed using a progressive Region Graph reduction based on both geometric and topological criteria which can be iteratively optimized to improve region boundaries. The skeleton has an analytical form with regularity inherited from the surface one. Such a form offers well-defined surface-skeleton and skeleton-surface projections. The resulting skeleton satisfies quality criteria which are relevant for skeleton-based modeling and processing. We propose applications that benefit from our skeleton model, including local thickness editing, inset surface creation for shell mapping, as well as a new mid-scale feature preserving smoothing.
ConnectedCharts: Explicit Visualization of Relationships between Data Graphics
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Viau, Christophe; McGuffin, Michael; S. Bruckner, S. Miksch, and H. Pfister
Multidimensional multivariate data can be visualized using many different well-known charts, such as bar charts, stacked bar charts, grouped bar charts, scatterplots, or pivot tables, or also using more advanced highdimensional techniques such as scatterplot matrices (SPLOMs) or parallel coordinate plots (PCPs). These many techniques have different advantages, and users may wish to use several charts or data graphics to understand a dataset from different perspectives. We present ConnectedCharts, a technique for displaying relationships between multiple charts. ConnectedCharts allow for hybrid combinations of bar charts, scatterplots, and parallel coordinates, with curves drawn to show the conceptual links between charts. The charts can be thought of as coordinated views, where linking is achieved not only through interactive brushing, but also with explicitly drawn curves that connect corresponding data tuples or axes. We present a formal description of a design space of many simple charts, and also identify different kinds of connections that can be displayed between related charts. Our prototype implementation demonstrates how the connections between multiple charts can make relationships clearer and can serve to document the history of a user's analytical process, leading to potential applications in visual analytics and dashboard design.

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