http://www.computer.org/tvcg The IEEE Transactions on Visualization and Computer Graphics (TVCG) is published bimonthly. Its Editorial Board strives to publish papers that present important research results and state-of-the-art seminal papers within TVCG's scope. These include subjects related to visualization and computer graphics techniques, systems, software, hardware, and user interface issues. Specific topics include, but are not limited to: a) visualization techniques and methodologies; b) visualization systems and software; c) volume visualization; d) flow visualization; e) information visualization; f) multivariate visualization; g) modeling and surfaces; h) rendering techniques and methodologies; i) graphics systems and software; j) animation and simulation; k) user interfaces; l) virtual reality; m) visual programming and program visualization; and n) applications. en-us Wed, 4 Jan 2012 11:00:01 GMT http://csdl.computer.org/common/images/logos/tvcg.gif List of recently published journal articles http://www.computer.org/tvcg http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.288 Maps offer a familiar way to present geographic data (continents, countries) and additional information (topography, geology), can be displayed with the help of contours and heat-map overlays. In this paper we consider visualizing large-scale dynamic relational data by taking advantage of the geographic map metaphor. We describe a map-based visualization system which uses animation to convey dynamics in large datasets, and which aims to preserve the viewer's mental map while also offering readable views at all times. Our system is fully functional and has been used to visualize user traffic on the Internet radio station last.fm, as well as TV-viewing patterns from an IPTV service. All map images in this paper are available on our web site in high-resolution. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.288 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.285 In this paper we describe a novel approach for applying texture mapping to volumetric data sets. In contrast to previous approaches, the presented technique enables a unified integration of 2D and 3D textures and thus allows to emphasize material boundaries as well as volumetric regions within a volumetric data set at the same time. One key contribution of this paper is a parametrization technique for volumetric data sets, which takes into account material boundaries and volumetric regions. Using this technique, the resulting parametrizations of volumetric data sets support texturing effects which create a higher degree of realism in volume rendered images. We evaluate the quality of the parametrization and demonstrate the usefulness of the proposed concepts by combining volumetric texturing with volumetric lighting models to generate photorealistic volume renderings. Furthermore, we show the applicability in the area of illustrative visualization. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.285 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.282 We present an algorithm to render objects of transparent materials with rough surfaces in real-time, under all-frequency distant illumination. Rough surfaces cause wide scattering as light enters and exits objects, which significantly complicates the rendering of such materials. We present two contributions to approximate the successive scattering events at interfaces, due to rough refraction: First, an approximation of the Bidirectional Transmittance Distribution Function (BTDF), using spherical Gaussians, suitable for real-time estimation of environment lighting using pre-convolution; second, a combination of cone tracing and macro-geometry filtering to efficiently integrate the scattered rays at the exiting interface of the object. We demonstrate the quality of our approximation by comparison against stochastic ray-tracing. Furthermore we propose two extensions to our method for supporting spatially varying roughness on object surfaces and local lighting for thin objects. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.282 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.279 We take a new, scenario based look at evaluation in information visualization. Our seven scenarios, evaluating visual data analysis and reasoning, evaluating user performance, evaluating user experience, evaluating environments and work practices, evaluating communication through visualization, evaluating visualization algorithms, and evaluating collaborative data analysis were derived through an extensive literature review of over 800 visualization publications. These scenarios distinguish different study goals and types of research questions and are illustrated through example studies. Through this broad survey and the distillation of these scenarios we make two contributions. One, we encapsulate the current practices in the information visualization research community and, two, we provide a different approach to reaching decisions about what might be the most effective evaluation of a given information visualization. Scenarios can be used to choose appropriate research questions and goals and the provided examples can be consulted for guidance on how to design one's own study. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.279 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.275 Redirected walking techniques allow people to walk in a larger virtual space than the physical extents of the laboratory. We describe two experiments conducted to investigate human sensitivity to walking on a curved path and to validate a new redirected walking technique. In a psychophysical experiment, we found that sensitivity to walking on a curved path was significantly lower for slower walking speeds (radius of 10 meters versus 22 meters). In an applied study, we investigated the influence of a velocity-dependent dynamic gain controller and an avatar controller on the average distance that participants were able to freely walk before needing to be reoriented. The mean walked distance was significantly greater in the dynamic gain controller condition, as compared to the static controller (22 meters versus 15 meters). Our results demonstrate that perceptually motivated dynamic redirected walking techniques, in combination with reorientation techniques, allow for unaided exploration of a large virtual city model. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.275 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.267 This paper presents a complete and robust solution for dense registration of partial nonrigid shapes. Its novel contributions are founded upon the newly-proposed heat kernel coordinates (HKCs) that can accurately position points on the shape, and the priority-vicinity search that ensures geometric compatibility during the registration. HKCs index points by computing heat kernels from multiple sources, and their magnitudes serve as priorities of queuing points in registration. We start with shape features as the sources of heat kernels via feature detection and matching. Following the priority order of HKCs, the dense registration is progressively propagated from feature sources to all points. Our method has a superior indexing ability that can produce dense correspondences with fewer flips. The diffusion nature of HKCs, which can be interpreted as a random walk on a manifold, makes our method robust to noise and small holes avoiding surface surgery and repair. Our method searches correspondence only in a small vicinity of registered points, which significantly improves the time performance. Through comprehensive experiments, our new method has demonstrated its technical soundness and robustness by generating highly compatible dense correspondences. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.267 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.265 Robust analysis of vector fields has been established as an important tool for deriving insights from the complex systems these fields model. Traditional analysis and visualization techniques rely primarily on computing streamlines through numerical integration. The inherent numerical errors of such approaches are usually ignored, leading to inconsistencies that cause unreliable visualizations and can ultimately prevent in-depth analysis. We propose a new representation for vector fields on surfaces that replaces numerical integration through triangles with maps from the triangle boundaries to themselves. This representation, called edge maps, permits a concise description of flow behaviors and is equivalent to computing all possible streamlines at a user defined error threshold. Independent of this error all streamlines computed using edge maps are guaranteed to be consistent up to floating point precision, enabling the stable extraction of features such as the topological skeleton. Furthermore, our representation explicitly stores the spatial and temporal errors which we use to produce more informative visualizations. This work describes the construction of edge maps, the error quantification, and a refinement procedure to adhere to a user defined error bound. Finally, we introduce new visualizations using the additional information provided by the edge maps to indicate the uncertainty involved in computing streamlines and topological structures. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.265 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.131 This paper presents a simple and efficient automatic mesh segmentation algorithm that solely exploits the shape concavity information. The method locates concave creases and seams using a set of concavity-sensitive scalar fields. These fields are computed by solving a Laplacian system with a novel concavity-sensitive weighting scheme. Isolines sampled from the concavity-aware fields naturally gather at concave seams, serving as good cutting boundary candidates. In addition, the fields provide sufficient information allowing efficient evaluation of the candidate cuts. We perform a summarization of all field gradient magnitudes to define a score for each isoline and employ a score-based greedy algorithm to select the best cuts. Extensive experiments and quantitative analysis have shown that the quality of our segmentations are better than or comparable with existing state-of-the-art more complex approaches. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.131 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.129 Multi-touch displays represent a promising technology for the display and manipulation of data. While the manipulation of 2D data has been widely explored, 3D manipulation with multi-touch displays remains largely uncovered. Based on an analysis of the integration and separation of degrees of freedom, we propose a taxonomy for 3D manipulation techniques with multi-touch displays. Using that taxonomy, we introduce DS3 (Depth-Separated Screen Space), a new 3D manipulation technique based on the separation of translation and rotation. In a controlled experiment, we compared DS3 with Sticky-Tools and Screen-Space. Results show that separating the control of translation and rotation significantly affects performance for 3D manipulation, with DS3 being at least 34% faster compared to the two other techniques. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.129 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.118 We introduce {\em hexagonal global parameterization}, a new type of surface parameterization in which parameter lines respect six-fold rotational symmetries (6-RoSy). Such parameterizations enable the tiling of surfaces with nearly regular hexagonal or triangular patterns, and can be used for triangular remeshing. Our framework to construct a hexagonal parameterization, referred to as HexCover, extends the QuadCover algorithm and formulates necessary conditions for hexagonal parameterization. We also provide an algorithm to automatically generate a $6$-RoSy field that respects directional and singularity features in the surface. We demonstrate the usefulness of our geometry-aware global parameterization with applications such as surface tiling with nearly regular textures and geometry patterns, as well as triangular and hexagonal remeshing. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.118 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.115 Given a set of corresponding user-specified anchor points on a pair of models having similar features and topologies, the cross-parameterization technique can establish a bijective mapping constrained by the anchor points. In this paper, we present an efficient algorithm to optimize the complexes and the shape of common base domains in cross-parameterization for reducing the distortion of the bijective mapping. The optimization is also constrained by the anchor points. Different from prior methods, we investigate a new signature, Length-Preserved Base Domain (LPBD), for measuring the level of stretch between surface patches in cross-parameterization. This new signature well balances the accuracy of measurement and the computational speed. Based on LPBD, a set of metrics are studied and compared. The best ones are employed in our domain optimization algorithm that consists of two major operators, boundary swapping and patch merging. Experimental results show that our optimization algorithm can reduce the distortion in cross-parameterization efficiently. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.115 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.106 MagnetViz was designed for the interactive manipulation of force-directed graph layouts, allowing the user to obtain visualizations based on the graph topology and/or the attributes of its nodes and edges. The user can introduce virtual magnets anywhere in the graph and these can be set to attract nodes and edges that fulfill user-defined criteria. When a magnet is placed, the force-directed nature of the layout forces it to reorganize itself in order to reflect the changes in the balance of forces, consequently changing the visualization into one that is more semantically relevant to the user. This paper describes MagnetViz's concepts, illustrating them with examples and a case study based on a usage scenario. We also describe how the MagnetViz has evolved since its original version and present the evaluation of its latest version. This evaluation consists of two user studies aiming at assessing generated layout quality and how well the concepts can be apprehended and employed, and a task taxonomy assessment focusing on establishing which graph visualization tasks the technique is able to handle. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.106 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.103 In this paper we present a novel isosurface visualization technique of guaranteed accuracy for directly visualizing isosurfaces of complex attribute data defined on (un-)structured (curvi-)linear hexahedral grids. Isosurfaces of high-order hexahedral based finite element solutions on both uniform grids (including MRI and CT scans) and more complex geometry representing a domain of interest can be rendered using our algorithm. An added property is that our technique can be used to directly visualize solutions and attributes in isogeometric analysis, an area based on using trivariate high order NURBS (Non-Uniform Rational B-splines) geometry and attribute representations for the analysis. Furthermore, our technique can be used to visualize isosurfaces of algebraic functions. Our approach combines subdivision and numerical root finding to form a robust and efficient isosurface visualization algorithm that does not miss surface features while finding all intersections between a view frustum and desired isosurfaces. This allows the use of view independent transparency in the rendering process. We demonstrate our technique through a straightforward CPU implementation on both complex structured and complex unstructured geometry with high-order simulation solutions, isosurfaces of medical data sets, and isosurfaces of algebraic functions. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.103 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.101 In this paper we present a framework for the groupwise processing of a set of meshes in dense correspondence. Such sets arise when modelling 3D shape variation or tracking surface motion over time. We extend a number of mesh processing tools to operate in a groupwise manner. Specifically, we present a geodesic-based surface flattening and spectral clustering algorithm which estimates a single class-optimal flattening. We also show how to modify an iterative edge collapse algorithm to perform groupwise simplification whilst retaining the correspondence of the data. Finally, we show how to compute class-optimal texture coordinates for the simplified meshes. We present alternative algorithms for topologically symmetric data which yields a symmetric flattening and low resolution mesh topology. We present flattening, simplification and texture mapping results on three different datasets and show that our approach allows the construction of low resolution 3D morphable models. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.101 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.78 We propose a new technique for visual exploration of streamlines in 3D vector fields. We construct a map from the space of all streamlines to points in $\RRR^n$ based on the preservation of the Hausdorff metric in streamline space. The image of a vector field under this map is a set of 2-manifolds in $\RRR^n$ with characteristic geometry and topology. Then standard clustering methods applied to the point sets in $\RRR^n$ yield a segmentation of the original vector field. Our approach provides a global analysis of 3D vector fields which incorporates the topological segmentation but yields additional information. In addition to a pure segmentation, the established map provides a natural parametrization visualized by the manifolds. We test our approach on a number of synthetic and real-world data sets. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.78 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.290 Design of time-varying vector fields, i.e., vector fields that can change over time, has a wide variety of important applications in computer graphics. Existing vector field design techniques do not address time-varying vector fields. In this paper, we present a framework for the design of time-varying vector fields, both for planar domains as well as manifold surfaces. Our system supports the creation and modification of various time-varying vector fields with desired spatial and temporal characteristics through several design metaphors including streamlines, pathlines, singularity paths, and bifurcations. These design metaphors are integrated into an element-based design to generate the time-varying vector fields via a sequence of basis field summations or spatial constrained optimizations at the sampled times. The key frame design and field deformation are also introduced to support other user design scenarios. Accordingly, a spatial-temporal constrained optimization and the time-varying transformation are employed to generate the desired fields for these two design scenarios, respectively. We apply the time-varying vector fields generated using our design system to a number of important computer graphics applications that require controllable dynamic effects such as evolving surface appearance, dynamic scene design, steerable crowd movement, and painterly animation, many of which are difficult or impossible to achieve via prior simulation-based methods. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.290 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.289 Redirected Free Exploration with Distractors (RFED) is a large-scale real-walking locomotion interface developed to enable people to walk freely in virtual environments that are larger than the tracked space in their facility. This paper describes the RFED system in detail and reports on a user study that evaluated RFED by comparing it to walking-in-place and joystick interfaces. The RFED system is composed of two major components, redirection and distractors. This paper discusses design challenges, implementation details, and lessons learned during the development of two working RFED systems. The evaluation study examined the effect of the locomotion interface on users' cognitive performance on navigation and wayfinding measures. The results suggest that participants using RFED were significantly better at navigating and wayfinding through virtual mazes than participants using walking-in-place and joystick interfaces. Participants traveled shorter distances, made fewer wrong turns, pointed to hidden targets more accurately and more quickly, and were able to place and label targets on maps more accurately, and more accurately estimate the virtual environment size. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.289 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.287 Co-located collaboration can be extremely valuable during complex visual analytics tasks. We present an extended analysis of an exploratory study of Cambiera, a system designed to support collaborative visual analysis tasks on a digital tabletop display. Fifteen participant pairs employed Cambiera to solve a problem involving 240 digital documents. Our analysis, supported by observations, system logs, questionnaires, and interview data, explores how pairs approached the problem on and around the table. We contribute a unique, rich understanding of how users made use of the digital tabletop workspace, how they worked together around the table, and how partners communicated with each other. We identify eight types of collaboration styles that can be used to identify how closely people work together while problem solving. We show how the closeness of teams' collaboration and communication influenced how they performed on the task overall. We further discuss the role of the tabletop for visual analytics tasks and derive design implications for future co-located collaborative tabletop problem solving systems. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.287 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.286 This paper presents analytic solutions to the integral moving least squares (MLS) equations originally proposed by Shen et al. by choosing another specific weighting function that renders the numerator in the MLS equation unitless. In addition, we analyze the original method to show that their approximation surfaces (i.e. enveloping surfaces with nonzero ε values in the weighting function) often form zero isosurfaces near concavities behind the triangle-soup models. This paper also presents error terms for the integral MLS formulations against signed distance fields. Based on our analytic solutions, we show that our method provides both interpolation and approximation surfaces faster and more efficiently. Because our method computes solutions for integral MLS equations directly, it does not rely on numerical steps that might have numerical-accuracy issues. In particular, and unlike the original method that deals with incorrect approximation surfaces by iteratively adjusting parameters, this paper proposes faster and more efficient approximations to surfaces without needing iterative routines. We also present computational efficiency comparisons, in which our method is 15-fold faster in computing integrations, even with conservative assumptions. Finally, we show that the surface normal vectors on the implicit surfaces formed by our analytic solutions are identical to the angle-weighted pseudo-normal vectors. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.286 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.284 The Morse-Smale complex is a useful topological data structure for the analysis and visualization of scalar data. This paper describes an algorithm that processes all mesh elements of the domain in parallel to compute the Morse-Smale complex of large two-dimensional data sets at interactive speeds. We employ a reformulation of the Morse-Smale complex using Forman's Discrete Morse Theory and achieve scalability by computing the discrete gradient using local accesses only. We also introduce a novel approach to merge gradient paths that ensures accurate geometry of the computed complex. We demonstrate that our algorithm performs well on both multicore environments and on massively parallel architectures such as the GPU. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.284 http://opac.ieeecomputersociety.org/opac?year=2012&volume=18&issue=02&acronym=tvcg IEEE Transactions on Visualization and Computer Graphics http://www.computer.org/portal/site/tvcg/ http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.283 We present Interactive Slice WIM, a framework for navigating and interrogating volumetric datasets using an interface enabled by a virtual reality environment made of two display surfaces: an interactive multi-touch table, and a stereoscopic display wall. The framework addresses two current challenges in immersive visualization: (1) providing an appropriate overview+detail style of visualization while navigating through volume data, and (2) supporting interactive querying and data exploration, i.e., interrogating volume data. The approach extends the World-in-Miniature (WIM) metaphor, simultaneously displaying a large-scale detailed data visualization and an interactive miniature. Leveraging the table+wall hardware, horizontal slices are projected (like a shadow) down onto the table surface, providing a useful 2D data overview to complement the 3D views as well as a data context for interpreting 2D multi-touch gestures made on the table. In addition to enabling effective navigation through complex geometries, extensions to the core Slice WIM technique support interacting with a set of multiple slices that persist on the table even as the user navigates around a scene and annotating and measuring data via points, paths, and volumes specified using interactive slices. Applications of the interface to two volume datasets are presented, and design decisions, limitations, and user feedback are discussed. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.283 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.281 Height-fields have become an important element of realistic real-time image synthesis to represent surface details. In this paper, we focus on the frequent case of static height-field data, for which we can precompute acceleration structures. While many rendering algorithms exist that impose trade-offs between speed and accuracy, we show that even accurate rendering can be combined with high performance. A careful analysis of the surface defined by the height values, leads to an efficient and accurate precomputation method. As a result, each texel stores a safety shape inside which a ray cannot cross the surface twice. This property ensures that no intersections are missed during the efficient marching method. Our analysis is general and can even consider visibility constraints that are robustly integrated into the precomputation. Further, we propose a particular instance of safety shapes with little memory overhead, which results in a rendering algorithm that outperforms existing methods, both in terms of accuracy and performance. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.281 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.278 In Virtual Reality, immersive systems such as the CAVE provide an important tool for the collaborative exploration of large 3D data. Unlike head-mounted displays, these systems are often only partially immersive due to space, access or cost constraints. The resulting loss of visual information becomes a major obstacle for critical tasks that need to utilize the users' entire field of vision. We have developed a conformal visualization technique that establishes a mapping between the full 360º field of view and the display geometry of a given visualization system. The mapping is provably angle-preserving and has the desirable property of preserving shapes locally, which is important for identifying shape-based features in the visual data. We apply the conformal visualization to both forward and backward rendering pipelines in a variety of retargeting scenarios, including CAVEs and angled arrangements of flat panel displays. Our user study shows that on the visual polyp detection task in Immersive Virtual Colonoscopy, conformal visualization leads to improved sensitivity at comparable examination times against the traditional rendering approach. We also develop a novel user interface based on the interactive recreation of the conformal mapping and the real-time regeneration of the view direction correspondence. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.278 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.277 Wide-SIMD hardware is power and area efficient, but it is challenging to efficiently map ray tracing algorithms to such hardware especially when the rays are incoherent. The two most commonly used schemes are either packet tracing, or relying on a separate traversal stack for each SIMD lane. Both work great for coherent rays, but suffer when rays are incoherent: The former experiences a dramatic loss of SIMD utilization once rays diverge; the latter requires a large local storage, and generates multiple incoherent streams of memory accesses. In this paper, we introduce a single-ray tracing scheme for incoherent rays on a 16-wide SIMD hardware. It uses a BVH with a branching factor of four as the acceleration structure, exploits four-wide SIMD in each box and primitive intersection test, and uses 16-wide SIMD by always performing four such tests in parallel. We then extend this scheme to a hybrid tracing scheme that automatically adapts to varying ray coherence by starting out with a 16-wide packet scheme and switching to the new single-ray scheme as soon as rays diverge. We show that on the Intel Many Integrated Core architecture this hybrid scheme consistently, and over a wide range of scenes and ray distributions, outperforms both packet and single-ray tracing. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.277 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.276 A curvature-adaptive implicit surface reconstruction for noisy and irregularly spaced points in 3-D is introduced. The reconstructed surface traces the zero-crossings of a signed field obtained from the sum of first-derivative anisotropic Gaussians centered at the points. The standard deviations of the anisotropic Gaussians are adapted to surface curvatures estimated from local data. A key characteristic of the formulation is its ability to smooth more along edges than across them, thereby preserving shape details while smoothing noise. The behavior of the proposed method under various density and organization of points is investigated and surface reconstruction results are compared with those obtained by well-known methods in the literature. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.276 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.274 Motion perception in immersive virtual environments significantly differs from the real world. For example, previous work has shown that users tend to underestimate travel distances in virtual environments (VEs). As a solution to this problem, researchers proposed to scale the mapped virtual camera motion relative to the tracked real-world movement of a user until real and virtual motion are perceived as equal, i.e., real-world movements could be mapped with a larger gain to the VE in order to compensate for the underestimation. However, introducing discrepancies between real and virtual motion can become a problem, in particular, due to misalignments of both worlds and distorted space cognition. In this article we describe a different approach that introduces apparent self-motion illusions by manipulating optic flow fields during movements in VEs. These manipulations can affect self-motion perception in VEs, but omit a quantitative discrepancy between real and virtual motions. In particular, we consider to which regions of the virtual view these apparent self-motion illusions can be applied, i.e., the ground plane or peripheral vision. Therefore, we introduce four illusions and show in experiments that optic flow manipulation can significantly affect users' self-motion judgments. Furthermore, we show that with such manipulations of optic flow fields the underestimation of travel distances can be compensated. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.274 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.273 In a virtual world, a group of virtual characters can interact with each other, and these characters may leave a group to join another. The interaction among individuals and groups often produces interesting events in a sequence of animation. The goal of this paper is to discover social events involving mutual interactions or group activities in multi-character animations and automatically plan a smooth camera motion to view interesting events suggested by our system or relevant events specified by a user. Inspired by sociology studies, we borrow the knowledge in Proxemics, social force, and social network analysis to model the dynamic relation among social events and the relation among the participants within each event. By analyzing the variation of relation strength among participants and spatiotemporal correlation among events, we discover salient social events in a motion clip and generate an overview video of these events with smooth camera motion using a simulated annealing optimization method. We tested our approach on different motions performed by multiple characters. Our user study shows that our results are preferred in 66.19% of the comparisons with those by the camera control approach without event analysis and are comparable (51.79%) to professional results by an artist. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.273 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.272 In this paper, we characterize the range of features that can be extracted from an Morse-Smale complex and describe a unified query language to extract them. We provide a visual dictionary to guide users when defining features in terms of these queries. We demonstrate our topology-rich visualization pipeline in a tool that interactively queries the MS complex to extract features at multiple resolutions, assigns rendering attributes, and combines traditional volume visualization with the extracted features. The flexibility and power of this approach is illustrated with examples showing novel features. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.272 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.271 In this paper, we present the first method for the geometric auto-calibration of multiple projectors on a set of CAVE-like immersive display surfaces including truncated domes and 4 or 5-wall CAVEs (three side walls, floor and/or ceiling). All such surfaces can be categorized as swept surfaces and multiple projectors can be registered on them using a single uncalibrated camera without using any physical markers on the surface. Our method can also handle non-linear distortion in the projectors, common in compact setups where a short throw lens is mounted on each projector. Further, when the whole swept surface is not visible from a single camera view, we can register the projectors using multiple pan and tilted views of the same camera. Thus, our method scales well with different size and resolution of the display. Since we recover the 3D shape of the display, we can achieve registration that is correct from any arbitrary viewpoint appropriate for head-tracked single-user virtual reality systems. We can also achieve wallpapered registration, more appropriate for multi-user collaborative explorations. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.271 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.270 Creating high-quality quad meshes from triangulated surfaces is a highly non trivial task that necessitates consideration of various application specific metrics of quality. In our work, we follow the premise that automatic reconstruction techniques may not generate outputs meeting all the subjective quality expectations of the user. Instead, we put the user at the center of the process by providing a flexible, interactive approach to quadrangulation design. By combining scalar field topology and combinatorial connectivity techniques, we present a new framework, following a coarse to fine design philosophy, which allows for explicit control of the subjective quality criteria on the output quad mesh, at interactive rates. Our quadrangulation framework uses the new notion of Reeb atlas editing, to define with a small amount of interactions a coarse quadrangulation of the model, capturing the main features of the shape, with user prescribed extraordinary vertices and alignment. Fine grain tuning is easily achieved with the notion of connectivity texturing, which allows for additional extraordinary vertices specification and explicit feature alignment, to capture the high-frequency geometries. Experiments demonstrate the interactivity and flexibility of our approach, as well as its ability to generate quad meshes of arbitrary resolution with high quality statistics, while meeting the user's own subjective requirements. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.270 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.269 We propose a combinatorial algorithm to track critical points of 2D time-dependent scalar fields. Existing tracking algorithms such as Feature Flow Fields apply numerical schemes utilizing derivatives of the data, which makes them prone to noise and involve a large number of computational parameters. In contrast, our method is robust against noise since it does not require derivatives, interpolation, and numerical integration. Furthermore, we propose an importance measure that combines the spatial persistence of a critical point with its temporal evolution. This leads to a time-aware feature hierarchy, which allows us to discriminate important from spurious features. Our method requires only a single, easy-to-tune computational parameter and is naturally formulated in an out-of-core fashion, which enables the analysis of large data sets. We apply our method to synthetic data and data sets from computational fluid dynamics and compare it to the stabilized continuous Feature Flow Field tracking algorithm. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.269 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.268 We propose to aid the interactive visualization of time-varying spatial datasets by simplifying node position data over the entire simulation as opposed to over individual states. Our approach is based on two observations. The first observation is that the trajectory of some nodes can be approximated well without recording the position of the node for every state. The second observation is that there are groups of nodes whose motion from one state to the next can be approximated well with a single transformation. We present dataset simplification techniques that take advantage of this node data redundancy. Our techniques are general, supporting many types of simulations, they achieve good compression factors, and they allow rigorous control of the maximum node position approximation error. We demonstrate our approach in the context of finite element analysis data, of liquid flow simulation data, and of fusion simulation data. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.268 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.266 We present a tool that is specifically designed to support a writer in revising a draft-version of a document. In addition to showing which paragraphs and sentences are difficult to read and understand, we assist the reader in understanding why this is the case. This requires features that are expressive predictors of readability, and are also semantically understandable. In the first part of the paper, we therefore discuss a semi-automatic feature selection approach that is used to choose appropriate measures from a collection of 141 candidate readability features. In the second part, we present the visual analysis tool VisRA, which allows the user to analyze the feature values across the text and within single sentences. Users can choose between different visual representations accounting for differences in the size of the documents and the availability of information about the physical and logical layout of the documents. We put special emphasis on providing as much transparency as possible to ensure that the user can purposefully improve the readability of a sentence. Several case-studies are presented that show the wide range of applicability of our tool. Furthermore, an in-depth evaluation assesses the quality of the measure and investigates how well users do in revising a text with the help of the tool. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.266 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.264 We propose a simple and efficient framework for simulating turbulent flow of an inhomogeneous fluid with spatially varying density and temperature. Instead of explicitly computing the complex motion of fluid dynamical instability, our method approximates the average motion of the fluid using an incompressible buoyancy-based solver. Then, the high-resolution dynamics is computed using a new extended version of the vortex particle method with baroclinity. By synthesizing these two solutions, we efficiently simulated a complex scene with turbulence effects. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.264 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.263 We present an inference-based surface reconstruction algorithm that is capable of identifying objects of interest amongst a cluttered scene, and reconstructing solid model representations even in the presence of occluded surfaces. Our proposed approach incorporates a predictive modeling framework that uses a set of user provided models for prior knowledge, and applies this knowledge to the iterative identification and construction process. Our approach uses a local to global construction process guided by rules for fitting high quality surface patches obtained from these prior models. We demonstrate the application of this algorithm on several example datasets containing heavy clutter and occlusion. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.263 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.262 This paper presents a geometric algorithm for the generation of uniform cubic B-spline curves interpolating a sequence of data points under tangent and curvature vectors constraints. To satisfy these constraints, knot insertion is used to generate additional control points which are progressively repositioned using corresponding geometric rules. Compared to existing schemes, our approach is capable of handling plane as well as space curves, has local control, and avoids the solution of the typical linear system. The effectiveness of the proposed algorithm is illustrated through several comparative examples. Applications of the method in NC machining and shape design are also outlined. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.262 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.158 The contribution of this paper is two-fold. First, we show how to extend the ESM algorithm to handle motion blur in 3D object tracking. ESM is a powerful algorithm for template matching-based tracking, but it can fail under motion blur. We introduce an image formation model that explicitly consider the possibility of blur, and show it results in a generalization of the original ESM algorithm. This allows to converge faster, more accurately and more robustly even under large amount of blur. Our second contribution is an efficient method for rendering the virtual objects under the estimated motion blur. It renders two images of the object under 3D perspective, and warps them to create many intermediate images. By fusing these images we obtain a final image for the virtual objects blurred consistently with the captured image. Because warping is much faster than 3D rendering, we can create realistically blurred images at a very low computational cost. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.158 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.157 The issue of transferring facial performance from one person’s face to another’s has been an area of interest for the movie industry and the computer graphics community for quite some time. In recent years, deformable face models, such as the Active Appearance Model (AAM), have made it possible to track and synthesise faces in real-time. Not surprisingly, deformable face model based approaches for facial performance transfer have gained tremendous interest in the computer vision and graphics community. In this paper, we focus on the problem of real-time facial performance transfer using the AAM framework. We propose a novel approach of learning the mapping between the parameters of two completely independent AAMs, using them to facilitate the facial performance transfer in a more realistic manner than previous approaches. The main advantage of modelling this parametric correspondence is that it allows a “meaningful” transfer of both the non-rigid shape and texture across faces irrespective of the speakers’ gender, shape and size of the faces, and illumination conditions. We explore linear and non-linear methods for modelling the parametric correspondence between the AAMs and show that the sparse linear regression method performs the best. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.157 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.156 This paper develops a novel surface fitting scheme for automatically reconstructing a genus-0 object into a continuous parametric spline surface. A key contribution for making such a fitting method both practical and accurate is our spherical generalization of the Delaunay configuration B-spline (DCB-spline), a new non-tensor-product spline. In this framework, we efficiently compute Delaunay configurations on sphere by the union of two planar Delaunay configurations. Also, we develop a hierarchical and adaptive method that progressively improves the fitting quality by new knot-insertion strategies guided by surface geometry and fitting error. Within our framework, a genus-0 model can be converted to a single spherical spline representation whose root mean square error is tightly bounded within a user-specified tolerance. The reconstructed continuous representation has many attractive properties such as global smoothness and no auxiliary knots. We conduct several experiments to demonstrate the efficacy of our new approach for reverse engineering and shape modeling. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.156 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.155 The main challenge for effective streamline visualization lies in seed placement, i.e., where to drop seeds and how many seeds should be placed. Seeding too many or too few streamlines is not able to reveal flow features and patterns either because it easily leads to visual clutter in rendering or it conveys little information about the flow field. Not only does the number of streamlines placed matter, their spatial relationships also play a key role in understanding the flow field. Therefore, effective flow visualization requires the streamlines to be placed in the right place and in the right amount. This paper introduces hierarchical streamline bundles, a novel approach to simplifying and visualizing 3D flow fields. By placing seeds and generating streamlines according to flow saliency, we produce a set of streamlines that captures important flow features near critical points without enforcing the dense seeding condition. We group spatially neighboring and geometrically similar streamlines to construct a hierarchy from which we extract streamline bundles at different levels of detail. Streamline bundles highlight multiscale flow features and patterns through clustered yet not cluttered display. This selective visualization strategy effectively reduces visual clutter while accentuating visual foci, and therefore is able to convey the desired insight into the flow data. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.155 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.154 This paper studies the design and application of a novel visual attention model meant to compute users gaze position automatically, i.e. without using a gaze-tracking ystem. The model we propose is specifically designed for real-time first-person exploration of 3D virtual environments. It is the first model adapted to this context which can compute, in real-time, a continuous gaze point position instead of a set of 3D objects potentially observed by the user. To do so, contrary to previous models which use a mesh-based representation of visual objects, we introduce a representation based on surface-elements. Our visual attention model combines the bottom-up and top-down components to compute a continuous gaze point position on screen that hopefully matches the user's one. We have conducted an experiment to study and compare the performance of our method with a state-of-the-art approach. Our results are found significantly better with more than 100% of accuracy gained. This suggests that computing in real-time a gaze point in a 3D virtual environment is possible and is a valid approach as compared to object-based approaches. Finally, we expose different applications of our model when exploring virtual environments. We present different algorithms which can improve or adapt the visual feedback of virtual environments based on gaze information. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.154 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.153 Events that happened in the past are important for understanding the ongoing processes, predicting future developments, and making informed decisions. Important and/or interesting events tend to attract many people. Some people leave traces of their attendance in the form of computer-processable data, such as records in the databases of mobile phone operators or photos on photo sharing web sites. We developed a suite of visual analytics methods for reconstructing past events from these activity traces. Our tools combine geocomputations, interactive geovisualizations and statistical methods to enable integrated analysis of the spatial, temporal, and thematic components of the data, including numeric attributes and texts. We also support interactive investigation of the sensitivity of the analysis results to the parameters used in the computations. For this purpose, statistical summaries of computation results obtained with different combinations of parameter values are visualized in a way facilitating comparisons. We demonstrate the utility of our approach on two large real data sets, mobile phone calls in Milano during 9 days and flickr photos made on British Isles during 5 years. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.153 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.152 This paper proposes an algorithm to build a set of orthogonal Point-Based Manifold Harmonic Bases (PB-MHB) for spectral analysis over point-sampled manifold surfaces. To ensure that PB-MHB are orthogonal to each other, it is necessary to have symmetrizable discrete Laplace-Beltrami Operator (LBO) over the surfaces. Existing converging discrete LBO for point clouds, as proposed by Belkin et al, is not guaranteed to be symmetrizable. We build a new point-wisely discrete LBO over the point-sampled surface that is guaranteed to be symmetrizable, and prove its convergence. By solving the eigen problem related to the new operator, we define a set of orthogonal bases over the point cloud. Experiments show that the new operator is converging better than other symmetrizable discrete Laplacian operators (such as graph Laplacian) defined on point-sampled surfaces, and can provide orthogonal bases for further spectral geometric analysis and processing tasks. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.152 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.151 Treating the interactions of soft tissue with rigid user-guided tools is a difficult problem. This is particularly true if the soft tissue has a \emph{slender} shape, \IE, resembling a thin shell, and if the underlying numerical time-integration scheme employs large time-steps. In this case, large mutual displacements of both the tool and the soft tissue occur frequently, resulting in deep interpenetrations or break-throughs. As a consequence, the computation of spatially and temporally coherent contact spaces turns out to be very challenging. In this paper, an approach is proposed that is tailored to these kinds of interactions. To solve this problem, a novel spatially reduced representation of the soft tissue geometry is employed where the dominant dimensions of the object are approximated by a two-dimensional triangle surface, while the third dimension is given in terms of nodal radii. To construct a feasible, non-penetrating configuration, a novel manifold projection scheme is presented where the colliding triangles are rasterized into a distance field in order to robustly estimate the contact spaces, even for large intersections. The method produces physically-plausible results, albeit it is purely geometric, and the material parameters are neglected at the collision response stage. Various examples, including an interactive prototype arthroscopy simulator, underline the wide applicability of the approach. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.151 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.144 Security Visualization is a very young term. It expresses the idea that common visualization techniques have been designed for use cases that are not supportive of security-related data, demanding novel techniques fine tuned for the purpose of thorough analysis. Significant amount of work has been published in this area, but little work has been done to study this emerging visualization discipline. We offer a comprehensive review of network security visualization and provide a taxonomy in the form of five use-case classes encompassing nearly all recent works in this area. We outline the incorporated visualization techniques and data sources and provide an informative table to display our findings. From the analysis of these systems, we examine issues and concerns regarding network security visualization and provide guidelines and directions for future researchers and visual system developers. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.144 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.143 Euler diagrams are often used to visualize intersecting data sets in applications such as criminology; genetics, medicine and computer file systems. One interesting aspect of these diagrams is that some data sets cannot be drawn without breaking one or more “wellformedness properties”, which are considered to reduce the user comprehension of the diagrams. However, it is possible to draw the same data with different diagrams, each of which breaks different wellformedness properties. Hence, some properties are “swappable”, so motivating the study of which of the alternatives would be best to use. This paper reports on the two empirical studies to determine how wellformedness properties affect comprehension. One study was with abstract data, the other was with concrete data that visualized students’ enrollment on university modules. We have results from both studies that imply that diagrams with concurrency or disconnected zones perform less well than other some other properties. Further, we have no results that imply that diagrams with brushing points adversely affect performance. Our data also indicates that non-simple curves are preferred less than diagrams with other properties. These results will inform both human diagram designers and the developers of automated drawing systems on the best way to visualize data using Euler diagrams. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.143 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.142 Networks are widely used to describe many natural and technological systems. Understanding how these evolve over time poses a challenge for existing visualization techniques originally developed for fixed network structures. We describe a method of incorporating the concept of aging into evolving networks, where nodes and edges store information related to the amount of local evolutionary change they have experienced. This property is used to generate visualizations that ensure stable substructures maintain relatively fixed spatial positions, allowing them to act as visual markers and providing context for evolutionary change elsewhere. By further supplementing these visualizations with color cues, the resultant animations enable a clearer portrayal of the underlying evolutionary process. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.142 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.141 A water drop behaves differently from a large water body because of its strong viscosity and surface tension under the small scale. Surface tension causes the motion of a water drop to be largely determined by its boundary surface. Meanwhile, viscosity makes the interior of a water drop less relevant to its motion, as the smooth velocity field can be well approximated by an interpolation of the velocity on the boundary. Consequently, we propose a fast deformable surface model to realistically animate water drops and their flowing behaviors on solid surfaces. Our system efficiently simulates water drop motions in a Lagrangian fashion, by reducing 3D fluid dynamics over the whole liquid volume to a deformable surface model. In each time step, the model uses an implicit mean curvature flow operator to produce surface tension effects, a contact angle operator to change droplet shapes on solid surfaces, and a set of mesh connectivity updates to handle topological changes and improve mesh quality over time. Our numerical experiments demonstrate a variety of physically plausible water drop phenomena at a real-time rate, including capillary waves when water drops collide, pinch-off of water jets, and droplets flowing over solid materials. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.141 http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.140 This paper presents a very easy-to-use interactive tool, which we call \emph{dot scissor}, for mesh segmentation. The user's effort is reduced to placing only a single-click where a cut is desired. Such a simple interface is made possible by a directional search strategy supported by a concave-aware harmonic field and a robust voting scheme which selects the best isoline as the final cut. With a concave-sensitive weighting scheme, the harmonic fields gather dense isolines along concave regions which are natural boundaries of semantic components. The voting scheme relies on an isoline-face scoring mechanism that considers both shape geometry and user intent. We show by extensive experiments and quantitative analysis that our tool advances the state-of-the-art segmentation methods in both simplicity of use and segmentation quality. http://doi.ieeecomputersociety.org/10.1109/TVCG.2011.140