IEEE Transactions on Visualization and Computer Graphics

PrePrint: Real-Time Physics-Based 3D Biped Character Animation Using an Inverted Pendulum Model

We present a physics-based approach to generate 3D biped character animation that can react to dynamical environments in real-time. Our approach utilizes an inverted pendulum model to online adjust the desired motion trajectory from the input motion capture data. This on-line adjustment produces a physically-plausible motion trajectory adapted to dynamic environments, which is then used as the desired motion for the motion controllers to track in dynamics simulation. Rather than using Proportional-Derivative controllers whose parameters usually cannot be easily set, our motion tracking adopts a velocity-driven method which computes joint torques based on the desired joint angular velocities. Physically-correct full body motion of the 3D character is computed in dynamics simulation using the computed torques and dynamical model of the character. Our experiments demonstrate that tracking motion capture data with real-time response animation can be achieved easily. In addition, physically-plausible motion style editing, automatic motion transition, and motion adaptation to different limb sizes can also be generated without difficulty.

PrePrint: A Programmable Display Layer for Virtual-Reality System Architectures

Display systems typically operate at a minimum rate of 60Hz. However, existing VR-architectures generally produce application updates at a lower rate. Consequently, the display is not updated by the application every display frame. This causes a number of undesirable perceptual artifacts. We describe an architecture that provides a programmable display layer (PDL) in order to generate updated display frames. This replaces the default display behavior of repeating application frames until an update is available. We will show three benefits of the architecture typical to VR. First, smooth motion is provided by generating intermediate display frames by per-pixel depth-image warping using 3D motion fields. Smooth motion eliminates various perceptual artifacts due to judder. Second, we implement fine-grained latency reduction at the display frame level using a synchronized prediction of simulation objects and the viewpoint. This improves the average quality and consistency of latency reduction. Third, a crosstalk reduction algorithm for consecutive display frames is implemented, which improves the quality of stereoscopic images. To evaluate the architecture, we compare image quality and latency to that of a classic level-of-detail approach.

PrePrint: Enhanced Voxelization and Representation of Objects with Sharp Details in Truncated Distance Fields

We present a new method for voxelization of solid objects containing sharp details. Voxelization is a sampling process which transforms a continuously defined object into a discrete one represented as a voxel field. The latter can be used for rendering or other purposes which often involve a reconstruction of a continuous approximation of the original object. The objects to be voxelized have to fulfill certain representability conditions for the later reconstruction to be as good as possible. Otherwise, disturbing reconstruction artifacts appear. Our method extends the traditional distance-based voxelization by an a-priori detection of sharp object details and their subsequent modification in such a way that the resulting object to be voxelized fulfills the representability conditions. We represent the resulting discrete objects by means of truncated (i.e. narrow-band) distance fields which provide for reduction of memory requirements and allow for further processing by level-set techniques. This approach is exemplified with two classes of solid objects frequently containing such sharp details: implicit solids and solids resulting from CSG operations. In both cases the sharp details are rounded to a specific curvature determined by the sampling distance.

PrePrint: Yet Faster Ray-Triangle Intersection (Using SSE4)

Ray-triangle intersection is an important algorithm, not only in the field of realistic rendering (based on ray tracing), but also in physics simulation, collision detection, modelling, etc. Obviously, the speed of this well-defined algorithm's implementations is important because calls to such a routine are numerous in rendering and simulation applications. Contemporary fast intersection algorithms, which use SIMD instructions, focus on the intersection of ray packets against triangles. For intersection between single rays and triangles, operations such as horizontal addition or dot product are required. The SSE4 instruction set adds the dot product instruction which can be used for this purpose. This article presents a new modification of the fast ray-triangle intersection algorithms commonly used, which – when implemented on SSE4 – outperforms the current state-of-the-art algorithms. It also allows both a single ray and ray packet intersection calculation with the same precomputed data. The speed gain measurements are described and discussed in the article.

PrePrint: RACBVHs: Random-Accessible Compressed Bounding Volume Hierarchies

We present a novel compressed bounding volume hierarchy (BVH) representation, random-accessible compressed bounding volume hierarchies (RACBVHs), for various applications requiring random access on BVHs of massive models. Our RACBVH representation is compact and transparently supports random access on the compressed BVHs without decompressing the whole BVH. We provide the general BVH access API to transparently access our RACBVH representation. At runtime, our decompression framework is guaranteed to provide correct BV nodes without decompressing the whole BVH. Also, our method is extended to support parallel random access that can utilize the multi-core CPU architecture. Our method can achieve up to a 12:1 compression ratio and, more importantly, can decompress 4.2M BV nodes (= 135 MB) per second by using a single CPU-core. To highlight the benefits of our approach, we apply our method to two different applications: ray tracing and collision detection. We can improve the runtime performance by more than a factor of 4 as compared to using the uncompressed original data. This improvement is a result of the fast decompression performance and reduced data access time by selectively fetching and decompressing small regions of the compressed BVHs requested by applications.

PrePrint: Robust Feature-Preserving Mesh Denoising Based on Consistent Sub-Neighborhoods

In this paper, we introduce a feature-preserving denoising algorithm. It is built on the premise that the underlying surface of a noisy mesh is piecewise smooth, and a sharp feature lies on the intersection of multiple smooth surface regions. A vertex close to a sharp feature is likely to have a neighborhood that includes distinct smooth segments. By defining the consistent sub-neighborhood as the segment whose geometry and normal orientation most consistent with those of the vertex, we can completely remove the influence from neighbors lying on other segments during denoising. Our method identifies piecewise smooth sub-neighborhoods using a robust density-based clustering algorithm based on shared nearest neighbors. In our method, we obtain an initial estimate of vertex normals and curvature tensors by robustly fitting a local quadric model. An anisotropic filter based on optimal estimation theory is further applied to smooth the normal field and the curvature tensor field. This is followed by second-order bilateral filtering, which better preserves curvature details and alleviates volume shrinkage during denoising. The support of these filters is defined by the consistent sub-neighborhood of a vertex. We have applied this algorithm to both generic and CAD models, and sharp features, such as edges and corners, are very well preserved.

PrePrint: Analyzing and Tracking Burning Structures in Lean Premixed Hydrogen Flames

This paper presents topology-based methods to robustly extract, analyze, and track features defined as subsets of isosurfaces. First, we demonstrate how features identified by thresholding isosurfaces can be defined in terms of the Morse complex. Second, we present a specialized hierarchy that encodes the feature segmentation independent of the threshold while still providing a flexible multi-resolution representation. Third, for a given parameter selection we create detailed tracking graphs representing the complete evolution of all features in a combustion simulation over several hundred time steps. Finally, we discuss a user interface that correlates the tracking information with interactive rendering of the segmented isosurfaces enabling an in-depth analysis of the temporal behavior.

PrePrint: Placegram: A Diagrammatic Map for Personal Geotagged Data Browsing

Geotagging personal data such as photos and videos is continuously becoming easier and more popular. Nevertheless, browsing such data on general purpose maps can be difficult, due to the frequent zoom and pan operations as well as extraneous information. This paper presents Placegram, a compact diagrammatic map visualization for personal geotagged data browsing based on cognitive map theories. An evaluation using real-life datasets shows that the speed of finding and pointing to places from the participants' own data increased by a factor of 2.1 to 2.9, and the number of interesting places discovered from others' data within a time limit increased by 48.8% in Placegram compared to a general purpose map. Placegram was even slightly faster than a simple text list, while at the same time preserving the geographic senses of direction and location. Subjective ratings and comments from participants support these results, indicating that Placegram is significantly preferred over both a general map and a text list.

PrePrint: Origamizing Polyhedral Surfaces

This paper presents the first practical method for "origamizing" or obtaining the folding pattern that folds a single sheet of material into a given polyhedral surface without any cut. The basic idea is to tuck fold a planar paper to form a three-dimensional shape. The main contribution is to solve the inverse problem; the input is an arbitrary polyhedral surface and the output is the folding pattern. Our approach is to convert this problem into a problem of laying out the polygons of the surface on a planar paper by introducing the concept of tucking molecules. We investigate the equality and inequality conditions required for constructing a valid crease pattern. We propose an algorithm based on two-step mapping and edge splitting to solve these conditions. The two-step mapping precalculates linear equalities and separates them from other conditions. This allows an interactive manipulation of the crease pattern in the system implementation. We present the first system for designing three-dimensional origami, enabling a user to interactively design complex spatial origami models that have not been realizable thus far.

PrePrint: Fluid Simulation with Articulated Bodies

We present an algorithm for creating realistic animations of characters that are swimming through fluids. Our approach combines dynamic simulation with data-driven kinematic motions (motion capture data) to produce realistic animation in a fluid. The interaction of the articulated body with the fluid is performed by incorporating joint constraints with rigid animation and by extending a solid/fluid coupling method to handle articulated chains. Our solver takes as input the current state of the simulation and calculates the angular and linear accelerations of the connected bodies needed to match a particular motion sequence for the articulated body. These accelerations are used to estimate the forces and torques that are then applied to each joint. Based on this approach, we demonstrate simulated swimming results for a variety of different strokes, including crawl, backstroke, breaststroke and butterfly. The ability to have articulated bodies interact with fluids also allows us to generate simulations of simple water creatures that are driven by simple controllers.

PrePrint: Route Visualization Using Detail Lenses

We present a method designed to address some limitations of typical route map displays of driving directions. The main goal of our system is to generate a printable version of a route map that shows the overview and detail views of the route within a single, consistent visual frame. Our proposed visualization provides a more intuitive spatial context than a simple list of turns. We present a novel multi-focus technique to achieve this goal, where the foci are defined by points-of-interest (POI) along the route. A detail lens that encapsulates the POI at a finer geospatial scale is created for each focus. The lenses are laid out on the map to avoid occlusion with the route and each other, and to optimally utilize the free space around the route. We define a set of layout metrics to evaluate the quality of a lens layout for a given route map visualization. We compare standard lens layout methods to our proposed method and demonstrate the effectiveness of our method in generating aesthetically pleasing layouts. Finally, we perform a user study to evaluate the effectiveness of our layout choices.

PrePrint: Texture Mapping via Optimal Mass Transport

In this paper, we present a novel method for texture mapping of closed surfaces. Our method is based on the technique of optimal mass transport (also known as the "earth-mover's metric"). This is a classical problem which concerns determining the optimal way, in the sense of minimal transportation cost, of moving a pile of soil from one site to another. In our context, the resulting mapping is area preserving and minimizes angle distortion in the optimal mass sense. Indeed, we first begin with an angle-preserving mapping (which may greatly distort area), and then correct it using the mass transport procedure derived via a certain gradient flow. In order to obtain fast convergence to the optimal mapping we incorporate a multiresolution scheme into our flow. We also use ideas from discrete exterior calculus in our computations.

PrePrint: Visualizing the Semantic Structure in Classical Music Works

One of the major obstacles to appreciating classical music is that extensive training is required to understand musical structure and compositional techniques towards comprehending the thoughts behind the musical work. In this paper, we propose an innovative visualization solution to reveal the semantic structure in classical orchestral work, such that users can gain insights into musical structure and appreciate the beauty of music. We formulate the semantic structure into macro-level layer interactions, microlevel theme variations, and macro-micro relationship between themes and layers to abstract the complicated construction of a music composition. The visualization has been applied with success in understanding some classical music work as supported by highly promising user study results with the general audience and very positive feedback from music students and experts, demonstrating its effectiveness in conveying the sophistication and beauty of classical music to novice users with informative and intuitive displays.

PrePrint: Estimation of Detection Thresholds for Redirected Walking Techniques

In immersive virtual environments users can control their virtual viewpoint by walking through the real world, and movements are mapped one-to-one to virtual camera motions. With redirection techniques, the virtual camera is manipulated by applying gains so that the virtual world moves differently than the real world. We have quantified how much humans can unknowingly be redirected on paths, which are different from the visually perceived paths. We tested 12 subjects in three different psychophysical experiments. In experiment E1, subjects performed rotations with different gains, and then had to choose whether the visually perceived rotation was smaller or greater than the physical rotation. In experiment E2, subjects chose whether the physical walk was shorter or longer than the visually perceived scaled travel distance. In experiment E3, subjects estimate the path curvature when walking a curved path in the real world while the visual display shows a straight path in the virtual world. Our results show that users can be turned physically about 49% more or 20% less than the perceived virtual rotation, distances can be downscaled by 14% and up-scaled by 26%, and users can be redirected on a circular arc with a radius greater than 22m.

PrePrint: Meshless Helmholtz-Hodge Decomposition

Vector fields analysis traditionally distinguishes conservative (curl-free) from mass preserving (divergence-free) components. The Helmholtz-Hodge decomposition allows separating any vector field into the sum of three uniquely defined components: curl-free, divergence-free and harmonic. This decomposition is usually achieved by using mesh-based methods such as finite differences or finite elements. This work presents a new meshless approach to the Helmholtz-Hodge decomposition for the analysis of 2D discrete vector fields. It embedds into the SPH particle-based framework. The proposed method is effficient and can be applied to extract features from a 2D discrete vector field and to multiphase fluid flow simulation to ensure incompressibility.

PrePrint: High Dynamic Range Texture Compression for Rendering Systems of Different Capacities

In this paper, we propose a novel approach for high dynamic range (HDR) texture compression (TC) suitable for rendering systems of different capacities. Based on the previously proposed DHTC scheme, we first work out an improved joint-channel compression framework, which is robust and flexible enough to provide compressed HDR textures at different bit-rates. Then, two compressed HDR texture formats based on the proposed framework are developed. The 8 bpp format is of near lossless visual quality, improving upon known state of the art algorithms. And to our knowledge, the 4 bpp format is the first workable 4 bpp solution with good quality. We also show that HDR textures in the proposed 4 bpp and 8 bpp formats can compose a layered architecture in the texture consumption pipeline, to significantly save the memory bandwidth and storage in real-time rendering. In addition, the 8 bpp format can also be used to handle traditional low dynamic range (LDR) RGBA textures. Our scheme exhibits a practical solution for compressing HDR textures at different rates and LDR textures with alpha maps.

PrePrint: Metric Driven RoSy Field Design and Remeshing

Designing rotational symmetry fields on surfaces is an important task for a wide range of graphics applications. This work introduces a rigorous and practical approach for automatic N-RoSy field design on arbitrary surfaces with user defined field topologies. The user has full control of the number, positions and indices of the singularities, the turning numbers of the loops, and is able to edit the field interactively. We formulate N-RoSy field construction as designing a Riemannian metric, such that the holonomy along any loop is compatible with the local symmetry of N-RoSy fields. We prove the compatibility condition using discrete parallel transport. The complexity of N-RoSy field design is caused by curvatures. In our work, we propose to simplify the Riemannian metric to make it flat almost everywhere. This approach greatly simplifies the process and improves the flexibility, such that, it can design N-RoSy fields with single singularity, and mixed-RoSy fields. This approach can also be generalized to construct regular remeshing on surfaces. To demonstrate the effectiveness of our approach, we apply our design system to pen-and-ink sketching and geometry remeshing. Furthermore, based on our remeshing results with high global symmetry, we generate Celtic knots on surfaces directly.

PrePrint: The Medical Exploration Toolkit - An Efficient Support for Visual Computing in Surgical Planning and Training

Application development is often guided by the usage of software libraries and toolkits. For medical applications, the toolkits currently available focus on image analysis and volume rendering. Advanced interactive visualizations and user interface issues are not adequately supported. Hence, we present a toolkit for application development in the field of medical intervention planning, training and presentation – the MedicalExplorationToolkit (METK). The METK is based on the rapid prototyping platform MeVisLab and offers a large variety of facilities for an easy and efficient application development process. We present dedicated techniques for advanced medical visualizations, exploration, standardized documentation and interface widgets for common tasks. These include, e.g., advanced animation facilities, viewpoint selection, several illustrative rendering techniques, and new techniques for object selection in 3D surface models. No extended programming skills are needed for application building, since a graphical programming approach can be used. The toolkit is freely available and well documented to facilitate the use and extension of the toolkit.

PrePrint: Tiled++ - An Enhanced Tiled Hi-Res Display Wall

In recent years, high-resolution displays have become increasingly important to decision makers and scientists, because large screens combined with a high pixel count facilitate content rich, simultaneous display of computer-generated imagery and high-definition video data from multiple sources. Tiled displays are attractive due to their extended screen real estate, scalability, and low cost. One of the drawbacks of LCD-based tiled displays is the fact that users sometimes get distracted by the screens’ bezels, which cause discontinuities in rendered images. Most conventional solutions either ignore the bezels and display all pixels, causing objects to become distorted, or eliminate pixels that would normally fall under the bezels, causing pixels to be missing in the display. In this paper we present a new scalable approach that neither leads to discontinuities nor significant loss of information. By projecting onto the bezels, we demonstrate that a combination of LCD-based tiled displays and projection significantly reduces the bezel problem. Our technique eliminates ambiguities that commonly occur on tiled displays. It improves the usability of multi-monitor systems by virtually eliminating the bezels. We describe a setup and provide results from an evaluation experiment conducted on a 3x3 and on a 10x5 tiled display wall.

PrePrint: All-frequency Lighting with Multiscale Spherical Radial Basis Functions

This paper proposes a novel multiscale spherical radial basis function (MSRBF) representation for all-frequency lighting. It supports the illumination of distant environment as well as the local illumination commonly used in practical applications, such as games. The key is to define a multiscale and hierarchical structure of spherical radial basis functions (SRBFs) with basis functions uniformly distributed over the sphere. The basis functions are divided into multiple levels according to their coverage (widths). Within the same level, SRBFs have the same width. Larger-width SRBFs are responsible for lower-frequency lighting while the smaller-width ones are responsible for the higher-frequency lighting. Hence our approach can achieve the true all-frequency lighting that is not achievable by the single-scale SRBF approach. Besides, the MSRBF approach is scalable as coarser rendering quality can be achieved without re-estimating the coefficients from the raw data. With the homogeneous form of basis functions, the rendering is highly efficient. The practicability of the proposed method is demonstrated with real-time rendering and effective compression for tractable storage.

PrePrint: Uncluttering Graph Layouts Using Anisotropic Diffusion and Mass Transport

Many graph layouts include very dense areas, making the layout difficult to understand. In this paper, we propose a technique for modifying an existing layout in order to reduce the clutter in dense areas. A physically-inspired evolution process, based on a modified heat equation is used to create an improved layout density image, making better use of available screen space. Using results from optimal mass transport problems, a warp to the improved density image is computed. The graph nodes are displaced according to the warp. The warp maintains the overall structure of the graph, thus preserving the mental map, while reducing the clutter in dense areas of the layout. The complexity of the algorithm depends mainly on the resolution of the image visualizing the graph and is linear in the size of the graph. This allows scaling the computation according to required running times. It is demonstrated how the algorithm can be significantly accelerated using a graphics processing unit (GPU), resulting in the ability to handle large graphs in a matter of seconds. Results on several layout algorithms and applications are demonstrated.

IEEE Transactions on Visualization and Computer Graphics - July/August 2009 (Vol. 15, No. 4)

IEEE Transactions on Visualization and Computer Graphics

PrePrint: Refilming with Depth-Inferred Videos

Compared to still image editing, content-based video editing faces the additional challenges of maintaining the spatio-temporal consistency with respect to geometry. This brings up difficulties of seamlessly modifying video content, for instance, inserting or removing an object. In this paper, we present a new video editing system for creating spatio-temporally consistent and visually appealing re-filming effects. Unlike the typical filming practice, our system requires no labor-intensive construction of 3D models/surfaces mimicking the real scene. Instead, it is based on an unsupervised inference of view-dependent depth maps for all video frames. We provide interactive tools requiring only a small amount of user input to perform elementary video content editing, such as separating video layers, completing background scene, and extracting moving objects. These tools can be utilized to produce a variety of visual effects in our system, including but not limited to video composition, "predator" effect, bullet-time, depth-of-field, and fog synthesis. Some of the effects can be achieved in real-time.

PrePrint: Virtual Rheoscopic Fluids

We present a visualization technique for simulated fluid dynamics data that visualizes the gradient of the velocity field in an intuitive way. Our work is inspired by {\em rheoscopic} particles, which are small, flat particles that, when suspended in fluid, align themselves with the shear of the flow. We adopt the physical principles of real rheoscopic particles and apply them, in model form, to 3D velocity fields. By simulating the behavior and reflectance of these particles we are able to render 3D simulations in a way that gives insight into the dynamics of the system. The results can be rendered in real-time, allowing the user to inspect the simulation from all perspectives. We achieve this by a combination of pre-computations and fast ray-tracing on the GPU. We demonstrate our method on several different simulations, showing their complex dynamics in the process.

PrePrint: Local Ambient Occlusion in Direct Volume Rendering

This paper presents a novel technique to efficiently compute illumination for Direct Volume Rendering using a local approximation of ambient occlusion to integrate the intensity of incident light for each voxel. An advantage with this local approach is that fully shadowed regions are avoided which is desired in many applications of volume rendering, such as medical visualization. Additional transfer function interactions are also presented, for instance to highlight specific structures with luminous tissue effects and to create an improved context for semi-transparent tissues with a separate absorption control for the illumination settings. Multi-resolution volume management and GPU-based computation are used to accelerate the calculations and to support large data sets. The scheme yields interactive frame rates with an adaptive sampling approach for incrementally refined illumination under arbitrary transfer function changes. The illumination effects can give a better understanding of the shape and density of tissues and so has the potential to increase the diagnostic value of medical volume rendering. Since the proposed method is gradient-free it is especially beneficial at the borders of clip planes, where gradients are undefined, and for noisy datasets.

PrePrint: Crease Surfaces: From Theory to Extraction and Application to Diffusion Tensor MRI

Crease surfaces are two-dimensional manifolds along which a scalar field assumes a local maximum (ridge) or a local minimum (valley) in a constrained space. Unlike isosurfaces, they are able to capture extremal structures in the data. Creases have a long tradition in image processing and computer vision, and have recently become a popular tool for visualization. When extracting crease surfaces, degeneracies of the Hessian (i.e., lines along which two eigenvalues are equal), have so far been ignored. We show that these loci, however, have two important consequences for the topology of crease surfaces: First, creases are bounded not only by a side constraint on eigenvalue sign, but also by Hessian degeneracies. Second, crease surfaces are not in general orientable. We describe an efficient algorithm for the extraction of crease surfaces which takes these insights into account and demonstrate that it produces more accurate results than previous approaches. Finally, we show that DT-MRI streamsurfaces, which were previously used for the analysis of planar regions in diffusion tensor MRI data, are mathematically ill-defined. As an example application of our method, creases in a measure of planarity are presented as a viable substitute.

PrePrint: Planetary-Scale Terrain Composition

Many inter-related planetary height map and surface image map data sets exist, and more data are collected each day. Broad communities of scientists require tools to compose these data interactively and explore them via real-time visualization. While related, these data sets are often unregistered with one another, having different projection, resolution, format, and type. We present a GPU-centric approach to the real-time composition and display of unregistered-but-related planetary-scale data. This approach employs a GPGPU process to tessellate spherical height fields. It uses a render-to-vertex-buffer technique to operate upon polygonal surface meshes in image space, allowing geometry processes to be expressed in terms of image processing. With height and surface map data processing unified in this fashion, a number of powerful composition operations may be uniformly applied to both. Examples include adaptation to non-uniform sampling due to projection, seamless blending of data of disparate resolution or transformation regardless of boundary, and the smooth interpolation of levels of detail in both geometry and imagery. Issues of scalability and precision are addressed, giving out-of-core access to giga-pixel data sources, and correct rendering at scales approaching one meter.

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PrePrint: Real-Time Ray-Tracing of Implicit Surfaces on the GPU

Compact representation of geometry using a suitable procedural or mathematical model and a ray-tracing mode of rendering fit the programmable graphics processor units (GPUs) well. Several such representations including parametric and subdivision surfaces have been explored in recent research. The important and widely applicable category of the general implicit surface has received less attention. In this paper, we present a ray-tracing procedure to render general implicit surfaces efficiently on the GPU. Though only the fourth or lower order surfaces can be rendered using analytical roots, our adaptive marching points algorithm can ray-trace arbitrary implicit surfaces without multiple roots, by sampling the ray at selected points till a root is found. Adapting the sampling step size based on a proximity measure and a horizon measure delivers high speed. The sign-test can handle any surface without multiple roots. The Taylor-test that uses ideas from interval analysis can ray-trace many surfaces with complex roots. Overall, a simple algorithm that fits the SIMD architecture of the GPU results in high performance. We demonstrate the ray-tracing of algebraic surfaces up to order 50 and non-algebraic surfaces including a Blinn's blobby with 75 spheres at better than interactive frame rates.

PrePrint: Multiresolution Mean Shift Clustering Algorithm for Shape Interpolation

In this paper, we solve the problem of 3D shape interpolation with significant pose variation. For an ideal 3D shape interpolation, especially the articulated model, the shape should follow the movement of the underlying articulated structure and be transformed in a way that is as rigid as possible. Given input shapes with compatible connectivity, we propose a novel multi-resolution mean shift (MMS) clustering algorithm to automatically extract their near-rigid components. Then, by building the hierarchical relationship among extracted components, we compute a common articulated structure for these input shapes. With the aid of this articulated structure, we solve the shape interpolation by combining 1) a global pose interpolation of near-rigid components from the source shape to the target shape with 2) a local gradient field interpolation for each pair of components, followed by solving a Poisson equation in order to reconstruct an interpolated shape. As a result, an aesthetically pleasing shape interpolation can be generated, with even the poses of shapes varying significantly. In contrast to a recent state-of-the-art work [19], the proposed approach can achieve comparable or even better results and have better computational efficiency as well.

PrePrint: Directable Weathering of Concave Rock Using Curvature Estimation

We address the problem of directable weathering of exposed concave rock for use in computer generated animation or games. Previous weathering models which do admit concave surfaces are computationally inefficient and difficult to control. In nature, the spheroidal and cavernous weathering rates depend on the surface curvature. Spheroidal weathering is fastest in areas with large positive mean curvature and cavernous weathering is fastest in areas with large negative mean curvature. We simulate both processes using an approximation of mean curvature on a voxel grid. Both weathering rates are also influenced by rock durability. The user controls rock durability by editing a durability graph before and during weathering simulation. Simulations of rock fall and colluvium deposition further improve realism. The profile of the final weathered rock matches the shape of the durability graph--up to the effects of weathering and colluvium deposition. We demonstrate the top-down directability and visual plausibility of the resulting model through a series of screen shots and rendered images. The results include the weathering of a cube into a sphere and a sheltered inside corner into a cavern as predicted by the underlying geomorphological models.

PrePrint: Compatible Embedding for 2D Shape Animation

We present new algorithms for the compatible embedding of 2D shapes. Such embeddings offer a convenient way to interpolate shapes having complex, detailed features. Compared to existing techniques, our approach requires less user input, is faster, more robust, and simpler to implement, making it ideal for interactive use in practical applications. Our new approach consists of three parts. First, our boundary matching algorithm locates salient features using the perceptually-motivated principles of scale-space and uses these as automatic correspondences to guide an elastic curve matching algorithm. Second, we simplify boundaries while maintaining their parametric correspondence and the embedding of the original shapes. Finally, we extend the mapping to shapes' interiors via a new compatible triangulation algorithm. The combination of our algorithms allows us to demonstrate 2D shape interpolation with instant feedback. The proposed algorithms exhibit a combination of simplicity, speed, and accuracy that has not been achieved in previous work.

PrePrint: Efficient LBM Visual Simulation on Face-Centered Cubic Lattices

The lattice Boltzmann method (LBM) for visual simulation of fluid flow generally employs cubic Cartesian (CC) lattices such as the D3Q13 and D3Q19 lattices for the particle transport. However, the CC lattices lead to sub-optimal representation of the simulation space. We introduce the face-centered cubic (FCC) lattice, fD3Q13, for LBM simulations. Compared to the CC lattices, the fD3Q13 lattice creates a more isotropic sampling of the simulation domain and its single lattice speed (i.e., link length) simplifies the computations and data storage. Furthermore, the fD3Q13 lattice can be decomposed into two independent interleaved lattices, one of which can be discarded, which doubles the simulation speed. The resulting LBM simulation can be efficiently mapped to the GPU, further increasing the computational performance. We show the numerical advantages of the FCC lattice on channeled flow in 2D and the flow-past-a-sphere benchmark in 3D. In both cases, the comparison is against the corresponding CC lattices using the analytic solutions for the systems as well as velocity field visualizations. We also demonstrate the performance advantages of the fD3Q13 lattice for interactive simulation and rendering of hot smoke in an urban environment using thermal LBM.

PrePrint: Enhancing Realism of Wet Surfaces in Temporal Bone Surgical Simulation

We present techniques to improve visual realism in an interactive surgical simulation application: a mastoidectomy simulator that offers a training environment for medical residents as a complement to using a cadaver. As well as displaying the mastoid bone through volume rendering, the simulation allows users to experience haptic feedback and appropriate sound cues while controlling a virtual bone drill and suction/irrigation device. The techniques employed to improve realism consist of a fluid simulation that allows for deformable boundaries based on volumetric bone data and a shading model that gives a wet look to the rendered bone to more closely duplicate the appearance bone has in a surgical environment. The fluid rendering includes bleeding effects, meniscus rendering, and refraction. We incorporate a planar computational fluid dynamics simulation into our three-dimensional rendering to effect realistic blood diffusion. Maintaining real-time performance while drilling away bone in the simulation is critical for engagement with the system.

PrePrint: TransCAIP: A Live 3D TV System Using a Camera Array and an Integral Photography Display with Interactive Control of Viewing Parameters

The system described in this paper provides a real-time 3D visual experience by using an array of 64 video cameras and an integral photography display with 60 viewing directions. The live 3D scene in front of the camera array is reproduced by the full-color, full-parallax autostereoscopic display with interactive control of viewing parameters. The main technical challenge is fast and flexible conversion of the data from the 64 multi-camera images to the integral photography format. Based on image-based rendering techniques, our conversion method first renders 60 novel images corresponding to the viewing directions of the display, and then arranges the rendered pixels to produce an integral photography image. For real-time processing on a single PC, all the conversion processes are implemented on a GPU with GPGPU techniques. The conversion method also allows a user to interactively control viewing parameters of the displayed image for reproducing the dynamic 3D scene with desirable parameters. This control is performed as a software process, without reconfiguring the hardware system, by changing the rendering parameters such as the convergence point of the rendering cameras and the interval between the viewpoints of the rendering cameras.

PrePrint: Efficient and Accurate Sound Propagation Using Adaptive Rectangular Decomposition

Accurate sound rendering can add significant realism to complement visual display in interactive applications, as well as facilitate acoustic predictions for many engineering applications, like accurate acoustic analysis for architectural design. Numerical simulation can provide this realism most naturally by modeling the underlying physics of wave propagation. However, wave simulation has traditionally posed a tough computational challenge. In this paper, we present a technique which relies on an adaptive rectangular decomposition of 3D scenes to enable efficient and accurate simulation of sound propagation in complex virtual environments. It exploits the known analytical solution of the Wave Equation in rectangular domains, and utilizes efficient implementation of Discrete Cosine Transform on the GPU to achieve at least a hundred-fold performance gain compared to a standard Finite Difference Time Domain (FDTD) implementation with comparable accuracy, while also being an order of magnitude more memory-efficient. Consequently, we are able to perform accurate numerical acoustic simulation on large, complex scenes in the kilohertz range. To the best of our knowledge, it was not previously possible to perform such simulations on a desktop computer. Our work thus enables acoustic analysis on large scenes and auditory display for complex virtual environments on commodity hardware.

PrePrint: Distribution-Driven Visualization of Volume Data

Feature detection and display are the essential goals of the visualization process. Most visualization software achieves these goals by mapping properties of sampled intensity values and their derivatives to color and opacity. In this work, we propose to explicitly study the local frequency distribution of intensity values in broader neighborhoods centered around each voxel. We have found frequency distributions to contain meaningful and quantitative information that is relevant for many kinds of feature queries. Our approach allows users to enter predicate-based hypotheses about relational patterns in local distributions and render visualizations that show how neighborhoods match the predicates. Distributions are a familiar concept to non-expert users, and we have built a simple graphical user interface for forming and testing queries interactively. The query framework readily applies to arbitrary spatial datasets and supports queries on time variant and multifield data. Users can directly query for classes of features previously inaccessible in general feature detection tools. Using several well-known datasets, we show new quantitative features that enhance our understanding of familiar visualization results.

PrePrint: Image-Based Modeling of the Human Eye

Rendering realistic organic materials is a challenging issue. The human eye is an important part of non-verbal communication which, consequently, requires specific modeling and rendering techniques to enhance the realism of virtual characters. We propose an image-based method for estimating both iris morphology and scattering features in order to generate convincing images of virtual eyes. In this regard, we develop a technique to unrefract iris photographs. We model the morphology of the human iris as an irregular multilayered tissue. We then approximate the scattering features of the captured iris. Finally, we propose a real-time rendering technique based on the subsurface texture mapping representation and introduce a precomputed refraction function as well as a caustic function, which accounts for the light interactions at the corneal interface.

PrePrint: A Survey of Radial Methods for Information Visualization

Radial visualization, or the practice of displaying data in a circular or elliptical pattern, is an increasingly common technique in information visualization research. In spite of its prevalence, little work has been done to study this visualization paradigm as a methodology in its own right. We provide a historical review of radial visualization, tracing it to its roots in centuries-old statistical graphics. We then identify the types of problem domains to which modern radial visualization techniques have been applied. A taxonomy for radial visualization is proposed, in the form of seven design patterns encompassing nearly all recent work in this area. From an analysis of these patterns, we distill a series of design considerations that system builders can use to create new visualizations that address aspects of the design space that have not yet been explored. It is hoped that our taxonomy will provide a framework for facilitating discourse among researchers and stimulate the development of additional theories and systems involving radial visualization as a distinct design metaphor.

PrePrint: Subdivision Analysis of the Trilinear Interpolant

Isosurfaces are fundamental volumetric visualization tools, and are generated by the well-known Marching Cubes cases that approximate contours of trilinearly-interpolated scalar fields. While a complete set of cases has recently been published by Nielson, the formal proof that these cases are the only ones possible and that they are topologically correct is difficult to follow. We present a more straightforward proof of the correctness and completeness of these cases based on a variation of the Dividing Cubes algorithm. Since this proof is based on topological arguments and a divide-and-conquer approach, this also sets the stage for developing tessellation cases for higher-order interpolants and for the quadrilinear interpolant in four dimensions. We also demonstrate that, apart from degenerate cases, Nielson's cases are in fact subsets of two basic configurations of the trilinear interpolant.

PrePrint: Case Study on Visualizing Hurricanes Using Illustration-Inspired Techniques

The devastating power of hurricanes was evident during the 2005 hurricane season, the most active season on record. This has prompted increased efforts by researchers to understand the physical processes that underlie the genesis, intensification, and tracks of hurricanes. This research aims at facilitating an improved understanding into the structure of hurricanes with the aid of visualization techniques. Our approach was developed by a mixed team of visualization and domain experts. To better understand these systems, and to explore their representation in Numerical Weather Prediction (NWP) models, we use a variety of illustration-inspired techniques to visualize their structure and time evolution. Illustration-inspired techniques aid in the identification of the amount of vertical wind shear in a hurricane which can help meteorologists predict dissipation. Illustration-style visualization, in combination with standard visualization techniques, helped explore the vortex rollup phenomena and the mesovortices contained within. We evaluated the effectiveness of our visualization with the help of six hurricane experts. The expert evaluation showed that the illustration-inspired techniques were preferred over existing tools. Visualization of the evolution of structural features is a prelude to a deeper visual analysis of the underlying dynamics.