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Now showing 1 - 10 of 36
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    Sketch-based Image-independent Editing of 3D Tumor Segmentations using Variational Interpolation
    (The Eurographics Association, 2012) Heckel, Frank; Braunewell, Stefan; Soza, Grzegorz; Tietjen, Christian; Hahn, Horst K.; Timo Ropinski and Anders Ynnerman and Charl Botha and Jos Roerdink
    In the past years sophisticated automatic segmentation algorithms for various medical image segmentation problems have been developed. However, there are always cases where automatic algorithms fail to provide an acceptable segmentation. In these cases the user needs efficient segmentation correction tools, a problem which has not received much attention in research. Cases to be manually corrected are often particularly difficult and the image does often not provide enough information for segmentation, so we present an image-independent method for intuitive sketch-based editing of 3D tumor segmentations. It is based on an object reconstruction using variational interpolation and can be used in any 3D modality, such as CT or MRI. We also discuss sketch-based editing in 2D as well as a hole-correction approach for variational interpolation. Our manual correction algorithm has been evaluated on 89 segmentations of tumors in CT by 2 technical experts with 6+ years of experience in tumor segmentation and assessment. The experts rated the quality of our correction tool as acceptable or better in 92.1% of the cases. They needed a median number of 4 correction steps with one step taking 0.4s on average.
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    Linear-Time Smoke Animation with Vortex Sheet Meshes
    (The Eurographics Association, 2012) Brochu, Tyson; Keeler, Todd; Bridson, Robert; Jehee Lee and Paul Kry
    We present the first quality physics-based smoke animation method which runs in time approximately linear in the size of the rendered two-dimensional visual detail. Our fundamental representation is a closed triangle mesh surface dividing space between clear air and a uniformly smoky region, on which we compute vortex sheet dynamics to accurately solve inviscid buoyant flow. We handle arbitrary moving no-stick solid boundaries and by default handle an infinite domain. The simulation itself runs in time linear to the number of triangles thanks to the use of a well-conditioned integral equation treatment together with a Fast Multipole Method for linear-time summations, providing excellent performance. Basic zero-albedo smoke rendering, with embedded solids, is easy to implement for interactive rates, and the mesh output can also serve as an extremely compact and detailed input to more sophisticated volume rendering.
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    Smoke Sheets for Graph-Structured Vortex Filaments
    (The Eurographics Association, 2012) Barnat, Alfred; Pollard, Nancy S.; Jehee Lee and Paul Kry
    Smoke is one of the core phenomena which fluid simulation techniques in computer graphics have attempted to capture. It is both well understood mathematically and important in lending realism to computer generated effects. In an attempt to overcome the diffusion inherent to Eulerian grid-based simulators, a technique has recently been developed which represents velocity using a sparse set of vortex filaments. This has the advantage of providing an easily understandable and controllable model for fluid velocity, but is computationally expensive because each filament affects the fluid velocity over an unbounded region of the simulation space. We present an alternative to existing techniques which merge adjacent filament rings, instead allowing filaments to form arbitrary structures, and we develop a new set of reconnection criteria to take advantage of this filament graph. To complement this technique, we also introduce a method for smoke surface tracking and rendering designed to minimize the number of sample points without introducing excessive diffusion or blurring. Though this representation lends itself to straightforward real-time rendering, we also present a method which renders the thin sheets and curls of smoke as diffuse volumes using any GPU capable of supporting geometry shaders.
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    Tessellation-Independent Smooth Shadow Boundaries
    (The Eurographics Association and Blackwell Publishing Ltd., 2012) Mattausch, Oliver; Scherzer, Daniel; Wimmer, Michael; Igarashi, Takeo; Fredo Durand and Diego Gutierrez
    We propose an efficient and light-weight solution for rendering smooth shadow boundaries that do not reveal the tessellation of the shadow-casting geometry. Our algorithm reconstructs the smooth contours of the underlying mesh and then extrudes shadow volumes from the smooth silhouettes to render the shadows. For this purpose we propose an improved silhouette reconstruction using the vertex normals of the underlying smooth mesh. Then our method subdivides the silhouette loops until the contours are sufficiently smooth and project to smooth shadow boundaries. This approach decouples the shadow smoothness from the tessellation of the geometry and can be used to maintain equally high shadow quality for multiple LOD levels. It causes only a minimal change to the fill rate, which is the well-known bottleneck of shadow volumes, and hence has only small overhead.
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    Computing Extremal Quasiconformal Maps
    (The Eurographics Association and Blackwell Publishing Ltd., 2012) Weber, Ofir; Myles, Ashish; Zorin, Denis; Eitan Grinspun and Niloy Mitra
    Conformal maps are widely used in geometry processing applications. They are smooth, preserve angles, and are locally injective by construction. However, conformal maps do not allow for boundary positions to be prescribed. A natural extension to the space of conformal maps is the richer space of quasiconformal maps of bounded conformal distortion. Extremal quasiconformal maps, that is, maps minimizing the maximal conformal distortion, have a number of appealing properties making them a suitable candidate for geometry processing tasks. Similarly to conformal maps, they are guaranteed to be locally bijective; unlike conformal maps however, extremal quasiconformal maps have sufficient flexibility to allow for solution of boundary value problems. Moreover, in practically relevant cases, these solutions are guaranteed to exist, are unique and have an explicit characterization. We present an algorithm for computing piecewise linear approximations of extremal quasiconformal maps for genus-zero surfaces with boundaries, based on Teichmüller's characterization of the dilatation of extremal maps using holomorphic quadratic differentials.We demonstrate that the algorithm closely approximates the maps when an explicit solution is available and exhibits good convergence properties for a variety of boundary conditions.
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    Visual Language Generalization for Procedural Modeling of Buildings
    (The Eurographics Association, 2012) Barroso, Santiago; Patow, Gustavo; Isabel Navazo and Gustavo Patow
    Procedural modeling has become the accepted standard for the creation of detailed large scenes, in particular urban landscapes. With the introduction of visual languages there has been a huge leap forward in terms of usability, but there is still need of more sophisticated tools to simplify the development process. In this paper we present extensions to the visual modeling of procedural buildings, which adapt concepts from general purpose programming languages, with the objective of providing higher descriptive power. In particular, we present the concepts of visual modules, parameter linking and the possibility to seamlessly add abstract parameter templates to the designer visual toolbox. We base our demonstrations on a new visual language created for volume-based models like historic architectonic structures (aqueducts, churches, cathedrals, etc.), which cannot be modeled as 2D facades because of the intrinsic volumetric structure of these construction (e.g. vaults or arches).
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    Can Mean-Curvature Flow be Modified to be Non-singular?
    (The Eurographics Association and Blackwell Publishing Ltd., 2012) Kazhdan, Michael; Solomon, Jake; Ben-Chen, Mirela; Eitan Grinspun and Niloy Mitra
    This work considers the question of whether mean-curvature flow can be modified to avoid the formation of singularities. We analyze the finite-elements discretization and demonstrate why the original flow can result in numerical instability due to division by zero. We propose a variation on the flow that removes the numerical instability in the discretization and show that this modification results in a simpler expression for both the discretized and continuous formulations. We discuss the properties of the modified flow and present empirical evidence that not only does it define a stable surface evolution for genus-zero surfaces, but that the evolution converges to a conformal parameterization of the surface onto the sphere.
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    Computing Curvature Information on 3D Meshes from Cultural Heritage Artifacts
    (The Eurographics Association, 2012) López, L.; Torres, J. C.; Soler, F.; Isabel Navazo and Gustavo Patow
    The curvature provides essential information about the features of a object surface. This poster presents an operation that makes it possible to generate the curvature information in the CHiSEL system.
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    Interactive Residual Stress Modeling for Soft Tissue Simulation
    (The Eurographics Association, 2012) Wu, Jun; Bürger, Kai; Westermann, Rüdiger; Dick, Christian; Timo Ropinski and Anders Ynnerman and Charl Botha and Jos Roerdink
    Residual stress is the stress which remains in a deformable body in the absence of external forces. Due to the release of residual stress after cutting, soft tissues will shrink and the wound will open. Thus, to realistically simulate soft tissue deformations due to cutting, a model for the residual stress in a patient body is needed. In this paper we present an interactive method to compute a physically meaningful patient-specific residual stress distribution. With our method, by using their experience doctors can sketch directional stress strokes and specify stress magnitudes at a few control points on the body surface. The residual stress is then immediately computed from these inputs and visualized by displaying the deformations of a set of control cuts on the body. In a visually guided session, the user can further modify the initial strokes and magnitudes until a satisfactory result is obtained. We demonstrate the potential of the proposed method for virtual cut simulation by showing the variations of wound openings depending on the residual stress distribution.
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    Stream Surface Parametrization by Flow-Orthogonal Front Lines
    (The Eurographics Association and Blackwell Publishing Ltd., 2012) Schulze, Maik; Germer, Tobias; Rössl, Christian; Theisel, Holger; Eitan Grinspun and Niloy Mitra
    The generation of discrete stream surfaces is an important and challenging task in scientific visualization, which can be considered a particular instance of geometric modeling. The quality of numerically integrated stream surfaces depends on a number of parameters that can be controlled locally, such as time step or distance of adjacent vertices on the front line. In addition there is a parameter that cannot be controlled locally: stream surface meshes tend to show high quality, well-shaped elements only if the current front line is "globally" approximately perpendicular to the flow direction. We analyze the impact of this geometric property and present a novel solution a stream surface integrator that forces the front line to be perpendicular to the flow and that generates quaddominant meshes with well-shaped and well-aligned elements. It is based on the integration of a scaled version of the flow field, and requires repeated minimization of an error functional along the current front line. We show that this leads to computing the 1-dimensional kernel of a bidiagonal matrix: a linear problem that can be solved efficiently. We compare our method with existing stream surface integrators and apply it to a number of synthetic and real world data sets.