Computer Graphics Forum

Cover image for Vol. 35 Issue 8

Early View (Online Version of Record published before inclusion in an issue)

Edited By: Hao (Richard) Zhang and Min Chen

Impact Factor: 1.542

ISI Journal Citation Reports © Ranking: 2015: 17/106 (Computer Science Software Engineering)

Online ISSN: 1467-8659

  1. Articles

    1. Data-Driven Shape Interpolation and Morphing Editing

      Lin Gao, Shu-Yu Chen, Yu-Kun Lai and Shihong Xia

      Version of Record online: 27 SEP 2016 | DOI: 10.1111/cgf.12991

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      Shape interpolation has many applications in computer graphics such as morphing for computer animation. In this paper, we propose a novel data-driven mesh interpolation method. We adapt patch-based linear rotational invariant coordinates to effectively represent deformations of models in a shape collection, and utilize this information to guide the synthesis of interpolated shapes. Unlike previous data-driven approaches, we use a rotation/translation invariant representation which defines the plausible deformations in a global continuous space. By effectively exploiting the knowledge in the shape space, our method produces realistic interpolation results at interactive rates, outperforming state-of-the-art methods for challenging cases.

  2. Major Revision from Eurographics Conference

    1. Real-Time Oil Painting on Mobile Hardware

      Tuur Stuyck, Fang Da, Sunil Hadap and Philip Dutré

      Version of Record online: 26 SEP 2016 | DOI: 10.1111/cgf.12995

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      Graphical Abstract: This paper presents a realistic digital oil painting system, specifically targeted at the real-time performance on highly resource-constrained portable hardware such as tablets and iPads. To effectively use the limited computing power, we develop an efficient adaptation of the shallow water equations that models all the characteristic properties of oil paint. The pigments are stored in a multi-layered structure to model the peculiar nature of pigment mixing in oil paint. The user experience ranges from thick shape-retaining strokes to runny diluted paint that reacts naturally to the gravity set by tablet orientation. Finally, the paint is rendered in real time using a combination of carefully chosen efficient rendering techniques.

  3. Articles

    1. Stress-Constrained Thickness Optimization for Shell Object Fabrication

      Haiming Zhao, Weiwei Xu, Kun Zhou, Yin Yang, Xiaogang Jin and Hongzhi Wu

      Version of Record online: 22 SEP 2016 | DOI: 10.1111/cgf.12986

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      We present an approach to fabricate shell objects with thickness parameters, which are computed to maintain the user-specified structural stability. Given a boundary surface and user-specified external forces, we optimize the thickness parameters according to stress constraints to extrude the surface. Our approach mainly consists of two technical components: First, we develop a patch-based shell simulation technique to efficiently support the static simulation of extruded shell objects using finite element methods. Second, we analytically compute the derivative of stress required in the sensitivity analysis technique to turn the optimization into a sequential linear programming problem.

    2. Group Modeling: A Unified Velocity-Based Approach

      Z. Ren, P. Charalambous, J. Bruneau, Q. Peng and J. Pettré

      Version of Record online: 20 SEP 2016 | DOI: 10.1111/cgf.12993

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      Crowd simulators are commonly used to populate movie or game scenes in the entertainment industry. Even though it is crucial to consider the presence of groups for the believability of a virtual crowd, most crowd simulations only take into account individual characters or a limited set of group behaviors. We introduce a unified solution that allows for simulations of crowds that have diverse group properties such as social groups, marches, tourists and guides, etc. We extend the Velocity Obstacle approach for agent-based crowd simulations by introducing Velocity Connection; the set of velocities that keep agents moving together while avoiding collisions and achieving goals. We demonstrate our approach to be robust, controllable, and able to cover a large set of group behaviors.

    3. Time-Continuous Quasi-Monte Carlo Ray Tracing

      C.J. Gribel and T. Akenine-Möller

      Version of Record online: 20 SEP 2016 | DOI: 10.1111/cgf.12985

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      Domain-continuous visibility determination algorithms have proved to be very efficient at reducing noise otherwise prevalent in stochastic sampling. Even though they come with an increased overhead in terms of geometrical tests and visibility information management, their analytical nature provides such a rich integral that the pay-off is often worth it. This paper presents a time-continuous, primary visibility algorithm for motion blur aimed at ray tracing.

    4. Virtual Inflation of the Cerebral Artery Wall for the Integrated Exploration of OCT and Histology Data

      S. Glaßer, T. Hoffmann, A. Boese, S. Voß, T. Kalinski, M. Skalej and B. Preim

      Version of Record online: 19 SEP 2016 | DOI: 10.1111/cgf.12994

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      Intravascular imaging provides new insights into the condition of vessel walls. This is crucial for cerebrovascular diseases including stroke and cerebral aneurysms, where it may present an important factor for indication of therapy. In this work, we provide new information of cerebral artery walls by combining ex vivo optical coherence tomography (OCT) imaging with histology data sets. To overcome the obstacles of deflated and collapsed vessels due to the missing blood pressure, the lack of co-alignment as well as the geometrical shape deformations due to catheter probing, we developed the new image processing method virtual inflation.

    5. DYVERSO: A Versatile Multi-Phase Position-Based Fluids Solution for VFX

      Iván Alduán, Angel Tena and Miguel A. Otaduy

      Version of Record online: 6 SEP 2016 | DOI: 10.1111/cgf.12992

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      Many impressive fluid simulation methods have been presented in research papers before. These papers typically focus on demonstrating particular innovative features, but they do not meet in a comprehensive manner the production demands of actual VFX pipelines. VFX artists seek methods that are flexible, efficient, robust and scalable, and these goals often conflict with each other. In this paper, we present a multi-phase particle-based fluid simulation framework, based on the well-known Position-Based Fluids (PBF) method, designed to address VFX production demands.

    6. Stream Line–Based Pattern Search in Flows

      Z. Wang, J. Martinez Esturo, H.-P. Seidel and T. Weinkauf

      Version of Record online: 12 AUG 2016 | DOI: 10.1111/cgf.12990

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      We propose a method that allows users to define flow features in form of patterns represented as sparse sets of stream line segments. Our approach finds similar occurrences in the same or other time steps. Related approaches define patterns using dense, local stencils or support only single segments. Our patterns are defined sparsely and can have a significant extent, i.e., they are integration-based and not local. This allows for a greater flexibility in defining features of interest. Similarity is measured using intrinsic curve properties only, which enables invariance to location, orientation, and scale.

    7. A Phase-Based Approach for Animating Images Using Video Examples

      Ekta Prashnani, Maneli Noorkami, Daniel Vaquero and Pradeep Sen

      Version of Record online: 1 AUG 2016 | DOI: 10.1111/cgf.12940

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      We present a novel approach for animating static images that contain objects that move in a subtle, stochastic fashion (e.g. rippling water, swaying trees, or flickering candles). To do this, our algorithm leverages example videos of similar objects, supplied by the user. Unlike previous approaches which estimate motion fields in the example video to transfer motion into the image, a process which is brittle and produces artefacts, we propose an Eulerian phase-based approach which uses the phase information from the sample video to animate the static image. As is well known, phase variations in a signal relate naturally to the displacement of the signal via the Fourier Shift Theorem. To enable local and spatially varying motion analysis, we analyse phase changes in a complex steerable pyramid of the example video.

  4. Major Revision from Eurographics Conference

    1. 4D Reconstruction of Blooming Flowers

      Qian Zheng, Xiaochen Fan, Minglun Gong, Andrei Sharf, Oliver Deussen and Hui Huang

      Version of Record online: 1 AUG 2016 | DOI: 10.1111/cgf.12989

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      Flower blooming is a beautiful phenomenon in nature as flowers open in an intricate and complex manner whereas petals bend, stretch and twist under various deformations. Flower petals are typically thin structures arranged in tight configurations with heavy self-occlusions. Thus, capturing and reconstructing spatially and temporally coherent sequences of blooming flowers is highly challenging. Early in the process only exterior petals are visible and thus interior parts will be completely missing in the captured data. Utilizing commercially available 3D scanners, we capture the visible parts of blooming flowers into a sequence of 3D point clouds. We reconstruct the flower geometry and deformation over time using a template-based dynamic tracking algorithm. To track and model interior petals hidden in early stages of the blooming process, we employ an adaptively constrained optimization. Flower characteristics are exploited to track petals both forward and backward in time. Our methods allow us to faithfully reconstruct the flower blooming process of different species.

  5. Articles

    1. Dynamically Enriched MPM for Invertible Elasticity

      Fei Zhu, Jing Zhao, Sheng Li, Yong Tang and Guoping Wang

      Version of Record online: 18 JUL 2016 | DOI: 10.1111/cgf.12987

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      We extend the material point method (MPM) for robust simulation of extremely large elastic deformation. This facilitates the application ofMPMtowards a unified solver since its versatility has been demonstrated lately with simulation of variedmaterials. Extending MPM for invertible elasticity requires accounting for several of its inherent limitations. MPM as a meshless method exhibits numerical fracture in large tensile deformations. We eliminate it by augmenting particles with connected material domains. Besides, constant redefinition of the interpolating functions between particles and grid introduces accumulated error which behaves like artificial plasticity. We address this problem by utilizing the Lagrangian particle domains as enriched degrees of freedom for simulation. We also novelly reformulate the computation in reference configuration and investigate inversion handling techniques to ensure the robustness of our method in regime of degenerated configurations.

    2. Visual Quantification of the Circle of Willis: An Automated Identification and Standardized Representation

      H. Miao, G. Mistelbauer, C. Našel and M. E. Gröller

      Version of Record online: 18 JUL 2016 | DOI: 10.1111/cgf.12988

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      This paper presents a method for the visual quantification of cerebral arteries, known as the Circle of Willis (CoW). It is an arterial structure with the responsibility of supplying the brain with blood, however, dysfunctions can lead to strokes. The diagnosis of such a time-critical/urgent event depends on the expertise of radiologists and the applied software tools. They use basic display methods of the volumetric data without any support of advanced image processing and visualization techniques. The goal of this paper is to present an automated method for the standardized description of cerebral arteries in stroke patients in order to provide an overview of the CoW's configuration. This novel representation provides visual indications of problematic areas as well as straightforward comparisons between multiple patients.

    3. Scalable Feature-Preserving Irregular Mesh Coding

      J. El Sayeh Khalil, A. Munteanu, L. Denis, P. Lambert and R. Van de Walle

      Version of Record online: 1 JUL 2016 | DOI: 10.1111/cgf.12938

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      This paper presents a novel wavelet-based transform and coding scheme for irregular meshes. The transform preserves geometric features at lower resolutions by adaptive vertex sampling and retriangulation, resulting in more accurate subsampling and better avoidance of smoothing and aliasing artefacts. By employing octree-based coding techniques, the encoding of both connectivity and geometry information is decoupled from any mesh traversal order, and allows for exploiting the intra-band statistical dependencies between wavelet coefficients. Improvements over the state of the art obtained by our approach are three-fold: (1) improved rate–distortion performance over Wavemesh and IPR for both the Hausdorff and root mean square distances at low-to-mid-range bitrates, most obvious when clear geometric features are present while remaining competitive for smooth, feature-poor models; (2) improved rendering performance at any triangle budget, translating to a better quality for the same runtime memory footprint; (3) improved visual quality when applying similar limits to the bitrate or triangle budget, showing more pronounced improvements than rate–distortion curves.

    4. Visualizing Group Structures in Graphs: A Survey

      Corinna Vehlow, Fabian Beck and Daniel Weiskopf

      Version of Record online: 29 JUN 2016 | DOI: 10.1111/cgf.12872

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      Graph visualizations encode relationships between objects. Abstracting the objects into group structures provides an overview of the data. Groups can be disjoint or overlapping, and might be organized hierarchically. However, the underlying graph still needs to be represented for analyzing the data in more depth. This work surveys research in visualizing group structures as part of graph diagrams. A particular focus is the explicit visual encoding of groups, rather than only using graph layout to indicate groups implicitly. We introduce a taxonomy of visualization techniques structuring the field into four main categories: visual node attributes vary properties of the node representation to encode the grouping, juxtaposed approaches use two separate visualizations, superimposed techniques work with two aligned visual layers, and embedded visualizations tightly integrate group and graph representation.

    5. Synthesis of Human Skin Pigmentation Disorders

      R. S. Barros and M. Walter

      Version of Record online: 28 JUN 2016 | DOI: 10.1111/cgf.12943

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      Changes in the human pigmentary system can lead to imbalances in the distribution of melanin in the skin resulting in artefacts known as pigmented lesions. Our work takes as departing point biological data regarding human skin, the pigmentary system and the melanocytes life cycle and presents a reaction–diffusion model for the simulation of the shape features of human-pigmented lesions. The simulation of such disorders has many applications in dermatology, for instance, to assist dermatologists in diagnosis and training related to pigmentation disorders. Our study focuses, however, on applications related to computer graphics. Thus, we also present a method to seamless blend the results of our simulation model in images of healthy human skin.

    6. Adaptive Physically Based Models in Computer Graphics

      P.-L. Manteaux, C. Wojtan, R. Narain, S. Redon, F. Faure and M.-P. Cani

      Version of Record online: 28 JUN 2016 | DOI: 10.1111/cgf.12941

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      One of the major challenges in physically based modelling is making simulations efficient. Adaptive models provide an essential solution to these efficiency goals. These models are able to self-adapt in space and time, attempting to provide the best possible compromise between accuracy and speed. This survey reviews the adaptive solutions proposed so far in computer graphics. Models are classified according to the strategy they use for adaptation, from time-stepping and freezing techniques to geometric adaptivity in the form of structured grids, meshes and particles.

    7. Spectral Processing of Tangential Vector Fields

      Christopher Brandt, Leonardo Scandolo, Elmar Eisemann and Klaus Hildebrandt

      Version of Record online: 22 JUN 2016 | DOI: 10.1111/cgf.12942

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      We propose a framework for the spectral processing of tangential vector fields on surfaces. The basis is a Fourier-type representation of tangential vector fields that associates frequencies with tangential vector fields. To implement the representation for piecewise constant tangential vector fields on triangle meshes, we introduce a discrete Hodge–Laplace operator that fits conceptually to the prominent cotan discretization of the Laplace–Beltrami operator. Based on the Fourier representation, we introduce schemes for spectral analysis, filtering and compression of tangential vector fields.

    8. Reevaluating Reconstruction Filters for Path-Searching Tasks in 3D

      D. A. T. Roberts and I. Ivrissimtzis

      Version of Record online: 21 JUN 2016 | DOI: 10.1111/cgf.12939

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      In this paper, we present an experiment on stereoscopic direct volume rendering, aiming at understanding the relationship between the choice of reconstruction filter and participant performance on tasks requiring spatial understanding such as 3D path-searching. The focus of our study is on the impact on task performance of the post-aliasing and smoothing produced by the reconstruction filters. We evaluated five reconstruction filters, each under two different transfer functions and two different displays with a wide range of behaviours in terms of post-aliasing and smoothing produced by the reconstruction filters.

    9. Visual Text Analysis in Digital Humanities

      S. Jänicke, G. Franzini, M. F. Cheema and G. Scheuermann

      Version of Record online: 20 JUN 2016 | DOI: 10.1111/cgf.12873

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      In 2005, Franco Moretti introduced Distant Reading to analyse entire literary text collections. This was a rather revolutionary idea compared to the traditional Close Reading, which focuses on the thorough interpretation of an individual work. Both reading techniques are the prior means of Visual Text Analysis. We present an overview of the research conducted since 2005 on supporting text analysis tasks with close and distant reading visualizations in the digital humanities.

    10. Intrinsic Image Decomposition Using Multi-Scale Measurements and Sparsity

      Shouhong Ding, Bin Sheng, Xiaonan Hou, Zhifeng Xie and Lizhuang Ma

      Version of Record online: 6 JUN 2016 | DOI: 10.1111/cgf.12874

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      Automatic decomposition of intrinsic images, especially for complex real-world images, is a challenging under-constrained problem. Thus, we propose a new algorithm that generates and combines multi-scale properties of chromaticity differences and intensity contrast. The key observation is that the estimation of image reflectance, which is neither a pixel-based nor a region-based property, can be improved by using multi-scale measurements of image content. The new algorithm iteratively coarsens a graph reflecting the reflectance similarity between neighbouring pixels.

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