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A Texture Synthesis Method for Liquid Animations

Adam W. Bargteil, Funshing Sin, Jonathan E. Michaels, Tolga G. Goktekin, and James F. O'Brien

Abstract

In this paper we present a method for synthesizing textures on animated liquid surfaces generated by a physically based fluid simulation system. Rather than advecting texture coordinates on the surface, we synthesize a new texture for every frame. We synthesize the texture with an optimization procedure which attempts to match the surface texture to an input sample texture. By synthesizing a new texture for every frame, our method is able to overcome the discontinuities and distortions of an advected parameterization. We achieve temporal coherence by initializing the surface texture with color values advected from the surface at the previous frame and including these colors in the energy function used during optimization.

Citation

Adam W. Bargteil, Funshing Sin, Jonathan E. Michaels, Tolga G. Goktekin, and James F. O'Brien. A texture synthesis method for liquid animations. In 2006 ACM SIGGRAPH / Eurographics Symposium on Computer Animation, September 2006. [BiBTeX]

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Physics-Based Motion Retiming

James McCann, Nancy S. Pollard, and Siddhartha S. Srinivasa

Abstract

By changing only the playback timing of a motion sequence, an animator can achieve a variety of effects that alter our perception of an event. In some scenarios, it may be important to consider physical properties of the motion when retiming (e.g., to preserve physical plausibility). However, existing retiming solutions can be quite time consuming when physical parameters are considered. This paper presents an interactive method for creating optimal motion retimings that takes into account physically based constraints and objective functions. We achieve fast performance through a precomputation phase where constraints are projected into the two-dimensional space of velocities and accelerations along the input motion path. Unlike previous approaches, our precomputation technique allows for rapid computation of plausible contact forces that result from retiming, and it also accommodates changing physical parameters. We demonstrate our approach by creating physically plausible results for changes in motion duration, manipulations of the gravity vector, and modifications of character limb masses.

Citation

James McCann, Nancy S. Pollard, and Siddhartha S. Srinivasa. Physics-based motion retiming. In 2006 ACM SIGGRAPH / Eurographics Symposium on Computer Animation, September 2006. [BiBTeX]

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Precomputed Search Trees: Planning for Interactive Goal-Driven Animation

Manfred Lau and James J. Kuffner

Abstract

We present a novel approach for interactively synthesizing motions for characters navigating in complex environments. We focus on the runtime efficiency for motion generation, thereby enabling the interactive animation of a large number of characters simultaneously. The key idea is to precompute search trees of motion clips that can be applied to arbitrary environments. Given a navigation goal relative to a current body position, the best available solution paths and motion sequences can be efficiently extracted during runtime through a series of table lookups. For distant start and goal positions, we first use a fast coarse-level planner to generate a rough path of intermediate sub-goals to guide each iteration of the runtime lookup phase. We demonstrate the efficiency of our technique across a range of examples in an interactive application with multiple autonomous characters navigating in dynamic environments. Each character responds in real-time to arbitrary user changes to the environment obstacles or navigation goals. The runtime phase is more than two orders of magnitude faster than existing planning methods or traditional motion synthesis techniques. Our technique is not only useful for autonomous motion generation in games, virtual reality, and interactive simulations, but also for animating massive crowds of characters offline for special effects in movies.

Citation

Manfred Lau and James J. Kuffner. Precomputed search trees: Planning for interactive goal-driven animation. In 2006 ACM SIGGRAPH / Eurographics Symposium on Computer Animation, pages 299–308, September 2006. [BiBTeX]

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Acquiring Scattering Properties of Participating Media by Dilution

Srinivasa Narasimhan, Mohit Gupta, Craig Donner, Ravi Ramamoorthi, Shree K. Nayar, and Henrik Wann Jensen

Abstract

The visual world around us displays a rich set of volumetric effects due to participating media. The appearance of these media is governed by several physical properties such as particle densities, shapes and sizes, which must be input (directly or indirectly) to a rendering algorithm to generate realistic images. While there has been significant progress in developing rendering techniques (for instance, volumetric Monte Carlo methods and analytic approximations), there are very few methods that measure or estimate these properties for media that are of relevance to computer graphics. In this paper, we present a simple device and technique for robustly estimating the properties of a broad class of participating media that can be either (a) diluted in water such as juices, beverages, paints and cleaning supplies, or (b) dissolved in water such as powders and sugar/salt crystals, or (c) suspended in water such as impurities. The key idea is to dilute the concentrations of the media so that single scattering effects dominate and multiple scattering becomes negligible, leading to a simple and robust estimation algorithm. Furthermore, unlike previous approaches that require complicated or separate measurement setups for different types or properties of media, our method and setup can be used to measure media with a complete range of absorption and scattering properties from a single HDR photograph. Once the parameters of the diluted medium are estimated, a volumetric Monte Carlo technique may be used to create renderings of any medium concentration and with multiple scattering. We have measured the scattering parameters of forty commonly found materials, that can be immediately used by the computer graphics community. We can also create realistic images of combinations or mixtures of the original measured materials, thus giving the user a wide flexibility in making realistic images of participating media.

Citation

Srinivasa Narasimhan, Mohit Gupta, Craig Donner, Ravi Ramamoorthi, Shree K. Nayar, and Henrik Wann Jensen. Acquiring scattering properties of participating media by dilution. ACM Transactions on Graphics (SIGGRAPH 2006), 25(3), August 2006. [BiBTeX]

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Capturing and Animating Skin Deformation in Human Motion

Sang Il Park and Jessica K. Hodgins

Abstract

During dynamic activities, the surface of the human body moves in many subtle but visually significant ways: bending, bulging, jiggling, and stretching. We present a technique for capturing and animating those motions using a commercial motion capture system and approximately 350 markers. Although the number of markers is significantly larger than that used in conventional motion capture, it is only a sparse representation of the true shape of the body. We supplement this sparse sample with a detailed, actor-specific surface model. The motion of the skin can then be computed by segmenting the markers into the motion of a set of rigid parts and a residual deformation (approximated first as a quadratic transformation and then with radial basis functions). We demonstrate the power of this approach by capturing flexing muscles, high frequency motions, and abrupt decelerations on several actors. We compare these results both to conventional motion capture and skinning and to synchronized video of the actors.

Citation

Sang Il Park and Jessica K. Hodgins. Capturing and animating skin deformation in human motion. ACM Transactions on Graphics (SIGGRAPH 2006), 25(3), August 2006. [BiBTeX]

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Hierarchical Simplification of City Models to Maintain Urban Legibility

Remco Chang, Thomas Butkiewicz, Caroline Ziemkiewicz, Zachary Wartell, Nancy S. Pollard, and William Ribarsky

Abstract

For 3D global visualization systems such as Google Earth, it is important to be able to render city-sized collections of relatively simple building models at fast speeds without losing spatial coherence. Since traditional mesh simplification algorithms are not designed for collections of simple models, we introduce a method of simplification through merging of similar objects. We incorporate the concept of "urban legibility" from architecture and city-planning as a guideline for simplifying city models. Our algorithm can be broken down into five steps. Hierarchical clustering, cluster merging, polyline simplification, and hierarchical texturing are performed during pre-processing, while at runtime, the levels-of-detail (LOD) process selects the appropriate models to render. It is our belief that many applications can benefit from our algorithm. Google Earth (and other 3D geographical information systems) as well as any spatial data visualization applications (including scatter plots) can all use logical, simplified clusters to represent large amounts of spatial information.

Citation

Remco Chang, Thomas Butkiewicz, Caroline Ziemkiewicz, Zachary Wartell, Nancy S. Pollard, and William Ribarsky. Hierarchical simplification of city models to maintain urban legibility. In Proceedings of the SIGGRAPH 2006 Conference on Sketches & Applications. ACM Press, August 2006. [BiBTeX]

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Mesh Ensemble Motion Graphs: Data-driven Mesh Animation with Constraints

Doug L. James, Christopher D. Twigg, Andrew Cove, and Robert Y. Wang

Abstract

We describe a technique for using space-time cuts to smoothly transition between stochastic mesh animation clips involving numerous deformable mesh groups while subject to physical constraints. These transitions are used to construct Mesh Ensemble Motion Graphs for interactive data-driven animation of high-dimensional mesh animation datasets, such as those arising from expensive physical simulations of deformable objects blowing in the wind. We formulate the transition computation as an integer programming problem, and introduce a novel randomized algorithm to compute transitions subject to geometric noninterpenetration constraints.

Citation

Doug L. James, Christopher D. Twigg, Andrew Cove, and Robert Y. Wang. Mesh ensemble motion graphs: Data-driven mesh animation with constraints. In Proceedings of the SIGGRAPH 2006 Conference on Sketches & Applications. ACM Press, August 2006. [BiBTeX]

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Precomputed Acoustic Transfer: Output-sensitive, accurate sound generation for geometrically complex vibration sources

Doug L. James, Jernej Barbič, and Dinesh K. Pai

Abstract

Simulating sounds produced by realistic vibrating objects is challenging because sound radiation involves complex diffraction and interreflection effects that are very perceptible and important. These wave phenomena are well understood, but have been largely ignored in computer graphics due to the high cost and complexity of computing them at audio rates. We describe a new algorithm for real-time synthesis of realistic sound radiation from rigid objects. We start by precomputing the linear vibration modes of an object, and then relate each mode to its sound pressure field, or acoustic transfer function, using standard methods from numerical acoustics. Each transfer function is then approximated to a specified accuracy using low-order multipole sources placed near the object. We provide a low-memory, multilevel, randomized algorithm for optimized source placement that is suitable for complex geometries. At runtime, we can simulate new interaction sounds by quickly summing contributions from each modes equivalent multipole sources. We can efficiently simulate global effects such as interreflection and changes in sound due to listener location. The simulation costs can be dynamically traded-off for sound quality. We present several examples of sound generation from physically based animations.

Citation

Doug L. James, Jernej Barbič, and Dinesh K. Pai. Precomputed acoustic transfer: Output-sensitive, accurate sound generation for geometrically complex vibration sources. ACM Transactions on Graphics (SIGGRAPH 2006), 25(3), August 2006. [BiBTeX]

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Quantitative Evaluation on Near Regular Texture Synthesis

Wen-Chieh Lin, James H. Hays, Chenyu Wu, Vivek Kwatra, and Yanxi Liu

Abstract

Near regular textures are pervasive in man-made and natural world. Their global regularity and local randomness pose new difficulties to the state of the art texture analysis and synthesis algorithms. We carry out a systematic comparison study on the performance of four texture synthesis algorithms on near-regular textures. Our results confirm that faithful near-regular texture synthesis remains a challenging problem for the state of the art general purpose texture synthesis algorithms. In addition, we provide comparison of human perception with computer evaluations on the quality of the texture synthesis results.

Citation

Wen-Chieh Lin, James H. Hays, Chenyu Wu, Vivek Kwatra, and Yanxi Liu. Quantitative evaluation on near regular texture synthesis. In Computer Vision and Pattern Recognition Conference (CVPR '06), volume 1, pages 427–434, June 2006. [BiBTeX]

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Discovering Texture Regularity as a Higher-Order Correspondence Problem

James H. Hays, Marius Leordeanu, Alexei A. Efros, and Yanxi Liu

Abstract

Understanding texture regularity in real images is a challenging computer vision task. We propose a higher-order feature matching algorithm to discover the lattices of near-regular textures in real images. The underlying lattice of a near-regular texture identifies all of the texels as well as the global topology among the texels. A key contribution of this paper is to formulate lattice-finding as a correspondence problem. The algorithm finds a plausible lattice by iteratively proposing texels and assigning neighbors between the texels. Our matching algorithm seeks assignments that maximize both pair-wise visual similarity and higher-order geometric consistency. We approximate the optimal assignment using a recently developed spectral method. We successfully discover the lattices of a diverse set of unsegmented, real-world textures with significant geometric warping and large appearance variation among texels.

Citation

James H. Hays, Marius Leordeanu, Alexei A. Efros, and Yanxi Liu. Discovering texture regularity as a higher-order correspondence problem. In 9th European Conference on Computer Vision, May 2006. [BiBTeX]

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