SCA 13: Eurographics/SIGGRAPH Symposium on Computer Animation

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https://diglib.eg.org/handle/10.2312/14887

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A Hybrid Lagrangian-Eulerian Formulation for Bubble Generation and Dynamics
(ACM SIGGRAPH / Eurographics Association, 2013) Patkar, Saket; Aanjaneya, Mridul; Karpman, Dmitriy; Fedkiw, Ronald; Theodore Kim and Robert Sumner
We present a hybrid Lagrangian-Eulerian framework for simulating both small and large scale bubble dynamics, where the bubbles can grow or shrink in volume as dictated by pressure forces in the surrounding fluid. Small under-resolved bubbles are evolved using Lagrangian particles that are monolithically two-way coupled to the surrounding flow in a manner that closely approximates the analytic bubble oscillation frequency while converging to the analytic volume as predicted by the well-known Rayleigh-Plesset equation. We present a novel scheme for interconverting between these underresolved Lagrangian bubbles and larger well-resolved bubbles that are modeled with a traditional Eulerian level set approach. We also present a novel seeding mechanism to realistically generate bubbles when simulating fluid structure interaction with complex objects such as ship propellers. Moreover, our framework for bubble generation is general enough to be incorporated into all grid-based as well as particle-based fluid simulation methods.
Diverse Motion Variations for Physics-based Character Animation
(ACM SIGGRAPH / Eurographics Association, 2013) Agrawal, Shailen; Shen, Shuo; Panne, Michiel van de; Theodore Kim and Robert Sumner
We present an optimization framework for generating diverse variations of physics-based character motions. This allows for the automatic synthesis of rich variations in style for simulated jumps, flips, and walks. While well-posed motion optimization problems result in a single optimal motion, we explore using underconstrained motion descriptions and then optimizing for diversity. As input, the method takes a parameterized controller for a successful motion instance, a set of constraints that should be preserved, and a pairwise distance metric between motions. An offline optimization then produces a highly diverse set of motion styles for the same task. We demonstrate results for a variety of 2D and 3D physics-based motions and show that this approach can generate compelling new motions.
A Level Set Method for Ductile Fracture
(ACM SIGGRAPH / Eurographics Association, 2013) Hegemann, Jan; Jiang, Chenfanfu; Schroeder, Craig; Teran, Joseph M.; Theodore Kim and Robert Sumner
We utilize the shape derivative of the classical Griffith's energy in a level set method for the simulation of dynamic ductile fracture. The level set is defined in the undeformed configuration of the object, and its evolution is designed to represent a transition from undamaged to failed material. No re-meshing is needed since the resulting topological changes are handled naturally by the level set method. We provide a new mechanism for the generation of fragments of material during the progression of the level set in the Griffith's energy minimization. Collisions between different material pieces are resolved with impulses derived from the material point method over a background Eulerian grid. This provides a stable means for colliding with embedded interfaces. Simulation of corotational elasticity is based on an implicit finite element discretization.
Data-driven Glove Calibration for Hand Motion Capture
(ACM SIGGRAPH / Eurographics Association, 2013) Wang, Yingying; Neff, Michael; Theodore Kim and Robert Sumner
Hand motion is an important component of human motion, playing a central role in communication. However, it is difficult to capture hand motion optically, especially in conjunction with full body motion. Due to a lack of appropriate calibration methods, data gloves also do not provide sufficiently accurate hand motion. In this paper, we present a novel glove calibration approach that can map raw sensor readings to hand motion data with both accurate joint rotations and fingertip positions. Our method elegantly handles the sensor coupling problem by treating calibration as a flexible mapping from sensor readings to joint rotations. A sampling process collects data tuples according to accuracy requirements, and organizes all the tuples in a training set. From these data, a specially designed Gaussian Process Regression model is trained to infer the calibration function, and the learned model can be used to calibrate new sensor readings. For real-time hand motion capture, a sparse approximation of the model is used to enhance performance. Evaluation experiments demonstrate that our approach provides significantly better results that have more accurate hand shapes and fingertip positions, compared to other calibration methods.
Control of Rotational Dynamics for Ground Behaviors
(ACM SIGGRAPH / Eurographics Association, 2013) Brown, David F.; Macchietto, Adriano; Yin, KangKang; Zordan, Victor; Theodore Kim and Robert Sumner
This paper proposes a physics-based framework to generate rolling behaviors with significant rotational components. The proposed technique is a general approach for guiding coordinated action that can be layered over existing control architectures through the purposeful regulation of specific whole-body features. Namely, we apply control for rotation through the specification and execution of specific desired 'rotation indices' for whole-body orientation, angular velocity and angular momentum control. We account for the stylistic components of behaviors through reference posture control. The novelty of the described work includes control over behaviors with considerable rotational components as well as a number of characteristics useful for general control, such as flexible posture tracking and contact control planning.
Multi-Domain Real-time Planning in Dynamic Environments
(ACM SIGGRAPH / Eurographics Association, 2013) Kapadia, Mubbasir; Beacco, Alejandro; Garcia, Francisco; Reddy, Vivek; Pelechano, Nuria; Badler, Norman I.; Theodore Kim and Robert Sumner
This paper presents a real-time planning framework for multicharacter navigation that enables the use of multiple heterogeneous problem domains of differing complexities for navigation in large, complex, dynamic virtual environments. The original navigation problem is decomposed into a set of smaller problems that are distributed across planning tasks working in these different domains. An anytime dynamic planner is used to efficiently compute and repair plans for each of these tasks, while using plans in one domain to focus and accelerate searches in more complex domains. We demonstrate the benefits of our framework by solving many challenging multi-agent scenarios in complex dynamic environments requiring space-time precision and explicit coordination between interacting agents, by accounting for dynamic information at all stages of the decision-making process.
SPREAD : Sound Propagation and Perception for Autonomous Agents in Dynamic Environments
(ACM SIGGRAPH / Eurographics Association, 2013) Huang, Pengfei; Kapadia, Mubbasir; Badler, Norman I.; Theodore Kim and Robert Sumner
The perception of sensory information and its impact on behavior is a fundamental component of being human. While visual perception is considered for navigation, collision, and behavior selection, the acoustic domain is relatively unexplored. Recent work in acoustics focuses on synthesizing sound in 3D environments; however, the perception of acoustic signals by a virtual agent is a useful and realistic adjunct to any behavior selection mechanism. In this paper, we present SPREAD, a novel agent-based sound perception model using a discretized sound packet representation with acoustic features including amplitude, frequency range, and duration. SPREAD simulates how sound packets are propagated, attenuated, and degraded as they traverse the virtual environment. Agents perceive and classify the sounds based on the locally-received packet set using a hierarchical clustering scheme, and have individualized hearing and understanding of their surroundings. Using this model, we demonstrate several simulations that greatly enrich controls and outcomes.
One-to-Many: Example-Based Mesh Animation Synthesis
(ACM SIGGRAPH / Eurographics Association, 2013) Zheng, Changxi; Theodore Kim and Robert Sumner
We propose an example-based approach for synthesizing diverse mesh animations. Provided a short clip of deformable mesh animation, our method synthesizes a large number of different animations of arbitrary length. Combining an automatically inferred linear blending skinning (LBS) model with a PCA-based model reduction, our method identifies possible smooth transitions in the example sequence. To create smooth transitions, we synthesize reduced deformation parameters based on a set of characteristic key vertices on the mesh. Furthermore, by analyzing cut nodes on a graph built upon the LBS model, we are able to decompose the mesh into independent components. Motions of these components are synthesized individually and assembled together. Our method has the complexity independent from mesh resolutions, enabling efficient generation of arbitrarily long animations without tedious parameter tuning and heavy computation. We evaluate our method on various animation examples, and demonstrate that numerous diverse animations can be generated from each single example.
Geodesic Voxel Binding for Production Character Meshes
(ACM SIGGRAPH / Eurographics Association, 2013) Dionne, Olivier; Lasa, Martin de; Theodore Kim and Robert Sumner
We propose a fully automatic method for specifying influence weights for closed-form skinning methods, such as linear blend skinning. Our method is designed to work with production meshes that may contain non-manifold geometry, be non-watertight, have intersecting triangles, or be comprise of multiple connected components. Starting from a character rest pose mesh and skeleton hierarchy, we first voxelize the input geometry. The resulting voxelization is then used to calculate binding weights, based on the geodesic distance between each voxel lying on a skeleton ''bone'' and all non-exterior voxels. This yields smooth weights at interactive rates, without time-constants, iteration parameters, or costly optimization at bind or pose time. By decoupling weight assignment from distance computation we make it possible to modify weights interactively, at pose time, without additional pre-processing or computation. This allows artists to assess impact of weight selection in the context in which they are used.
Differential Blending for Expressive Sketch-Based Posing
(ACM SIGGRAPH / Eurographics Association, 2013) Öztireli, A. Cengiz; Baran, Ilya; Popa, Tiberiu; Dalstein, Boris; Sumner, Robert W.; Gross, Markus; Theodore Kim and Robert Sumner
Generating highly expressive and caricatured poses can be difficult in 3D computer animation because artists must interact with characters indirectly through complex character rigs. Furthermore, since caricatured poses often involve large bends and twists, artifacts arise with traditional skinning algorithms that are not designed to blend large, disparate rotations and cannot represent extremely large rotations. To overcome these problems, we introduce a differential blending algorithm that can successfully encode and blend large transformations, overcoming the inherent limitation of previous skeletal representations. Based on this blending method, we illustrate a sketch-based interface that supports curved bones and implements the line-of-action concept from hand-drawn animation to create expressive poses in 3D animation. By interpolating stored differential transformations across temporal keyframes, our system also generates caricatured animation. We present a detailed technical analysis of our differential blending algorithm and show several posing and animation results created using our system to demonstrate the utility of our method in practice.
Relationship Descriptors for Interactive Motion Adaptation
(ACM SIGGRAPH / Eurographics Association, 2013) Al-Asqhar, Rami Ali; Komura, Taku; Choi, Myung Geol; Theodore Kim and Robert Sumner
This paper presents an interactive motion adaptation scheme for close interactions between skeletal characters and mesh structures, such as moving through restricted environments, and manipulating objects. This is achieved through a new spatial relationship-based representation, which describes the kinematics of the body parts by the weighted sum of translation vectors relative to points selectively sampled over the surfaces of the mesh structures. In contrast to previous discrete representations that either only handle static spatial relationships, or require offline, costly optimization processes, our continuous framework smoothly adapts the motion of a character to large updates of the mesh structures and character morphologies on-the-fly, while preserving the original context of the scene. The experimental results show that our method can be used for a wide range of applications, including motion retargeting, interactive character control and deformation transfer for scenes that involve close interactions. Our framework is useful for artists who need to design animated scenes interactively, and modern computer games that allow users to design their own characters, objects and environments.
Fast Deformation of Volume Data Using Tetrahedral Mesh Rasterization
(ACM SIGGRAPH / Eurographics Association, 2013) Gascon, Jorge; Espadero, Jose M.; Perez, Alvaro G.; Torres, Rosell; Otaduy, Miguel A.; Theodore Kim and Robert Sumner
Many inherently deformable structures, such as human anatomy, are often represented using a regular volumetric discretization, e.g., in medical imaging. While deformation algorithms employ discretizations that deform themselves along with the material, visualization algorithms are optimized for regular undeformed discretizations. In this paper, we propose a method to transform highresolution volume data embedded in a deformable tetrahedral mesh. We cast volume deformation as a problem of tetrahedral rasterization with 3D texture mapping. Then, the core of our solution to volume data deformation is a very fast algorithm for tetrahedral rasterization. We perform rasterization as a massively parallel operation on target voxels, and we minimize the number of voxels to be handled using a multi-resolution culling approach. Our method allows the deformation of volume data with over 20 million voxels at interactive rates.
Virtual Character Performance From Speech
(ACM SIGGRAPH / Eurographics Association, 2013) Marsella, Stacy; Xu, Yuyu; Lhommet, Margaux; Feng, Andrew; Scherer, Stefan; Shapirok, Ari; Theodore Kim and Robert Sumner
We demonstrate a method for generating a 3D virtual character performance from the audio signal by inferring the acoustic and semantic properties of the utterance. Through a prosodic analysis of the acoustic signal, we perform an analysis for stress and pitch, relate it to the spoken words and identify the agitation state. Our rule-based system performs a shallow analysis of the utterance text to determine its semantic, pragmatic and rhetorical content. Based on these analyses, the system generates facial expressions and behaviors including head movements, eye saccades, gestures, blinks and gazes. Our technique is able to synthesize the performance and generate novel gesture animations based on coarticulation with other closely scheduled animations. Because our method utilizes semantics in addition to prosody, we are able to generate virtual character performances that are more appropriate than methods that use only prosody. We perform a study that shows that our technique outperforms methods that use prosody alone.
Preface and Table of Contents
(ACM SIGGRAPH / Eurographics Association, 2013) Theodore Kim and Robert Sumner
Chimera Grids for Water Simulation
(ACM SIGGRAPH / Eurographics Association, 2013) English, R. Elliot; Qiu, Linhai; Yu, Yue; Fedkiw, Ronald; Theodore Kim and Robert Sumner
We introduce a new method for large scale water simulation using Chimera grid embedding, which discretizes space with overlapping Cartesian grids that translate and rotate in order to decompose the domain into different regions of interest with varying spatial resolutions. Grids can track both fluid features and solid objects, allowing for dynamic spatial adaptivity without remeshing or repartitioning the domain. We solve the inviscid incompressible Navier- Stokes equations with an arbitrary-Lagrangian-Eulerian style semi- Lagrangian advection scheme and a monolithic SPD Poisson solver. We modify the particle level set method in order to adapt it to Chimera grids including particle treatment across grid boundaries with disparate cell sizes, and strategies to deal with locality in the implementation of the level set and fast marching algorithms. We use a local Voronoi mesh construction to solve for pressure and address a number of issues that arise with the treatment of the velocity near the interface. The resulting method is highly scalable on distributed parallel architectures with minimal communication costs.
Modeling and Animating Myriapoda: A Real-Time Kinematic/Dynamic Approach
(ACM SIGGRAPH / Eurographics Association, 2013) Fang, Jingyi; Jiang, Chenfanfu; Terzopoulos, Demetri; Theodore Kim and Robert Sumner
Unlike two, four, six, and eight legged animals, Myriapoda-i.e., centipedes, millipedes, etc.-have been largely overlooked in the computer graphics literature. We present an artificial life framework for modeling these arthropods and animating their locomotive behavior over regular or irregular surfaces in real time with compelling physical and biological realism. Our hybrid approach combines kinematic and dynamic simulation, as well as a decentralized, distributed leg control system whose emergent behavior is suitable for animating simulated myriapoda of different morphologies with the characteristically vivid leg wave patterns of their biological counterparts. The simulated creature's antennae sense its virtual environment and the sensory information guides its adaptive behaviors, including obstacle avoidance and foraging.
Velocity-Based Modeling of Physical Interactions in Multi-Agent Simulations
(ACM SIGGRAPH / Eurographics Association, 2013) Kim, Sujeong; Guy, Stephen J.; Manocha, Dinesh; Theodore Kim and Robert Sumner
We present an interactive algorithm to model physics-based interactions in multi-agent simulations. Our approach is capable of modeling both physical forces and interactions between agents and obstacles, while allowing the agents to anticipate and avoid collisions for local navigation. We combine velocity-based collision-avoidance algorithms with external physical forces. The overall formulation can approximately simulate various physical e?ects, including collisions, pushing, deceleration and resistive forces. We have integrated our approach with an open-source physics engine and use the resulting system to model plausible behaviors of and interactions among large numbers of agents in dense environments. Our algorithm can simulate a few thousand agents at interactive rates and can generate many emergent behaviors. The overall approach is useful for interactive applications that require plausible physical behavior, including games and virtual worlds.
Consistent Surface Model for SPH-based Fluid Transport
(ACM SIGGRAPH / Eurographics Association, 2013) Orthmann, Jens; Hochstetter, Hendrik; Bader, Julian; Bayraktar, Serkan; Kolb, Andreas; Theodore Kim and Robert Sumner
Surface effects play an essential role in fluid simulations. A vast number of dynamics including wetting of surfaces, cleansing, and foam dynamics are based on surface-surface and surface-bulk interactions, which in turn rely on a robust surface computation. In this paper we introduce a conservative Lagrangian formulation of surface effects based upon incompressible smoothed particle hydrodynamics (SPH). The key concept of our approach is to realize an implicit definition of the fluid's (free) surface by assigning each particle a value estimating its surface area. Based on this consistent surface representation, a conservative coupling of bulk and surface is achieved. We demonstrate the applicability and robustness of our approach for several types of surface-relevant effects including adsorption, diffusion and reaction kinetics.
Rod Constraints for Simplified Ragdolls
(ACM SIGGRAPH / Eurographics Association, 2013) Lewin, Chris; Thorman, Matt; Waterson, Tom; Williams, Chris; Willis, Phil; Theodore Kim and Robert Sumner
Physics-based animation has become a standard feature in modern games. Typically, the bones in a character's animation rig are each associated with a simulated rigid body, leading to a jointed assembly commonly called a ragdoll. The high density of animation bones in the spine area can cause instability and performance issues, so we are motivated to find a simplified physical representation for this region. We approximate the spine region of a ragdoll as an inextensible elastic curve, building a circular arc constraint based on the Kirchhoff rod model. Our simplified spine shows improved performance and stability over the standard group of socket joints, and proves to be more controllable. To model general elastic rods we use soft position constraints in place of forces, leading to a stable maximal coordinate formulation of inextensible Kirchhoff rods.
Accurate Surface Embedding for Higher Order Finite Elements
(ACM SIGGRAPH / Eurographics Association, 2013) Suwelack, Stefan; Lukarski, Dimitar; Heuveline, Vincent; Dillmann, Rüdiger; Speidel, Stefanie; Theodore Kim and Robert Sumner
In this paper we present a novel approach to efficiently simulate the deformation of highly detailed meshes using higher order finite elements (FE). An efficient algorithm based on non-linear optimization is proposed in order to find the closest point in the curved computational FE mesh for each surface vertex. In order to extrapolate deformations to surface points outside the FE mesh, we introduce a mapping scheme that generates smooth surface deformations and preserves local shape even for low-resolution computational meshes. The mapping is constructed by representing each surface vertex in terms of points on the computational mesh and its distance to the FE mesh in normal direction. A numerical analysis shows that the mapping can be robustly constructed using the proposed non-linear optimization technique. Furthermore it is demonstrated that the numerical complexity of the mapping scheme is linear in the number of surface nodes and independent of the size of the coarse computational mesh.

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