SCA 09: Eurographics/SIGGRAPH Symposium on Computer Animation

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https://diglib.eg.org/handle/10.2312/427

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Accurate Tangential Velocities For Solid Fluid Coupling
(ACM SIGGRAPH / Eurographics Association, 2009) Robinson-Mosher, Avi; English, R. Elliot; Fedkiw, Ronald; Eitan Grinspun and Jessica Hodgins
We propose a novel method for obtaining more accurate tangential velocities for solid fluid coupling. Our method works for both rigid and deformable objects as well as both volumetric objects and thin shells. The fluid can be either one phase such as smoke or two phase such as water with a free surface. The coupling between the solid and the fluid can either be one-way with kinematic solids or fully two-way coupled. The only previous scheme that was general enough to handle both two-way coupling and thin shells required a mass lumping strategy that did not allow for freely flowing tangential velocities. Similar to that previous work, our method prevents leaking of fluid across a thin shell, however unlike that work our method does not couple the tangential velocities in any fashion, allowing for the proper slip independently on each side of the body. Moreover, since it accurately and directly treats the tangential velocity, it does not rely on grid refinement to obtain a reasonable solution. Therefore, it gives a highly improved result on coarse meshes.
Anisotropic Friction for Deformable Surfaces and Solids
(ACM SIGGRAPH / Eurographics Association, 2009) Pabst, Simon; Thomaszewski, Bernhard; Straßer, Wolfgang; Eitan Grinspun and Jessica Hodgins
This paper presents a method for simulating anisotropic friction for deforming surfaces and solids. Frictional contact is a complex phenomenon that fuels research in mechanical engineering, computational contact mechanics, composite material design and rigid body dynamics, to name just a few. Many real-world materials have anisotropic surface properties. As an example, most textiles exhibit direction-dependent frictional behavior, but despite its tremendous impact on visual appearance, only simple isotropic models have been considered for cloth and solid simulation so far. In this work, we propose a simple, application-oriented but physically sound model that extends existing methods to account for anisotropic friction. The sliding properties of surfaces are encoded in friction tensors, which allows us to model frictional resistance freely along arbitrary directions. We also consider heterogeneous and asymmetric surface roughness and demonstrate the increased simulation quality on a number of two- and three-dimensional examples. Our method is computationally efficient and can easily be integrated into existing systems.
Automatic Construction of a Minimum Size Motion Graph
(ACM SIGGRAPH / Eurographics Association, 2009) Zhao, Liming; Normoyle, Aline; Khanna, Sanjeev; Safonova, Alla; Eitan Grinspun and Jessica Hodgins
Motion capture data have been used effectively in many areas of human motion synthesis. Among those, motion graph-based approaches have shown great promise for novice users due to their ability to generate long motions and the fully automatic process of motion synthesis. The performance of motion graph based approaches, however, relies heavily on selecting a good set of motions used to build the graph. This motion set needs to contain enough motions to achieve good connectivity and smooth transitions. At the same time, the motion set needs to be small for fast motion synthesis. Manually selecting a good motion set that achieves these requirements is difficult, especially given that motion capture databases are growing larger to provide a richer variety of human motions. Therefore we propose an automatic approach to select a good motion set. We cast the motion selection problem as a search for a minimum size subgraph from a large motion graph representing the motion capture database and propose an efficient algorithm, called the Iterative Sub-graph Algorithm, to find a good approximation to the optimal solution. Our approach especially benefits novice users who desire simple and fully automatic motion synthesis tools, such as motion graph-based techniques.
Bounded Normal Trees for Reduced Deformations of Triangulated Surfaces
(ACM SIGGRAPH / Eurographics Association, 2009) Schvartzman, Sara C.; Gascón, Jorge; Otaduy, Miguel A.; Eitan Grinspun and Jessica Hodgins
Several reduced deformation models in computer animation, such as linear blend skinning, point-based animation, embedding in finite element meshes, cage-based deformation, or subdivision surfaces, define surface vertex positions through convex combination of a rather small set of linear transformations. In this paper, we present an algorithm for computing tight normal bounds for a surface patch with an arbitrary number of triangles, with a cost linear in the number of governor linear transformations. This algorithm for normal bound computation constitutes the key element of the Bounded Normal Tree (BN-Tree), a novel culling data structure for hierarchical self-collision detection. In situations with sparse self-contact, normal-based culling can be performed with a small output-sensitive cost, regardless of the number of triangles in the surface.
ClearPath: Highly Parallel Collision Avoidance for Multi-Agent Simulation
(ACM SIGGRAPH / Eurographics Association, 2009) Guyy, Stephen. J.; Chhugani, Jatin; Kim, Changkyu; Satish, Nadathur; Lin, Ming; Manocha, Dinesh; Dubey, Pradeep; Eitan Grinspun and Jessica Hodgins
We present a new local collision avoidance algorithm between multiple agents for real-time simulations. Our approach extends the notion of velocity obstacles from robotics and formulates the conditions for collision free navigation as a quadratic optimization problem. We use a discrete optimization method to efficiently compute the motion of each agent. This resulting algorithm can be parallelized by exploiting data-parallelism and thread-level parallelism. The overall approach, ClearPath, is general and can robustly handle dense scenarios with tens or hundreds of thousands of heterogeneous agents in a few milli-seconds. As compared to prior collision avoidance algorithms, we observe more than an order of magnitude performance improvement.
Efficient and Robust Annotation of Motion Capture Data
(ACM SIGGRAPH / Eurographics Association, 2009) Müller, Meinard; Baak, Andreas; Seidel, Hans-Peter; Eitan Grinspun and Jessica Hodgins
In view of increasing collections of available 3D motion capture (mocap) data, the task of automatically annotating large sets of unstructured motion data is gaining in importance. In this paper, we present an efficient approach to label mocap data according to a given set of motion categories or classes, each specified by a suitable set of positive example motions. For each class, we derive a motion template that captures the consistent and variable aspects of a motion class in an explicit matrix representation. We then present a novel annotation procedure, where the unknown motion data is segmented and annotated by locally comparing it with the available motion templates. This procedure is supported by an efficient keyframe-based preprocessing step, which also significantly improves the annotation quality by eliminating false positive matches. As a further contribution, we introduce a genetic learning algorithm to automatically learn the necessary keyframes from the given example motions. For evaluation, we report on various experiments conducted on two freely available sets of motion capture data (CMU and HDM05).
Energy Stability and Fracture for Frame Rate Rigid Body Simulations
(ACM SIGGRAPH / Eurographics Association, 2009) Su, Jonathan; Schroeder, Craig; Fedkiw, Ronald; Eitan Grinspun and Jessica Hodgins
Our goal is to design robust algorithms that can be used for building real-time systems, but rather than starting with overly simplistic particle-based methods, we aim to modify higher-end visual effects algorithms. A major stumbling block in utilizing these visual effects algorithms for real-time simulation is their computational intensity. Physics engines struggle to fully exploit available resources to handle high scene complexity due to their need to divide those resources among many smaller time steps, and thus to obtain the maximum spatial complexity we design our algorithms to take only one time step per frame. This requires addressing both accuracy and stability issues for collisions, contact, and evolution in a manner significantly different from a typical simulation in which one can rely on shrinking the time step to ameliorate accuracy and stability issues. In this paper we present a novel algorithm for conserving both energy and momentum when advancing rigid body orientations, as well as a novel technique for clamping energy gain during contact and collisions. We also introduce a technique for fast and realistic fracture of rigid bodies using a novel collision-centered prescoring algorithm.
Exact volume preserving skinning with shape control
(ACM SIGGRAPH / Eurographics Association, 2009) Rohmer, Damien; Hahmann, Stefanie; Cani, Marie-Paule; Eitan Grinspun and Jessica Hodgins
In the real world, most objects do not loose volume when they deform: they may for instance compensate a local compression by inflating in the orthogonal direction, or, in the case of a character, preserve volume through specific bulges and folds. This paper presents a novel extension to smooth skinning, which not only offers an exact control of the object volume, but also enables the user to specify the shape of volume-preserving deformations through intuitive 1D profile curves. The method, a geometric post-processing to standard smooth skinning, perfectly fits into the usual production pipeline. It can be used whatever the desired locality of volume correction and does not bring any constraint on the original mesh. Several behaviors mimicking the way rubber-like materials and organic shapes respectively deform can be modeled. An improved algorithm for robustly computing skinning weights is also provided, making the method directly usable on complex characters, even for non-experts.
Experiment-based Modeling, Simulation and Validation of Interactions between VirtualWalkers
(ACM SIGGRAPH / Eurographics Association, 2009) Pettré, Julien; Ondrej, Jan; Olivier, Anne-Hélène; Cretual, Armel; Donikian, Stéphane; Eitan Grinspun and Jessica Hodgins
An interaction occurs between two humans when they walk with converging trajectories. They need to adapt their motion in order to avoid and cross one another at respectful distance. This paper presents a model for solving interactions between virtual humans. The proposed model is elaborated from experimental interactions data. We first focus our study on the pair-interaction case. In a second stage, we extend our approach to the multiple interactions case. Our experimental data allow us to state the conditions for interactions to occur between walkers, as well as each one's role during interaction and the strategies walkers set to adapt their motion. The low number of parameters of the proposed model enables its automatic calibration from available experimental data. We validate our approach by comparing simulated trajectories with real ones. We also provide comparison with previous solutions. We finally discuss the ability of our model to be extended to complex situations.
Face/Off: Live Facial Puppetry
(ACM SIGGRAPH / Eurographics Association, 2009) Weise, Thibaut; Li, Hao; Gool, Luc Van; Pauly, Mark; Eitan Grinspun and Jessica Hodgins
We present a complete integrated system for live facial puppetry that enables high-resolution real-time facial expression tracking with transfer to another person's face. The system utilizes a real-time structured light scanner that provides dense 3D data and texture. A generic template mesh, fitted to a rigid reconstruction of the actor's face, is tracked offline in a training stage through a set of expression sequences. These sequences are used to build a person-specific linear face model that is subsequently used for online face tracking and expression transfer. Even with just a single rigid pose of the target face, convincing real-time facial animations are achievable. The actor becomes a puppeteer with complete and accurate control over a digital face.
Fast and Robust Tracking of Fluid Surfaces
(ACM SIGGRAPH / Eurographics Association, 2009) Müller, Matthias; Eitan Grinspun and Jessica Hodgins
Surface tracking is an important problem with applications in many research fields. Among the most famous examples in computer graphics is the simulation and rendering of liquids with free surfaces. A surface that is advected by a general velocity field constantly changes its topology. This is the main reason why moving surfaces are typically defined implicitly as the zero set of a scalar field rather than by an explicit representation such as a mesh for instance. In this paper we present a method for tracking fluid surfaces using triangle meshes. This is done in two steps. First, the vertices are advected by the velocity field of the fluid. Second, self-penetrations are fixed using marching cubes triangle templates. The technique is efficient in terms of computation and memory consumption, it is simple to implement and allows for direct control of volume and feature preservation.
Fitting Behaviors to Pedestrian Simulations
(ACM SIGGRAPH / Eurographics Association, 2009) Lerner, Alon; Fitusi, Eitan; Chrysanthou, Yiorgos; Cohen-Or, Daniel; Eitan Grinspun and Jessica Hodgins
In this paper we present a data-driven approach for fitting behaviors to simulated pedestrian crowds. Our method annotates agent trajectories, generated by any crowd simulator, with action-tags. The aggregate effect of animating the agents according to the tagged trajectories enhances the impression that the agents are interacting with one another and with the environment. In a preprocessing stage, the stimuli which motivated a person to perform an action, as observed in a crowd video, are encoded into examples. Using the examples, non-linear, action specific influence functions are encoded into two-dimensional maps which evaluate, for each action, the relative importance of a stimulus within a configuration. At run time, given an agents stimuli configuration, the importance of each stimulus is determined and compared to the examples. Thus, the probability of performing each action is approximated and an action-tag is chosen accordingly. We fit behaviors to pedestrian crowds, thereby enhancing their natural appearance.
Guiding of Smoke Animations Through Variational Coupling of Simulations at Different Resolutions
(ACM SIGGRAPH / Eurographics Association, 2009) Nielsen, Michael B.; Christensen, Brian B.; Zafar, Nafees Bin; Roble, Doug; Museth, Ken; Eitan Grinspun and Jessica Hodgins
We propose a novel approach to guiding of Eulerian-based smoke animations through coupling of simulations at different grid resolutions. Specifically we present a variational formulation that allows smoke animations to adopt the low-frequency features from a lower resolution simulation (or non-physical synthesis), while simultaneously developing higher frequencies. The overall motivation for this work is to address the fact that art-direction of smoke animations is notoriously tedious. Particularly a change in grid resolution can result in dramatic changes in the behavior of smoke animations, and existing methods for guiding either significantly lack high frequency detail or may result in undesired features developing over time. Provided that the bulk movement can be represented satisfactorily at low resolution, our technique effectively allows artists to prototype simulations at low resolution (where computations are fast) and subsequently add extra details without altering the overall look and feel . Our implementation is based on a customized multi-grid solver with memory-efficient data structures.
Interactive Editing of Motion Style Using Drives and Correlations
(ACM SIGGRAPH / Eurographics Association, 2009) Neff, Michael; Kim, Yejin; Eitan Grinspun and Jessica Hodgins
Animation data, from motion capture or other sources, is becoming increasingly available and provides high quality motion, but is difficult to customize for the needs of a particular application. This is especially true when stylistic changes are needed, for example, to reflect a character's changing mood, differentiate one character from another or meet the precise desires of an animator. We introduce a system for editing animation data that is particularly well suited to making stylistic changes. Our approach transforms the joint angle representation of animation data into a set of pose parameters more suitable for editing. These motion drives include position data for the wrists, ankles and center of mass, as well as the rotation of the pelvis. We also extract correlations between drives and body movement, specifically between wrist position and the torso angles. The system solves for the pose at each frame based on the current values of these drives and correlations using an efficient set of inverse kinematics and balance algorithms. An animator can interactively edit the motion by performing linear operations on the motion drives or extracted correlations, or by layering additional correlations. We demonstrate the effectiveness of the approach with various examples of gesture and locomotion.
Interactive Synthesis of Human-Object Interaction
(ACM SIGGRAPH / Eurographics Association, 2009) Jain, Sumit; Liu, C. Karen; Eitan Grinspun and Jessica Hodgins
We present an interactive motion editing tool for creating dynamic scenes with human and object interaction. Our approach solves for an optimal control problem that leverages methods for physics-based rigid body control and kinematics-based human motion editing. Because the humans and the objects are coupled via physical contacts, our interface allows the animator to directly change the trajectories of humans or objects and simultaneously ren- der the effect of the edits on the entire scene. Consequently, the animator can efficiently create complex interaction that requires precise synchronization, such as juggling. Our framework is generic to the choice of human motion editing method, as long as the differential information of the motion can be computed.
Leveraging the Talent of Hand Animators to Create Three-Dimensional Animation
(ACM SIGGRAPH / Eurographics Association, 2009) Jain, Eakta; Sheikh, Yaser; Hodgins, Jessica; Eitan Grinspun and Jessica Hodgins
The skills required to create compelling three-dimensional animation using computer software are quite different from those required to create compelling hand animation with pencil and paper. The three-dimensional medium has several advantages over the traditional medium-it is easy to relight the scene, render it from different viewpoints, and add physical simulations. In this work, we propose a method to leverage the talent of traditionally trained hand animators to create three-dimensional animation of human motion, while allowing them to work in the medium that is familiar to them. The input to our algorithm is a set of hand-animated frames. Our key insight is to use motion capture data as a source of domain knowledge and 'lift' the two-dimensional animation to three dimensions, while maintaining the unique style of the input animation. A motion capture clip is projected to two dimensions. First, time alignment is done to match the timing of the hand-drawn frames and then, the limbs are aligned to better match the pose in the hand-drawn frames. Finally the motion is reconstructed in three dimensions. We demonstrate our algorithm on a variety of hand animated motion sequences on different characters, including ballet, a stylized sneaky walk, and a sequence of jumping jacks.
A Point-Based Method for Animating Elastoplastic Solids
(ACM SIGGRAPH / Eurographics Association, 2009) Gerszewski, Dan; Bhattacharya, Haimasree; Bargteil, Adam W.; Eitan Grinspun and Jessica Hodgins
In this paper we describe a point-based approach for animating elastoplastic materials. Our primary contribution is a simple method for computing the deformation gradient for each particle in the simulation. The deformation gradient is computed for each particle by finding the affine transformation that best approximates the motion of neighboring particles over a single timestep. These transformations are then composed to compute the total deformation gradient that describes the deformation around a particle over the course of the simulation. Given the deformation gradient we can apply arbitrary constitutive models and compute the resulting elastic forces. Our method has two primary advantages: we do not store or compare to an initial rest configuration and we work directly with the deformation gradient. The first advantage avoids poor numerical conditioning and the second naturally leads to a multiplicative model of deformation appropriate for finite deformations. We demonstrate our approach on a number of examples that exhibit a wide range of material behaviors.
A Point-based Method for Animating Incompressible Flow
(ACM SIGGRAPH / Eurographics Association, 2009) Sin, Funshing; Bargteil, Adam W.; Hodgins, Jessica K.; Eitan Grinspun and Jessica Hodgins
In this paper, we present a point-based method for animating incompressible flow. The advection term is handled by moving the sample points through the flow in a Lagrangian fashion. However, unlike most previous approaches, the pressure term is handled by performing a projection onto a divergence-free field. To perform the pressure projection, we compute a Voronoi diagram with the sample points as input. Borrowing from Finite Volume Methods, we then invoke the divergence theorem and ensure that each Voronoi cell is divergence free. To handle complex boundary conditions, Voronoi cells are clipped against obstacle boundaries and free surfaces. The method is stable, flexible and combines many of the desirable features of point-based and grid-based methods. We demonstrate our approach on several examples of splashing and streaming liquid and swirling smoke.
Pose-Timeline for Propagating Motion Edits
(ACM SIGGRAPH / Eurographics Association, 2009) Mukai, Tomohiko; Kuriyama, Shigeru; Eitan Grinspun and Jessica Hodgins
Motion editing often requires repetitive operations for modifying similar action units to give a similar effect or impression. This paper proposes a system for efficiently and flexibly editing the sequence of iterative actions by a few intuitive operations. Our system visualizes a motion sequence on a summary timeline with editable pose-icons, and drag-and-drop operations on the timeline enable intuitive controls of temporal properties of the motion such as timing, duration, and coordination. This graphical interface is also suited to transfer kinematical and temporal features between two motions through simple interactions with a quick preview of the resulting poses. Our method also integrates the concept of edit propagation by which the manual modification of one action unit is automatically transferred to the other units that are robustly detected by similarity search technique. We demonstrate the efficiency of our pose-timeline interface with a propagation mechanism for the timing adjustment of mutual actions and for motion synchronization with a music sequence.
Preface and Table of Contents
(ACM SIGGRAPH / Eurographics Association, 2009) Eitan Grinspun and Jessica Hodgins

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