SCA 07: Eurographics/SIGGRAPH Symposium on Computer Animation

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https://diglib.eg.org/handle/10.2312/429

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On the Beat! Timing and Tension for Dynamic Characters
(The Eurographics Association, 2007) Allen, Brian; Chu, Derek; Shapiro, Ari; Faloutsos, Petros; Dimitris Metaxas and Jovan Popovic
Dynamic simulation is a promising complement to kinematic motion synthesis, particularly in cases where simulated characters need to respond to unpredictable interactions. Moving beyond simple rag-doll effects, though, requires dynamic control. The main issue with dynamic control is that there are no standardized techniques that allow an animator to precisely specify the timing of the motion while still providing natural response to external disturbances. The few proposed techniques that address this problem are based on heuristically or manually tuning proportional-derivative (PD) control parameters and do not generalize easily. We propose an approach to dynamic character control that is able to honor timing constraints, to provide naturallooking motion and to allow for realistic response to perturbations. Our approach uses traditional PD control to interpolate between key-frames. The key innovation is that the parameters of the PD controllers are computed for each joint analytically. By continuously updating these parameters over time, the controller is able to respond naturally to both external perturbations and changes in the state of the character
Animation Collage
(The Eurographics Association, 2007) Theobalt, Christian; Roessl, Christian; Aguiar, Edilson de; Seidel, Hans-Peter; Dimitris Metaxas and Jovan Popovic
We propose a method to automatically transform mesh animations into animation collages, i.e. moving assemblies of shape primitives from a database given by an artist. An animation collage is a complete reassembly of the original animation in a new abstract visual style that imitates the spatio-temporal shape and deformation of the input. Our algorithm automatically decomposes input animations into plausible approximately rigid segments and fits to each segment one shape from the database by means of a spatio-temporal matching procedure. The collage is then animated in compliance with the original s shape and motion. Apart from proposing solutions to a number of spatio-temporal alignment problems, this work is an interesting add-on to the graphics artist s toolbox with many applications in arts, non-photorealistic rendering, and animated movie productions. We exemplify the beauty of animation collages by showing results created with our software prototype.
Multiobjective Control with Frictional Contacts
(The Eurographics Association, 2007) Abe, Yeuhi; Silva, Marco da; Popovic, Jovan; Dimitris Metaxas and Jovan Popovic
Standing is a fundamental skill mastered by humans and animals alike. Although easy for adults, it requires careful and deliberate manipulation of contact forces. The variation in contact configuration (e.g., standing on one foot, on uneven ground, or while holding on for support) presents a difficult challenge for interactive simulation of humans and animals, especially while performing tasks in the presence of external disturbances. We describe an analytic approach for control of standing in three-dimensional simulations based upon local optimization. At any point in time, the control system solves a quadratic program to compute actuation by maximizing the performance of multiple motion objectives subject to constraints imposed by actuation limits and contact configuration. This formulation is suitable for interactive animation and it adapts to the proportions of any character model in any non-planar, frictional contact configuration.
Smooth Movers: Perceptually Guided Human Motion Simulation
(The Eurographics Association, 2007) McDonnell, Rachel; Newell, Fiona; O'Sullivan, Carol; Dimitris Metaxas and Jovan Popovic
To animate a character, a number of poses are displayed in quick succession in order to create the illusion of motion. For most real-time applications, such as games, the pose update rate is largely constrained by the available hardware and overall simulation complexity. To date, no analysis of the factors that affect the perceived smoothness of animated virtual characters has been presented. In the first perceptual studies aimed at identifying such factors and their interactions, we have determined some thresholds that could be used to produce acceptably smooth human animations in a variety of conditions. Some interesting results were found, e.g., that character type, clothing, scene complexity or motion synchronicity had no effect on smoothness perception in our experiments, but cycle rate, linear velocity, motion complexity and group size all had a significant effect, with slower or lower intensity movements generally requiring fewer updates. Our results should be of real practical use to character animators in various application areas, but in particular to developers of real-time applications where Simulation Levels Of Detail (SLOD) need to be employed.
Dynamic, Expressive Speech Animation From a Single Mesh
(The Eurographics Association, 2007) Wampler, Kevin; Sasaki, Daichi; Zhang, Li; Popovic, Zoran; Dimitris Metaxas and Jovan Popovic
In this work we present a method for human face animation which allows us to generate animations for a novel person given just a single mesh of their face. These animations can be of arbitrary text and may include emotional expressions. We build a multilinear model from data which encapsulates the variation in dynamic face motions over changes in identity, expression, and over different texts. We then describe a synthesis method consisting of a phoneme planning and a blending stage which uses this model as a base and attempts to preserve both face shape and dynamics given a novel text and an emotion at each point in time.
Controlling Individual Agents in High-Density Crowd Simulation
(The Eurographics Association, 2007) Pelechano, N.; Allbeck, J.M.; Badler, N.I.; Dimitris Metaxas and Jovan Popovic
Simulating the motion of realistic, large, dense crowds of autonomous agents is still a challenge for the computer graphics community. Typical approaches either resemble particle simulations (where agents lack orientation controls) or are conservative in the range of human motion possible (agents lack psychological state and aren t allowed to push each other). Our HiDAC system (for High-Density Autonomous Crowds) focuses on the problem of simulating the local motion and global wayfinding behaviors of crowds moving in a natural manner within dynamically changing virtual environments. By applying a combination of psychological and geometrical rules with a social and physical forces model, HiDAC exhibits a wide variety of emergent behaviors from agent line formation to pushing behavior and its consequences; relative to the current situation, personalities of the individuals and perceived social density.
Arbitrary Cutting of Deformable Tetrahedralized Objects
(The Eurographics Association, 2007) Sifakis, Eftychios; Der, Kevin G.; Fedkiw, Ronald; Dimitris Metaxas and Jovan Popovic
We propose a flexible geometric algorithm for placing arbitrary cracks and incisions on tetrahedralized deformable objects. Although techniques based on remeshing can also accommodate arbitrary fracture patterns, this flexibility comes at the risk of creating sliver elements leading to models that are inappropriate for subsequent simulation. Furthermore, interactive applications such as virtual surgery simulation require both a relatively low resolution mesh for efficient simulation of elastic deformation and highly detailed surface geometry to facilitate accurate manipulation and cut placement. Thus, we embed a high resolution material boundary mesh into a coarser tetrahedral mesh using our cutting algorithm as a meshing tool, obtaining meshes that can be efficiently simulated while preserving surface detail. Our algorithm is similar to the virtual node algorithm in that we avoid sliver elements and their associated stringent timestep restrictions, but it is significantly more general allowing for the arbitrary cutting of existing cuts, sub-tetrahedron resolution (e.g. we cut a single tetrahedron into over a thousand pieces), progressive introduction of cuts while the object is deforming, and moreover the ability to accurately cut the high resolution embedded mesh.
Harmonic Skeleton for Realistic Character Animation
(The Eurographics Association, 2007) Aujay, Gregoire; Hetroy, Franck; Lazarus, Francis; Depraz, Christine; Dimitris Metaxas and Jovan Popovic
Current approaches to skeleton generation are based on topological and geometrical information only; this can be insufficient for realistic character animation, since the location of the joints does not usually match the real bone structure of the model. This paper proposes the use of anatomical information to enhance the skeleton. Using a harmonic function, this information can be recovered from the skeleton itself, which is guaranteed not to have undesired endpoints. The skeleton is computed as a Reeb graph of such a function over the surface of the model. Starting from one point selected on the head of the character, the entire process is fast, automatic and robust; it generates skeletons whose joints can be associated with the character s anatomy. Results are provided, including a quantitative validation of the generated skeletons.
Time-critical distributed contact for 6-DoF haptic rendering of adaptively sampled reduced deformable models
(The Eurographics Association, 2007) Barbic, Jernej; James, Doug; Dimitris Metaxas and Jovan Popovic
Real-time evaluation of distributed contact forces for rigid or deformable 3D objects is important for providing multi-sensory feedback in emerging real-time applications, such as 6-DoF haptic force-feedback rendering. Unfortunately, at very high temporal rates (1 kHz for haptics), there is often insufficient time to resolve distributed contact between geometrically complex objects. In this paper, we present a spatially and temporally adaptive sample-based approach to approximate contact forces under hard real-time constraints. The approach is CPU based, and supports contact between a rigid and a reduced deformable model with complex geometry. Penalty-based contact forces are efficiently resolved using a multi-resolution point-based representation for one object, and a signed-distance field for the other. Hard realtime approximation of distributed contact forces uses multi-level progressive point-contact sampling, and exploits temporal coherence, graceful degradation and other optimizations. We present several examples of 6-DoF haptic rendering of geometrically complex rigid and deformable objects in distributed contact at real-time kilohertz rates.
Liquid Simulation on Lattice-Based Tetrahedral Meshes
(The Eurographics Association, 2007) Chentanez, Nuttapong; Feldman, Bryan E.; Labelle, François; O Brien, James F.; Shewchuk, Jonathan R.; Dimitris Metaxas and Jovan Popovic
We describe a method for animating incompressible liquids with detailed free surfaces. For each time step, semi- Lagrangian contouring computes a new fluid boundary (represented as a fine surface triangulation) from the previous time step s fluid boundary and velocity field. Then a mesh generation algorithm called isosurface stuffing discretizes the region enclosed by the new fluid boundary, creating a tetrahedral mesh that grades from a fine resolution at the surface to a coarser resolution in the interior. The mesh has a structure, based on the body centered cubic lattice, that accommodates graded tetrahedron sizes but is regular enough to aid efficient point location and to save memory used to store geometric properties of identical tetrahedra. Although the mesh is warped to conform to the liquid boundary, it has a mathematical guarantee on tetrahedron quality, and is generated very rapidly. Each successive time step entails creating a new triangulated liquid surface and a new tetrahedral mesh. Semi-Lagrangian advection computes velocities at the current time step on the new mesh. We use a finite volume discretization to perform pressure projection required to enforce the fluid s incompressibility, and we solve the linear system with algebraic multigrid. A novel thickening scheme prevents thin sheets and droplets of liquid from vanishing when their thicknesses drop below the mesh resolution. Examples demonstrate that the method captures complex liquid motions and fine details on the free surfaces without suffering from excessive volume loss or artificial damping.
Solving General ShallowWave Equations on Surfaces
(The Eurographics Association, 2007) Wang, Huamin; Miller, Gavin; Turk, Greg; Dimitris Metaxas and Jovan Popovic
We propose a new framework for solving General Shallow Wave Equations (GSWE) in order to efficiently simulate water flows on solid surfaces under shallow wave assumptions. Within this framework, we develop implicit schemes for solving the external forces applied to water, including gravity and surface tension. We also present a two-way coupling method to model interactions between fluid and floating rigid objects. Water flows in this system can be simulated not only on planar surfaces by using regular grids, but also on curved surfaces directly without surface parametrization. The experiments show that our system is fast, stable, physically sound, and straightforward to implement on both CPUs and GPUs. It is capable of simulating a variety of water effects including: shallow waves, water drops, rivulets, capillary events and fluid/floating rigid body coupling. Because the system is fast, we can also achieve real-time water drop control and shape design.
Guided TimeWarping for Motion Editing
(The Eurographics Association, 2007) Hsu, Eugene; Silva, Marco da; Popovic, Jovan; Dimitris Metaxas and Jovan Popovic
Time warping allows users to modify timing without affecting poses. It has many applications in animation systems for motion editing, such as refining motions to meet new timing constraints or modifying the acting of animated characters. However, time warping typically requires many manual adjustments to achieve the desired results. We present a technique which simplifies this process by allowing time warps to be guided by a provided reference motion. Given few timing constraints, it computes a warp that both satisfies these constraints and maximizes local timing similarities to the reference. The algorithm is fast enough to incorporate into standard animation workflows. We apply the technique to two common tasks: preserving the natural timing of motions under new time constraints and modifying the timing of motions for stylistic effects.
Weakly compressible SPH for free surface flows
(The Eurographics Association, 2007) Becker, Markus; Teschner, Matthias; Dimitris Metaxas and Jovan Popovic
We present a weakly compressible form of the Smoothed Particle Hydrodynamics method (SPH) for fluid flow based on the Tait equation. In contrast to commonly employed projection approaches that strictly enforce incompress- ibility, time-consuming solvers for the Poisson equation are avoided by allowing for small, user-defined density fluctuations. We also discuss an improved surface tension model that is particularly appropriate for single-phase free-surface flows. The proposed model is compared to existing models and experiments illustrate the accuracy of the approach for free surface flows. Combining the proposed methods, volume-preserving low-viscosity liquids can be efficiently simulated using SPH. The approach is appropriate for medium-scale and small-scale phenomena. Effects such as splashing and breaking waves are naturally handled.
Kinodynamic skinning using volume-preserving deformations
(The Eurographics Association, 2007) Angelidis, Alexis; Singh, Karan; Dimitris Metaxas and Jovan Popovic
We present a new approach to character skinning where divergence-free vector fields induced by skeletal motion, describe the velocity of skin deformation. The joint transformations for a pose relative to a rest pose create a bend deformation field, resulting in pose-dependent or kinematic skin deformations, varying smoothly across joints. The bend deformation parameters are interactively controlled to capture the varying deformability of bone and other anatomic tissue within an overall fold-over free and volume-preserving skin deformation. Subsequently, we represent the dynamics of skeletal motion, tissue elasticity, muscular tension and the environment as forces that are mapped to vortices at tissue interfaces. A simplified Biot-Savart law in the context of elastic deformation recovers a divergence-free velocity field from the vorticity. Finally, we apply a new stable technique to efficiently integrate points along their deformation trajectories. Adding these dynamic forces over a window of time prior to a given pose provides a continuum of user controllable kinodynamic skinning. A comprehensive implementation using a typical animator workflow in Maya shows our approach to be effective for complex character skinning.
Screen Space Meshes
(The Eurographics Association, 2007) Mueller, Matthias; Schirm, Simon; Duthaler, Stephan; Dimitris Metaxas and Jovan Popovic
We present a simple yet powerful approach for the generation and rendering of surfaces defined by the boundary of a three-dimensional point cloud. First, a depth map plus internal and external silhouettes of the surface are generated in screen space. These are used to construct a 2D screen space triangle mesh with a new technique that is derived from Marching Squares. The resulting mesh is transformed back to 3D world space for the computation of occlusions, reflections, refraction, and other shading effects. One of the main applications for screen space meshes is the visualization of Lagrangian, particle-based fluids models. Our new method has several advantages over the full 3D Marching Cubes approach. The algorithm only generates surface where it is visible, view-dependent level of detail comes for free, and interesting visual effects are possible by filtering in screen space.
Face Poser: Interactive Modeling of 3D Facial Expressions Using Model Priors
(The Eurographics Association, 2007) Lau, Manfred; Chai, Jinxiang; Xu, Ying-Qing; Shum, Heung-Yeung; Dimitris Metaxas and Jovan Popovic
In this paper, we present an intuitive interface for interactively posing 3D facial expressions. The user can create and edit facial expressions by drawing freeform strokes, or by directly dragging facial points in 2D screen space. Designing such an interface for face modeling and editing is challenging because many unnatural facial expressions might be consistent with the ambiguous user input. The system automatically learns a model prior from a prerecorded facial expression database and uses it to remove the ambiguity. We formulate the problem in a maximum a posteriori (MAP) framework by combining the prior with user-defined constraints. Maximizing the posterior allows us to generate an optimal and natural facial expression that satisfies the user-defined constraints. Our system is interactive; it is also simple and easy to use. A first-time user can learn to use the system and start creating a variety of natural face models within minutes. We evaluate the performance of our approach with cross validation tests, and by comparing with alternative techniques.
Hybrid Simulation of Deformable Solids
(The Eurographics Association, 2007) Sifakis, Eftychios; Shinar, Tamar; Irving, Geoffrey; Fedkiw, Ronald; Dimitris Metaxas and Jovan Popovic
Although mesh-based methods are efficient for simulating simple hyperelasticity, maintaining and adapting a mesh-based representation is less appealing in more complex scenarios, e.g. collision, plasticity and fracture. Thus, meshless or point-based methods have enjoyed recent popularity due to their added flexibility in dealing with these situations. Our approach begins with an initial mesh that is either conforming (as generated by one s favorite meshing algorithm) or non-conforming (e.g. a BCC background lattice). We then propose a framework for embedding arbitrary sample points into this initial mesh allowing for the straightforward handling of collisions, plasticity and fracture without the need for complex remeshing. A straightforward consequence of this new framework is the ability to naturally handle T-junctions alleviating the requirement for a manifold initial mesh. The arbitrarily added embedded points are endowed with full simulation capability allowing them to collide, interact with each other, and interact with the parent geometry in the fashion of a particle-centric simulation system. We demonstrate how this formulation facilitates tasks such as arbitrary refinement or resampling for collision processing, the handling of multiple and possibly conflicting constraints (e.g. when cloth is nonphysically pinched between two objects), the straightforward treatment of fracture, and sub-element resolution of elasticity and plasticity.
Group Behavior from Video: A Data-Driven Approach to Crowd Simulation
(The Eurographics Association, 2007) Lee, Kang Hoon; Choi, Myung Geol; Hong, Qyoun; Lee, Jehee; Dimitris Metaxas and Jovan Popovic
Crowd simulation techniques have frequently been used to animate a large group of virtual humans in computer graphics applications. We present a data-driven method of simulating a crowd of virtual humans that exhibit behaviors imitating real human crowds. To do so, we record the motion of a human crowd from an aerial view using a camcorder, extract the two-dimensional moving trajectories of each individual in the crowd, and then learn an agent model from observed trajectories. The agent model decides each agent s actions based on features of the environment and the motion of nearby agents in the crowd. Once the agent model is learned, we can simulate a virtual crowd that behaves similarly to the real crowd in the video. The versatility and flexibility of our approach is demonstrated through examples in which various characteristics of group behaviors are captured and reproduced in simulated crowds.
Legendre Fluids: A Unified Framework for Analytic Reduced Space Modeling and Rendering of Participating Media
(The Eurographics Association, 2007) Gupta, Mohit; Narasimhan, Srinivasa G.; Dimitris Metaxas and Jovan Popovic
In this paper, we present a unified framework for reduced space modeling and rendering of dynamic and non- homogenous participating media, like snow, smoke, dust and fog. The key idea is to represent the 3D spatial variation of the density, velocity and intensity fields of the media using the same analytic basis. In many situa- tions, natural effects such as mist, outdoor smoke and dust are smooth (low frequency) phenomena, and can be compactly represented by a small number of coefficients of a Legendre polynomial basis. We derive analytic ex- pressions for the derivative and integral operators in the Legendre coefficient space, as well as the triple product integrals of Legendre polynomials. These mathematical results allow us to solve both the Navier-Stokes equations for fluid flow and light transport equations for single scattering efficiently in the reduced Legendre space. Since our technique does not depend on volume grid resolution, we can achieve computational speedups as compared to spatial domain methods while having low memory and pre-computation requirements as compared to data- driven approaches. Also, analytic definition of derivatives and integral operators in the Legendre domain avoids the approximation errors inherent in spatial domain finite difference methods. We demonstrate many interesting visual effects resulting from particles immersed in fluids as well as volumetric scattering in non-homogenous and dynamic participating media, such as fog and mist.
Adaptive Deformations with Fast Tight Bounds
(The Eurographics Association, 2007) Otaduy, Miguel A.; Germann, Daniel; Redon, Stephane; Gross, Markus; Dimitris Metaxas and Jovan Popovic
Simulation of deformations and collision detection are two highly intertwined problems that are often treated sepa- rately. This is especially true in existing elegant adaptive simulation techniques, where standard collision detection algorithms cannot leverage the adaptively selected degrees of freedom.We propose a seamless integration of multi- grid algorithms and collision detection that identifies boundary conditions while inherently exploiting adaptivity. We realize this integration through multiscale bounding hierarchies, a novel unified hierarchical representation, together with an adaptive multigrid algorithm for irregular meshes and an adaptivity-aware hierarchical collision detection algorithm. Our solution produces detailed deformations with adapted computational cost, but it also enables robust interactive simulation of self-colliding deformable objects with high-resolution surfaces.

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