SCA 07: Eurographics/SIGGRAPH Symposium on Computer Animation

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

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Cubic Shells
(The Eurographics Association, 2007) Garg, Akash; Grinspun, Eitan; Wardetzky, Max; Zorin, Denis; Dimitris Metaxas and Jovan Popovic
Hinge-based bending models are widely used in the physically-based animation of cloth, thin plates and shells. We propose a hinge-based model that is simpler to implement, more efficient to compute, and offers a greater number of effective material parameters than existing models. Our formulation builds on two mathematical observations: (a) the bending energy of curved flexible surfaces can be expressed as a cubic polynomial if the surface does not stretch; (b) a general class of anisotropic materials those that are orthotropic is captured by appropriate choice of a single stiffness per hinge. Our contribution impacts a general range of surface animation applications, from isotropic cloth and thin plates to orthotropic fracturing thin shells.
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.
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.
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.
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.
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.
Real-time Simulations of Bubbles and Foam within a ShallowWater Framework
(The Eurographics Association, 2007) Thuerey, N.; Sadlo, F.; Schirm, S.; Mueller-Fischer, M.; Gross, M.; Dimitris Metaxas and Jovan Popovic
Bubbles and foam are important fluid phenomena on scales that we encounter in our lives every day. While different techniques to handle these effects were developed in the past years, they require a full 3D fluid solver with free surfaces and surface tension. We present a shallow water based particle model that is coupled with a smoothed particle hydrodynamics simulation to demonstrate that real-time simulations of bubble and foam effects are possible with high frame rates. A shallow water simulation is used to represent the overall water volume. It is coupled to a particle-based bubble simulation with a flow field of spherical vortices. This bubble simulation is interacting with a smoothed particle hydrodynamics simulation including surface tension to handle foam on the fluid surface. The realism and performance of our approach is demonstrated with several test cases that run with high frame rates on a standard PC.
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.
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.
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.
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.
Deformation Styles for Spline-based Skeletal Animation
(The Eurographics Association, 2007) Forstmann, Sven; Ohya, Jun; Krohn-Grimberghe, Artus; McDougall, Ryan; Dimitris Metaxas and Jovan Popovic
We present a novel skinned skeletal animation system based on spline-aligned deformations for providing high quality and fully designable deformations in real-time. Our ambition is to allow artists the easy creation of abstract, pose-dependent deformation behaviors that might directly be assigned to a large variety of target objects simultaneously. To achieve this goal, we introduce the usage of deformation styles and demonstrate their applicability by our animation system. We therefore enhance spline-skinned skeletal animation with two sweep-based free-form-deformation (FFD) variants. The two FFD variants are pose-dependent, driven by three textures and three curves, which can be designed by the artist. As the three textures are similar to height-maps, their creation is very intuitive. Once designed, the deformation styles can be directly applied to any number of targets for imitating material behaviors of cloth, metal or even muscles. Our GPU based implementation shows promising results for real-time usage, as about 30 Million vertices per second can be animated. The basic spline-skinning even reaches more than twice the speed and gets close to the performance of skeletal subspace deformation (SSD). Furthermore, our method can easily be combined along with other existing deformation techniques as pose space deformation or SSD.
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.
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.
Animation of Chemically Reactive Fluids Using a Hybrid Simulation Method
(The Eurographics Association, 2007) Kang, Byungkwon; Jang, Yoojin; Ihm, Insung; Dimitris Metaxas and Jovan Popovic
Chemical phenomena abound in the real world, and often comprise indispensable elements of visual effects that are routinely created in the film industry. In this paper, we present a hybrid technique for simulating chemically reactive fluids, based on the theory of chemical kinetics. Our method makes synergistic use of both Eulerian grid-based methods and Lagrangian particle methods to simulate real and hypothetical chemical mechanisms effectively and efficiently. We demonstrate that by modeling chemical reactions using a particle system, an established, physically based fluid system can be extended easily to generate a wide range of chemical phenomena, ranging from catalysis and erosion to fire and explosions, with only a small additional cost.
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.
A Decision Network Framework for the Behavioral Animation of Virtual Humans
(The Eurographics Association, 2007) Yu, Qinxin; Terzopoulos, Demetri; Dimitris Metaxas and Jovan Popovic
We introduce a framework for advanced behavioral animation in virtual humans, which addresses the challenging open problem of simulating social interactions between pedestrians in urban settings. Based on hierarchical decision networks, our novel framework combines probability, decision, and graph theories for complex behavior modeling and intelligent action selection subject to manifold internal and external factors in the presence of uncertain knowledge. It yields autonomous characters that can make nontrivial interpretations and arrive at rational decisions dependent on multiple considerations. We demonstrate our framework in behavioral animation scenarios involving interacting autonomous pedestrians, including an elaborate emergency response animation.
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.
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.
Flipping with Physics: Motion Editing for Acrobatics
(The Eurographics Association, 2007) Majkowska, Anna; Faloutsos, Petros; Dimitris Metaxas and Jovan Popovic
Complex acrobatic stunts, such as double or triple flips, can be performed only by highly skilled athletes. On the other hand, simpler tricks, such as single-flip jumps, are relatively easy to master.We present a method for creating complex, multi-flip ballistic motions from simple, single-flip jumps. Our approach also allows an animator to interact with the system by introducing modifications to a ballistic phase of a motion. Our method automatically adjusts motion trajectories, to assure physical validity of the motion after the modifications. The presented technique is efficient and produces physically valid results without resorting to computationally expensive optimization. To validate our approach we present the results of a study of user sensitivity to errors in angular momentum and take-off angle. The study shows that small changes of these parameters introduced by our method are not perceptible to a viewer.

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