SCA 15: Eurographics/SIGGRAPH Symposium on Computer Animation
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Item Animating Articulated Characters using Wiggly Splines(ACM Siggraph, 2015) Schulz, Christian; Tycowicz, Christoph von; Seidel, Hans-Peter; Hildebrandt, Klaus; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaWe propose a new framework for spacetime optimization that can generate artistic motion with a long planning horizon for complex virtual characters. The scheme can be used for generating general types of motion and neither requires motion capture data nor an initial motion that satisfies the constraints. Our modeling of the spacetime optimization combines linearized dynamics and a novel warping scheme for articulated characters. We show that the optimal motions can be described using a combination of vibration modes, wiggly splines, and our warping scheme. This enables us to restrict the optimization to low-dimensional spaces of explicitly parametrized motions. Thereby the computation of an optimal motion is reduced to a low-dimensional non-linear least squares problem, which can be solved with standard solvers. We show examples of motions created by specifying only a few constraints for positions and velocities.Item BSwarm: Biologically-Plausible Dynamics Model of Insect Swarms(ACM Siggraph, 2015) Wang, Xinjie; Ren, Jiaping; Jin, Xiaogang; Manocha, Dinesh; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaWe present a biologically plausible dynamics model to simulate swarms of flying insects. Our formulation, which is based on biological conclusions and experimental observations, is designed to simulate large insect swarms of varying densities. We use a hybrid formulation that combines a force-based model to capture different interactions between the insects with a data-driven noise model, and computes collision-free trajectories. We introduce a quantitative metric to evaluate the accuracy of such multi-agent systems and model the inherent noise. We highlight the performance of our dynamics model for simulating large flying swarms of midges, fruit fly, locusts and moths. In practice, our approach can generate many collective behaviors, including aggregation, migration, phase transition, and escape responses, and we highlight the benefits over prior methods.Item Computational Design of Walking Automata(ACM Siggraph, 2015) Bharaj, Gaurav; Coros, Stelian; Thomaszewski, Bernhard; Tompkin, James; Bickel, Bernd; Pfister, Hanspeter; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaCreating mechanical automata that can walk in stable and pleasing manners is a challenging task that requires both skill and expertise. We propose to use computational design to offset the technical difficulties of this process. A simple drag-and-drop interface allows casual users to create personalized walking toys from a library of pre-defined template mechanisms. Provided with this input, our method leverages physical simulation and evolutionary optimization to refine the mechanical designs such that the resulting toys are able to walk. The optimization process is guided by an intuitive set of objectives that measure the quality of the walking motions. We demonstrate our approach on a set of simulated mechanical toys with different numbers of legs and various distinct gaits. Two fabricated prototypes showcase the feasibility of our designs.Item Constraint-based Wrinkle Simulation in Texture Space(ACM Siggraph, 2015) Silva, Paulo; Rungjiratananon, Witawat; Elcott, Sharif; Driancourt, Remi; Igarashi, Takeo; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaAn important factor in increasing realism of synthetic images, wrinkles have received a considerable amount of attention from the computer graphics research community. The method Wrinkle Meshes, targets real-time simulation of wrinkles [M¨uller and Chentanez 2010]. This method adds wrinkles to a fine mesh surface representation, by applying a constraint based method guided by a coarse physical simulation. It provides a framework which is simple to implement and unconditionally stable, and therefore desirable for game simulations. A problem with that method is that it requires a high-resolution mesh for simulation and rendering. This usually implies that artists need to create an additional version of assets for the purpose of wrinkling, which is not desirable in video-game production codes where the CPU and memory resources are severely limited. To address this problem we propose a texture-based Wrinkle Meshes. We leverage the idea of a high-resolution constraintbased simulation, but modify it to use a texture space solver. We also propose an encoding for the simulation data which reduces the memory requirements considerably.Item Discovering Human Postures from Non-human Silhouette(ACM Siggraph, 2015) Won, Jungdam; Lee, Jehee; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaThe shadow theatre is an unique performing arts, which utilizes a shadow as an main communication tool. This can be understood easily by an extension of traditional shadow play. Key difference comes from the fact that entire human bodies are used to make interesting shapes on screen during the play, whereas puppets or human hands are only used in the traditional one. Modern technologies on projection devices facilitate larger stage area, which enables the actors to use human motions in the play. Viewers often prefer full body shadow play over the traditional one because it gives energetic and delicate feelings simultaneously and it is less trivial to conjecture original performers' postures. Our goal is to automatically discover human postures from nonhuman silhouettes or shadow images. We are particularly interested in the situation where multiple actors and environments(e.g., floor, platform or props) exist similar to the true performance. The actors' posing process is therefore greatly accelerated by our system for new shapes which haven't been handled yet. The system can also be used to generate articulated sculptures imitating target shadow whereas freeform geometry was made in [Mitra and Pauly 2009]. A light source, screen and actors are substructional elements in the shadow theatre. If their positions and orientations are fixed, one scene is determined accordingly. Precise and complex coordination between the elements in spatial domain is therefore key to choreograph. To do so, we present a novel approach that takes as input in the form of non-human 2D silhouettes or any types of images which can represent target shadows. From this input our system discovers a set of human poses.Item Divergence-Free Smoothed Particle Hydrodynamics(ACM Siggraph, 2015) Bender, Jan; Koschier, Dan; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaIn this paper we introduce an efficient and stable implicit SPH method for the physically-based simulation of incompressible fluids. In the area of computer graphics the most efficient SPH approaches focus solely on the correction of the density error to prevent volume compression. However, the continuity equation for incompressible flow also demands a divergence-free velocity field which is neglected by most methods. Although a few methods consider velocity divergence, they are either slow or have a perceivable density fluctuation. Our novel method uses an efficient combination of two pressure solvers which enforce low volume compression (below 0:01 %) and a divergence-free velocity field. This can be seen as enforcing incompressibility both on position level and velocity level. The first part is essential for realistic physical behavior while the divergence-free state increases the stability significantly and reduces the number of solver iterations. Moreover, it allows larger time steps which yields a considerable performance gain since particle neighborhoods have to be updated less frequently. Therefore, our divergence-free SPH (DFSPH) approach is significantly faster and more stable than current state-of-the-art SPH methods for incompressible fluids. We demonstrate this in simulations with millions of fast moving particles.Item Effect of Appearance on Perception of Deformation(ACM Siggraph, 2015) Han, Donghui; Keyser, John; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaWe provide an experimental validation that appearance of an object can influence the perception of its level of deformation in a 3D simulation. Our study provides helpful insights in how to improve visual plausibility of deformation, which may allow artists to adjust their designs to enhance or minimize the perceived deformation in a model. We use a physically-based deformation model to simulate simple geometric shapes undergoing deformation. We apply a number of different appearance and rendering parameters to these objects, and then use two user studies to measure whether appearance used for an object can have a statistically significant effect on the perception of its deformation. In another study, we adjust the number of objects simulated and investigate how this can influence the effect of appearance. We find that appearance can potentially influence people's sensitivity to differences of deformation as well as subjective rating of softness in our studies. Further analysis shows that, in simple scenarios, the effect of low-level cues in appearance can be dominant, even if high-level information delivered by appearance has the opposite implication. The third study shows that as the number of objects in a scenario increases, objects are perceived to be stiffer. Also, the effect of low-level cues is weaker.Item Efficient Simulation of Knitted Cloth Using Persistent Contacts(ACM Siggraph, 2015) Cirio, Gabriel; Lopez-Moreno, Jorge; Otaduy, Miguel A.; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaKnitted cloth is made of yarns that are stitched in regular patterns, and its macroscopic behavior is dictated by the contact interactions between such yarns. We propose an efficient representation of knitted cloth at the yarn level that treats yarn-yarn contacts as persistent, thereby avoiding expensive contact handling altogether. We introduce a compact representation of yarn geometry and kinematics, capturing the essential deformation modes of yarn loops and stitches with a minimum cost. Based on this representation, we design force models that reproduce the characteristic macroscopic behavior of knitted fabrics. We demonstrate the efficiency of our method on simulations with millions of degrees of freedom (hundreds of thousands of yarn loops), almost one order of magnitude faster than previous techniques.Item Energy-efficient mid-term strategies for collision avoidance in crowd simulation(ACM Siggraph, 2015) Bruneau, Julien; Pettré, Julien; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaWhen navigating in crowds, humans are able to move efficiently between people. They look ahead to know which path would reduce the complexity of their interactions with others. Current navigation systems for virtual agents consider the long-term planning to find a path in the static environment and the short term reaction to avoid collision with close obstacles. Recently some mid-term considerations have been added to avoid high density areas. However, there is no mid-term planning among static and dynamic obstacles that would enable the agent to look ahead and avoid difficult paths or find easy ones as human do. In this paper we present a system for such mid-term planning. This system is added to the navigation process between the path finding and the local avoidance to improve the navigation of virtual agents. We show the capacities of such system on several case studies. Finally we use an energy criterion to compare trajectories computed with and without the mid-term planning.Item Eulerian-on-Lagrangian Cloth(ACM Siggraph, 2015) Piddington, Kyle; Levin, David I.W.; Pai, Dinesh K.; Sueda, Shinjiro; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaWe present a new, Eulerian-on-Lagrangian approach for modeling cloth. When a cloth modeled using the traditional Lagrangian approach is moved around an object with sharp corners, such as the edge of a table, the cloth cannot always bend smoothly around the object because it can bend only at its nodes. With our method, these constraints are built into the discretization of the cloth, giving us an equation of motion that directly honors these constraints. This allows the cloth to bend and move smoothly around such constraints. We show how our method can efficiently handle challenging simulations, such as pulling a table cloth from under wine glasses without knocking them over.Item Fast Cloth Simulation with Implicit Contact and Exact Coulomb Friction(ACM Siggraph, 2015) Daviet, Gilles; Bertails-Descoubes, Florence; Casati, Romain; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaCloth dynamics plays a major role in the visual appearance of moving characters. Properly accounting for frictional contact is of utmost importance to avoid cloth-body penetration and to capture folding behavior due to dry friction. We present here the first method able to account for contact with exact Coulomb friction between a cloth and the underlying character. Our key contribution is to formulate and solve the frictional contact problem merely on velocity variables, by leveraging some tools of convex analysis. Our method is both fast and robust, allowing us to simulate full-size garments with more realistic body-cloth interactions compared to former methods, while maintaining similar computational timings.Item Fully Automatic Generation of Anatomical Face Simulation Models(ACM Siggraph, 2015) Cong, Matthew; Bao, Michael; E, Jane L.; Bhat, Kiran S.; Fedkiw, Ronald; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaWe present a fast, fully automatic morphing algorithm for creating simulatable flesh and muscle models for human and humanoid faces. Current techniques for creating such models require a significant amount of time and effort, making them infeasible or impractical. In fact, the vast majority of research papers use only a floating mask with no inner lips, teeth, tongue, eyelids, eyes, head, ears, etc.-and even those that build the full visual model would typically still lack the cranium, jaw, muscles, and other internal anatomy. Our method requires only the target surface mesh as input and can create a variety of models in only a few hours with no user interaction. We start with a symmetric, high resolution, anatomically accurate template model that includes auxiliary information such as feature points and curves. Then given a target mesh, we automatically orient it to the template, detect feature points, and use these to bootstrap the detection of corresponding feature curves. These curve correspondences are used to deform the surface mesh of the template model to match the target mesh. Then, the calculated displacements of the template surface mesh are used to drive a three-dimensional morph of the full template model including all interior anatomy. The resulting target model can be simulated to generate a large range of expressions that are consistent across characters using the same muscle activations. Full automation of this entire process makes it readily available to a wide range of users.Item Fully Momentum-Conserving Reduced Deformable Bodies with Collision, Contact, Articulation, and Skinning(ACM Siggraph, 2015) Sheth, Rahul; Lu, Wenlong; Yu, Yue; Fedkiw, Ronald; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaWe propose a novel framework for simulating reduced deformable bodies that fully accounts for linear and angular momentum conservation even in the presence of collision, contact, articulation, and other desirable effects. This was motivated by the observation that the mere excitation of a single mode in a reduced degree of freedom model can adversely change the linear and angular momentum. Although unexpected changes in linear momentum can be avoided during basis construction, adverse changes in angular momentum appear unavoidable, and thus we propose a robust framework that includes the ability to compensate for them. Enabled by this ability to fully account for linear and angular momentum, we introduce an impulse-based formulation that allows us to precisely control the velocity of any node in spite of the fact that we only have access to a lower-dimensional set of degrees of freedom. This allows us to model collision, contact, and articulation in a robust and high visual fidelity manner, especially when compared to penalty-based forces that merely aim to coerce local velocities. In addition, we propose a new ''deformable bones'' framework wherein we leverage standard skinning technology for ''bones,'' ''bone'' placement, blending operations, etc. even though each of our ''deformable bones'' is a fully simulated reduced deformable model.Item Functional Thin Films on Surfaces(ACM Siggraph, 2015) Azencot, Omri; Vantzos, Orestis; Wardetzky, Max; Rumpf, Martin; Ben-Chen, Mirela; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaThe motion of a thin viscous film of fluid on a curved surface exhibits many intricate visual phenomena, which are challenging to simulate using existing techniques. A possible alternative is to use a reduced model, involving only the temporal evolution of the mass density of the film on the surface. However, in this model, the motion is governed by a fourth-order nonlinear PDE, which involves geometric quantities such as the curvature of the underlying surface, and is therefore difficult to discretize. Inspired by a recent variational formulation for this problem on smooth surfaces, we present a corresponding model for triangle meshes. We provide a discretization for the curvature and advection operators which leads to an efficient and stable numerical scheme, requires a single sparse linear solve per time step, and exactly preserves the total volume of the fluid. We validate our method by qualitatively comparing to known results from the literature, and demonstrate various intricate effects achievable by our method, such as droplet formation, evaporation, droplets interaction and viscous fingering.Item Gaze Driven Animation of Eyes(ACM Siggraph, 2015) Neog, Debanga Raj; Ranjan, Anurag; Cardoso, João L.; Pai, Dinesh K.; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaWe present a data driven model of eye movement, that includes movement of the globes, the periorbital soft tissues and eyelids and also the formation of wrinkles in the tissues. We describe a pipeline for measurement and estimation of tissue movement around the eyes using monocular high speed video capture. We use dense optical flow techniques to simultaneously estimate skin and globe motion, as well as high resolution texture images. Our methods are robust to transient occlusions. Finally, we present a system for interactive animation of eyes using a small number of animation parameters, including gaze. These parameters can be obtained from any source, such as keyframe animation or an actor's performance.Item Hands On: Interactive Animation of Precision Manipulation and Contact(ACM Siggraph, 2015) Humberston, Ben; Pai, Dinesh K.; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaHumans show effortless dexterity while manipulating objects using their own hands. However, specifying the motion of a virtual character's hand or of a robotic manipulator remains a difficult task that requires animation expertise or extensive periods of offline motion capture. We present Hands On: a real-time, adaptive animation interface, driven by compliant contact and force information, for animating contact and precision manipulations of virtual objects. Using our interface, an animator controls an abstract grasper trajectory while the full hand pose is automatically shaped by proactive adaptation and compliant scene interactions. Haptic force feedback enables intuitive control by mapping interaction forces from the full animated hand back to the reduced animator feedback space, invoking the same human sensorimotor processes utilized in natural precision manipulations. We provide an approach for online, adaptive shaping of the animated manipulator based on prior interactions, resulting in more functional and appealing motions. The importance of haptic feedback for authoring virtual object manipulations is verified in a user study with nonexpert participants that examines contact force trajectories while using our interface. Comparing the quality of motions produced with and without force rendering, haptic feedback is shown to be critical for efficiently communicating contact forces and dynamic events to the user.Item Hierarchical Planning and Control for Complex Motor Tasks(ACM Siggraph, 2015) Zimmermann, Daniel; Coros, Stelian; Ye, Yuting; Sumner, Robert W.; Gross, Markus; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaWe present a planning and control framework that enables physically simulated characters to perform various types of motor tasks. To create physically-valid motion plans, our method uses a hierarchical set of simplified models. Computational resources are therefore focused where they matter most: motion plans for the immediate future are generated using higher-fidelity models, while coarser models are used to create motion plans with longer time horizons. Our framework can be used for different types of motor skills, including ones where the actions of the arms and legs must be precisely coordinated. We demonstrate controllers for tasks such as getting up from a chair, crawling onto a raised platform, or using a handrail while climbing stairs. All of the motions are simulated using a black-box physics engine from high level user commands, without requiring any motion capture data.Item A Hybrid Crowd Simulation Framework Towards Modeling Behavior of Individual Avoidance of Crowds(ACM Siggraph, 2015) Liu, Haiying; Yan, Zhixin; Lindeman, Robert W.; Ding, Gangyi; Huang, Tianyu; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaCrowd movement is a common but complicated phenomenon in our daily lives. The behaviors of crowds can be affected by both individual and crowd. Most previous research could be categorized as either agent-based methods [van den Berg et al. 2009], which have advantage on simulating individual behaviors, or continuous methods [Narain et al. 2009] which are efficient for simulating crowds with large population. To take advantage of both, [Golas et al. 2013] proposed a hybrid solution which combined and blended both methods. [Bruneau et al. 2015] proposed a virtual reality based study to measure the behavior of individual avoidance of crowds. Their study showed that people have different choices of going through or around based on the density, moving direction and type of crowd. During their experiments, they found significant individual difference between subjects, but not studied as a factor. In this poster, we focus on simulating individual differences on choices of going through or avoiding crowds. We introduce an empirical agent model for individual avoidance behaviors. By integrating personality trait into our agent model, we are able to simulate individual difference of avoid or join behavior. We also present our hybrid crowd simulation framework which can automatically identify individuals and crowds, and explicitly trigger individual avoidance of crowds during simulation.Item Improving Naturalness of Locomotion of Many-Muscle Humanoids(ACM Siggraph, 2015) Yu, Ri; Jo, Dongchul; Lee, Jehee; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaFor many decades, researchers have worked on the simulation of biped locomotion such as human walking. As simulation models have evolved, the simulation using a musculoskeletal model has also become possible [Lee et al. 2014]. Since the number of muscles of models they use is greater than the number of DoF of the models, the optimization problem is undetermined. In order to solve this problem, they use the 2-norm of muscle activations as an objective of optimization and minimize it. However, to obtain more realistic simulation results, real mechanisms of human movement should be applied. In spite of development of the humanoid locomotion simulation, it is still not natural enough due to the lack of knowledge in the mechanisms of human movement. Discussions about this topic have been made, and many people believe that human movement tries to minimize one of the followings; muscle activation, derivatives of muscle activation, joint torque, derivatives of joint torque, metabolic energy expenditure or some combination of these. In this study, we did experiments for each minimization case and compared the results of kinematic data and energy consumption.Item Learning an Inverse Rig Mapping for Character Animation(ACM Siggraph, 2015) Holden, Daniel; Saito, Jun; Komura, Taku; Florence Bertails-Descoubes and Stelian Coros and Shinjiro SuedaWe propose a general, real-time solution to the inversion of the rig function - the function which maps animation data from a character's rig to its skeleton. Animators design character movements in the space of an animation rig, and a lack of a general solution for mapping motions from the skeleton space to the rig space keeps the animators away from the state-of-the-art character animation methods, such as those seen in motion editing and synthesis. Our solution is to use non-linear regression on sparse example animation sequences constructed by the animators, to learn such a mapping offline. When new example motions are provided in the skeleton space, the learned mapping is used to estimate the rig space values that reproduce such a motion. In order to further improve the precision, we also learn the derivative of the mapping, such that the movements can be fine-tuned to exactly follow the given motion. We test and present our system through examples including full-body character models, facial models and deformable surfaces. With our system, animators have the freedom to attach any motion synthesis algorithms to an arbitrary rigging and animation pipeline, for immediate editing. This greatly improves the productivity of 3D animation, while retaining the flexibility and creativity of artistic input.