SCA 15: Eurographics/SIGGRAPH Symposium on Computer Animation

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https://diglib.eg.org/handle/10.2312/14634

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Divergence-Free Smoothed Particle Hydrodynamics
(ACM Siggraph, 2015) Bender, Jan; Koschier, Dan; Florence Bertails-Descoubes and Stelian Coros and Shinjiro Sueda
In 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.
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 Sueda
Crowd 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.
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 Sueda
We 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.
Simulation of Fluid Mixing with Interface Control
(ACM Siggraph, 2015) He, Xiaowei; Wang, Huamin; Zhang, Fengjun; Wang, Hongan; Wang, Guoping; Zhou, Kun; Wu, Enhua; Florence Bertails-Descoubes and Stelian Coros and Shinjiro Sueda
The simulation of fluid mixing under the Eulerian framework often su ers from numerical dissipation issues. In this paper, we present a mass-preserving convection scheme that o ers direct control on the shape of the interface. The key component of this scheme is a sharpening term built upon the di usive flux of a userspecified kernel function. To determine the thickness of the ideal interface during fluid mixing, we perform theoretical analysis on a one-dimensional di usive model using the Fick's law of di usion. By explicitly controlling the interface thickness using a spatio-temporally varying kernel variable, we can use our scheme to produce realistic fluid mixing e ects without numerical dissipation artifacts. We can also use the scheme to control interface changes between two fluids, due to temperature, pressure, or external energy input. This convection scheme is compatible with many advection methods and it has a small computational overhead.
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 Sueda
Creating 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.
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 Sueda
Knitted 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.
Effect of Appearance on Perception of Deformation
(ACM Siggraph, 2015) Han, Donghui; Keyser, John; Florence Bertails-Descoubes and Stelian Coros and Shinjiro Sueda
We 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.
Multifarious Hierarchies of Mechanical Models for Artist Assigned Levels-of-Detail
(ACM Siggraph, 2015) Malgat, Richard; Gilles, Benjamin; Levin, David I.W.; Nesme, Matthieu; Faure, François; Florence Bertails-Descoubes and Stelian Coros and Shinjiro Sueda
We present a new framework for artist driven level of detail in solid simulations. Simulated objects are simultaneously embedded in several, separately designed deformation models with their own independent degrees of freedom. The models are ordered to apply their deformations hierarchically, and we enforce the uniqueness of the dynamics solutions using a novel kinetic filtering operator designed to ensure that each child only adds detail motion to its parent without introducing redundancies. This new approach allows artists to easily add fine-scale details without introducing unnecessary degrees-of-freedom to the simulation or resorting to complex geometric operations like anisotropic volume meshing. We illustrate the utility of our approach with several detail enriched simulation examples.
Tracking Control for Streaming Input Motion Using Segmented Foot Model
(ACM Siggraph, 2015) Lee, Seokjae; Park, Hwangpil; Lee, Jehee; Florence Bertails-Descoubes and Stelian Coros and Shinjiro Sueda
Motion capture data has been widely used to make realistic animation. Especially, in physics-based character simulation, motion data provides plausible reference trajectory of the character. However, it requires post-processing of the unnatural motion data such as foot-skating and use specified controller with different parameters for each motion. Recent advancements in motion capture hardware such as Kinect allow us to obtain various motion data low-cost and easy way in real-time, but still it remains challenge to simulate unpredictable motion with common controller. The human foot is complex structure containing 26 bones and it allows complex movements with balance. However, widely used human dynamics model has simple foot model which consist of one or two body, so it has functional limitations such as keeping flatfoot while static balancing. In our work, we propose new foot model that mimics the human foot to control various motion in realtime. Because foot contact condition with the ground is critical to maintain balance, more contact points and additional DOFs from segmented foot provide ability to control foot and whole body to be desired state in various foot contact condition.
Trajectory-Free Reactive Stepping of Physics-based Character Using Momentum Control
(ACM Siggraph, 2015) Lee, Sukwon; Choi, Hyunchul; Jin, Taeil; Lee, Sung-Hee; Florence Bertails-Descoubes and Stelian Coros and Shinjiro Sueda
We introduce a trajectory-free reactive stepping controller using momentum control. The controller is characterized by moving passively in the direction of external pushes without attempting to follow some prescribed trajectory, thereby achieving a natural reactive stepping behavior adaptive to various perturbations.
A new sharp-crease bending element for folding and wrinkling surfaces and volumes
(ACM Siggraph, 2015) Patkar, Saket; Jin, Ning; Fedkiw, Ronald; Florence Bertails-Descoubes and Stelian Coros and Shinjiro Sueda
We present a novel sharp-crease bending element for the folding and wrinkling of surfaces and volumes. Based on a control curve specified by an artist or derived from internal stresses of a simulation, we create a piecewise linear curve at the resolution of the computational mesh. Then, the key idea is to cut the object along the curve using the virtual node algorithm creating new degrees of freedom, while subsequently reattaching the resulting pieces eliminating the translational degrees of freedom so that adjacent pieces may only rotate or bend about the cut. Motivated by an articulated rigid body framework, we utilize the concepts of pre-stabilization and post-stabilization in order to enforce these reattachment constraints. Our cuts can be made either razor sharp or relatively smooth via the use of bending springs. Notably, our sharp-crease bending elements can not only be used to create pleats in cloth or folds in paper but also to create similar buckling in volumetric objects. We illustrate this with examples of forehead wrinkles and nasolabial folds for facial animation. Moreover, our sharp-crease bending elements re- quire minimal extra simulation time as compared to the underlying mesh, and tend to reduce simulation times by an order of magnitude when compared to the alternative of mesh refinement.
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 Sueda
The 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.
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 Sueda
We 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.
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 Sueda
We 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.
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 Sueda
For 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.
Discovering Human Postures from Non-human Silhouette
(ACM Siggraph, 2015) Won, Jungdam; Lee, Jehee; Florence Bertails-Descoubes and Stelian Coros and Shinjiro Sueda
The 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.
Sketch Abstractions for Character Posing
(ACM Siggraph, 2015) Hahn, Fabian; Mutzel, Frederik; Coros, Stelian; Thomaszewski, Bernhard; Nitti, Maurizio; Gross, Markus; Sumner, Robert W.; Florence Bertails-Descoubes and Stelian Coros and Shinjiro Sueda
We propose a sketch-based posing system for rigged 3D characters that allows artists to create custom sketch abstractions on top of a character's actual shape. A sketch abstraction is composed of rigged curves that form an iconographic 2D representation of the character from a particular viewpoint. When provided with a new input sketch, our optimization system minimizes a nonlinear iterative closest point energy to find the rigging parameters that best align the character's sketch abstraction to the input sketch. A custom regularization term addresses the underconstrained nature of the problem to select favorable poses. Although our system supports arbitrary black-box rigs, we show how to optimize computations when rigging formulas and derivatives are available. We demonstrate our system's flexibility with examples showing different artist-designed sketch abstractions for both full body posing and the customization of individual components of a modular character. Finally, we show that simple sketch abstractions can be built on the fly by projecting a drawn curve onto the character's mesh. Redrawing the curve allows the user to dynamically pose the character. Taken together, our system enables a new form of intuitive sketch-based posing in which the character designer has the freedom to prescribe the sketch abstraction that is most meaningful for the character.
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 Sueda
When 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.
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 Sueda
We 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.
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 Sueda
Humans 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.

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