SCA 15: Eurographics/SIGGRAPH Symposium on Computer Animation

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

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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 Sueda
We 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.
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 Sueda
Cloth 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.
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 Sueda
An 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.
Learning Reduced-Order Feedback Policies for Motion Skills
(ACM Siggraph, 2015) Ding, Kai; Liu, Libin; Panne, Michiel van de; Yin, KangKang; Florence Bertails-Descoubes and Stelian Coros and Shinjiro Sueda
We introduce a method for learning low-dimensional linear feedback strategies for the control of physics-based animated characters around a given reference trajectory. This allows for learned low-dimensional state abstractions and action abstractions, thereby reducing the need to rely on manually designed abstractions such as the center-of-mass state or foot-placement actions. Once learned, the compact feedback structure allow simulated characters to respond to changes in the environment and changes in goals. The approach is based on policy search in the space of reduced-order linear output feedback matrices. We show that these can be used to replace or further reduce manually-designed state and action abstractions. The approach is sufficiently general to allow for the development of unconventional feedback loops, such as feedback based on ground reaction forces. Results are demonstrated for a mix of 2D and 3D systems, including tilting-platform balancing, walking, running, rolling, targeted kicks, and several types of ballhitting tasks.
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 Sueda
We 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.
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 Sueda
We 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.

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