SCA 08: Eurographics/SIGGRAPH Symposium on Computer Animation

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https://diglib.eg.org/handle/10.2312/428

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Accurate Viscous Free Surfaces for Buckling, Coiling, and Rotating Liquids
(The Eurographics Association, 2008) Batty, Christopher; Bridson, Robert; Markus Gross and Doug James
We present a fully implicit Eulerian technique for simulating free surface viscous liquids which eliminates artifacts in previous approaches, efficiently supports variable viscosity, and allows the simulation of more compelling viscous behaviour than previously achieved in graphics. Our method exploits a variational principle which automatically enforces the complex boundary condition on the shear stress at the free surface, while giving rise to a simple discretization with a symmetric positive definite linear system. We demonstrate examples of our technique capturing realistic buckling, folding and coiling behavior. In addition, we explain how to handle domains whose boundary comprises both ghost fluid Dirichlet and variational Neumann parts, allowing correct behaviour at free surfaces and solid walls for both our viscous solve and the variational pressure projection of Batty et al. [BBB07].
Achieving Good Connectivity in Motion Graphs
(The Eurographics Association, 2008) Zhao, Liming; Safonova, Alla; Markus Gross and Doug James
Motion graphs provide users with a simple yet powerful way to synthesize human motions. While motion graphbased synthesis has been widely successful, the quality of the generatedmotion depends largely on the connectivity of the graph and the quality of transitions in it. However, achieving both of these criteria simultaneously in motion graphs is difficult. Good connectivity requires transitions between less similar poses, while good motion quality results only when transitions happen between very similar poses. This paper introduces a new method for building motion graphs. The method first builds a set of interpolated motion clips, which contain many more similar poses than the original dataset. Using this set, the method then constructs a motion graph and reduces its size by minimizing the number of interpolated poses present in the graph. The outcome of the algorithm is a motion graph called a well-connected motion graph with very good connectivity and only smooth transitions. Our experimental results show that well-connected motion graphs outperform standardmotion graphs across a number of measures, result in very good motion quality, allow for high responsiveness when used for interactive control, and even do not require post-processing of the synthesizedmotions
Action Capture with Accelerometers
(The Eurographics Association, 2008) Slyper, Ronit; Hodgins, Jessica; Markus Gross and Doug James
We create a performance animation system that leverages the power of low-cost accelerometers, readily available motion capture databases, and construction techniques from e-textiles. Our system, built with only off-theshelf parts, consists of five accelerometers sewn into a comfortable shirt that streams data to a computer. The accelerometer readings are continuously matched against accelerations computed from existing motion capture data, and an avatar is animated with the closest match. We evaluate our system visually and using simultaneous motion and accelerometer capture.
Composite Agents
(The Eurographics Association, 2008) Yeh, Hengchin; Curtis, Sean; Patil, Sachin; Berg, Jur van den; Manocha, Dinesh; Lin, Ming; Markus Gross and Doug James
We introduce the concept of composite agents to effectively model complex agent interactions for agent-based crowd simulation. Each composite agent consists of a basic agent that is associated with one or more proxy agents. This formulation allows an agent to exercise influence over other agents greater than that implied by its physical properties. Composite agents can be added to most agent-based simulation systems and used to model emergent behaviors among individuals. In practice, there is negligible overhead of introducing composite agents in the simulation. We highlight their application to modeling aggression, social priority, authority, protection and guidance in complex scenes.
Density Contrast SPH Interfaces
(The Eurographics Association, 2008) Solenthaler, Barbara; Pajarola, Renato; Markus Gross and Doug James
To simulate multiple fluids realistically many important interaction effects have to be captured accurately. Smoothed Particle Hydrodynamics (SPH) has shown to be a simple, yet flexible method to cope with many fluid simulation problems in a robust way. Unfortunately, the results obtained when using SPH to simulate miscible fluids are severely affected, especially if density ratios become large. The undesirable effects reach from unphysical density and pressure variations to spurious and unnatural interface tensions, as well as severe numerical instabilities. In this work, we present a formulation based on SPH which can handle density discontinuities at interfaces between multiple fluids correctly without increasing the computational costs compared to standard SPH. The basic idea is to replace the density computation in SPH by a measure of particle densities and consequently derive new formulations for pressure and viscous forces. The new method enables the user to select the desired amount of interface tension according to the simulation problem at hand. We succeed to stably simulate multiple fluids with high density contrasts without the above described artifacts apparent in standard SPH simulations.
DrivenShape - a Data-driven Approach for Shape Deformation
(The Eurographics Association, 2008) Kim, Tae-Yong; Vendrovsky, Eugene; Markus Gross and Doug James
DrivenShape is a data-driven technique that exploits known correspondence between two sets of shape deformations (e.g. a character's pose and her shirt). It allows users to drive deformation of secondary object simply by animating the pose shape. The tool is especially useful when the corresponding shapes are highly correlated and the space of all the possible shapes is limited. We have successfully used this technique in our recent productions, and it enabled artists to save on both computation time and man hours.
Elevation Cable Modeling for Interactive Simulation of Cranes
(The Eurographics Association, 2008) García-Fernandez, Ignacio; Pla-Castells, Marta; Martinez-Dura, Rafael J.; Markus Gross and Doug James
In this paper, the way to simulate hoisting cables in real time is addressed. We overcome instability in such simulation by considering a two-layered model: a model for the dynamics of a cable passing through a set of pulleys and an oscillation model based on the classical one-dimensional wave equation. The first layer considers the interaction between the cable and pulleys with the elevation equipment, while the second layer simulates cable oscillation. Numerical instability is avoided by suspending the oscillation layer when required. Due to the system properties, this can be carried out in such a way that does not cause significant loss in the system quality. It considers the oscillation of the cable between every pair of pulleys, collision detection and the variation of the cable length very efficiently. Rendering issues are discussed, with remarks on how to prevent aliasing artifacts in the cable. Efficiency is analyzed, including performance tests which show that the model can be run very efficiently. The paper also covers how to integrate the model in a complex multibody simulation with a high degree of interactivity
Evolving Sub-Grid Turbulence for Smoke Animation
(The Eurographics Association, 2008) Schechter, Hagit; Bridson, Robert; Markus Gross and Doug James
We introduce a simple turbulence model for smoke animation, qualitatively capturing the transport, diffusion, and spectral cascade of turbulent energy unresolved on a typical simulation grid. We track the mean kinetic energy per octave of turbulence in each grid cell, and a novel 'net rotation' variable for modeling the self-advection of turbulent eddies. These additions to a standard fluid solver drive a procedural post-process, layering plausible dynamically evolving turbulent details on top of the large-scale simulated motion. Finally, to make the most of the simulation grid before jumping to procedural sub-grid models, we propose a new multistep predictor to alleviate the nonphysical dissipation of angular momentum in standard graphics fluid solvers.
Fast Adaptive Shape Matching Deformations
(The Eurographics Association, 2008) Steinemann, Denis; Otaduy, Miguel A.; Gross, Markus; Markus Gross and Doug James
We present a new shape-matching deformation model that allows for efficient handling of topological changes and dynamic adaptive selection of levels of detail. Similar to the recently presented Fast Lattice Shape Matching (FLSM), we compute the position of simulation nodes by convolution of rigid shape matching operators on many overlapping regions, but we rely instead on octree-based hierarchical sampling and an interval-based region definition. Our approach enjoys the efficiency and robustness of shape-matching deformation models, and the same algorithmic simplicity and linear cost as FLSM, but it eliminates its dense sampling requirements. Our method can handle adaptive spatial discretizations, allowing the simulation of more degrees of freedom in arbitrary regions of interest at little additional cost. The method is also versatile, as it can simulate elastic and plastic deformation, it can handle cuts interactively, and it reuses the underlying data structures for efficient handling of (self-)collisions. All this makes it especially useful for interactive applications such as videogames.
Flexible Simulation of Deformable Models Using Discontinuous Galerkin FEM
(The Eurographics Association, 2008) Kaufmann, Peter; Martin, Sebastian; Botsch, Mario; Gross, Markus; Markus Gross and Doug James
We propose a simulation technique for elastically deformable objects based on the discontinuous Galerkin finite element method (DG FEM). In contrast to traditional FEM, it overcomes the restrictions of conforming basis functions by allowing for discontinuous elements with weakly enforced continuity constraints. This added flexibility enables the simulation of arbitrarily shaped, convex and non-convex polyhedral elements, while still using simple polynomial basis functions. For the accurate strain integration over these elements we propose an analytic technique based on the divergence theorem. Being able to handle arbitrary elements eventually allows us to derive simple and efficient techniques for volumetric mesh generation, adaptive mesh refinement, and robust cutting
Globally Coupled Collision Handling Using Volume Preserving Impulses
(The Eurographics Association, 2008) Sifakis, Eftychios; Marino, Sebastian; Teran, Joseph; Markus Gross and Doug James
We present a novel algorithm for collision processing on triangulated meshes. Our method robustly maintains a collision free state on complex geometries while resorting to collision resolution at time intervals often comparable to the frame rate. Our approach is motivated by the behavior of a thin layer of fluid inserted in the empty space between nearly-colliding parts of the simulated surface, acting as a cushioning mechanism. Point-triangle or edge-edge pairs on a collision course are naturally resolved by the incompressible response of this fluid buffer. This response is formulated into a globally coupled nonlinear system which we solve using Newton iteration and symmetric, positive definite solvers. The globally coupled treatment of collisions allows us to resolve up to two orders of magnitude more collisions than traditional greedy algorithms (e.g. Gauss-Seidel collision response) and take substantially larger time steps without compromising the visual quality of the simulation.
Image-based Collision Detection and Response between Arbitrary Volume Objects
(The Eurographics Association, 2008) Faure, Francois; Barbier, Sebastien; Allard, Jeremie; Falipou, Florent; Markus Gross and Doug James
We present a new image-based method to process contacts between objects bounded by triangular surfaces. Unlike previous methods, it relies on image-based volume minimization, which eliminates complex geometrical computations and robustly handles deep intersections. The surfaces are rasterized in three orthogonal directions, and intersections are detected based on pixel depth and normal orientation. Per-pixel contact forces are computed and accumulated at the vertices. We show how to compute pressure forces which serve to minimize the intersection volume, as well as friction forces. No geometrical precomputation is required, which makes the method efficient for both deformable and rigid objects. We demonstrate it on rigid, skinned, and particle-based physical models with detailed surfaces in contacts at interactive frame rates.
Interactive Terrain Modeling Using Hydraulic Erosion
(The Eurographics Association, 2008) Stava, Ondrej; Benes, Bedrich; Brisbin, Matthew; Krivanek, Jaroslav; Markus Gross and Doug James
We present a step toward interactive physics-based modeling of terrains. A terrain, composed of layers of materials, is edited with interactive modeling tools built upon different physics-based erosion and deposition algorithms. First, two hydraulic erosion algorithms for running water are coupled. Areas where the motion is slow become more eroded by the dissolution erosion, whereas in the areas with faster motion, the force-based erosion prevails. Second, when the water under-erodes certain areas, slippage takes effect and the river banks fall into the water. A variety of local and global editing operation is provided. The user has a great level of control over the process and receives immediate feedback since the GPU-based erosion simulation runs at least at 20 fps on off-the-shelf computers for scenes with grid resolution of 2048×1024 and four layers of material. We also present a divide and conquer approach to handle large terrain erosion, where the terrain is tiled, and each tile calculated independently on the GPU. We show a wide variety of erosion-based modeling features such as forming rivers, drying flooded areas, rain, interactive manipulation with rivers, spring, adding obstacles into the water, etc.
Low Viscosity Flow Simulations for Animation
(The Eurographics Association, 2008) Molemaker, Jeroen; Cohen, Jonathan M.; Patel, Sanjit; Noh, Jonyong; Markus Gross and Doug James
We present a combination of techniques to simulate turbulent fluid flows in 3D. Flow in a complex domain is modeled using a regular rectilinear grid with a finite-difference solution to the incompressible Navier-Stokes equations. We propose the use of the QUICK advection algorithm over a globally high resolution grid. To calculate pressure over the grid, we introduce the Iterated Orthogonal Projection (IOP) framework. In IOP a series of orthogonal projections ensures that multiple conditions such as non-divergence and boundary conditions arising through complex domains shapes or moving objects will be satisfied simultaneously to specified accuracy. This framework allows us to use a simple and highly efficient multigrid method to enforce non-divergence in combination with complex domain boundary conditions. IOP is amenable to GPU implementation, resulting in over an order of magnitude improvement over a CPU-based solver. We analyze the impact of these algorithms on the turbulent energy cascade in simulated fluid flows and the resulting visual quality
Meshless Modeling of Deformable Shapes and their Motion
(The Eurographics Association, 2008) Adams, Bart; Ovsjanikov, Maks; Wand, Michael; Seidel, Hans-Peter; Guibas, Leonidas J.; Markus Gross and Doug James
We present a new framework for interactive shape deformation modeling and key frame interpolation based on a meshless finite element formulation. Starting from a coarse nodal sampling of an object's volume, we formulate rigidity and volume preservation constraints that are enforced to yield realistic shape deformations at interactive frame rates. Additionally, by specifying key frame poses of the deforming shape and optimizing the nodal displacements while targeting smooth interpolated motion, our algorithm extends to a motion planning framework for deformable objects. This allows reconstructing smooth and plausible deformable shape trajectories in the presence of possibly moving obstacles. The presented results illustrate that our framework can handle complex shapes at interactive rates and hence is a valuable tool for animators to realistically and efficiently model and interpolate deforming 3D shapes.
Motion-Motif Graphs
(The Eurographics Association, 2008) Beaudoin, Philippe; Coros, Stelian; Panne, Michiel van de; Poulin, Pierre; Markus Gross and Doug James
We present a technique to automatically distill a motion-motif graph from an arbitrary collection of motion capture data. Motion motifs represent clusters of similar motions and together with their encompassing motion graph they lend understandable structure to the contents and connectivity of large motion datasets. They can be used in support of motion compression, the removal of redundant motions, and the creation of blend spaces. This paper develops a string-based motif-finding algorithm which allows for a user-controlled compromise between motif length and the number of motions in a motif. It allows for time warps within motifs and assigns the majority of the input data to relevant motifs. Results are demonstrated for large datasets (more than 100,000 frames) with computation times of tens of minutes.
Perceptually Guided Expressive Facial Animation
(The Eurographics Association, 2008) Deng, Zhigang; Ma, Xiaohan; Markus Gross and Doug James
Most of current facial animation approaches largely focus on the accuracy or efficiency of their algorithms, or how to optimally utilize pre-collected facial motion data. However, human perception, the ultimate measuring stick of the visual fidelity of synthetic facial animations, was not effectively exploited in these approaches. In this paper, we present a novel perceptually guided computational framework for expressive facial animation, by bridging objective facial motion patterns with subjective perceptual outcomes. First, we construct a facial perceptual metric (FacePEM) using a hybrid of region-based facial motion analysis and statistical learning techniques. The constructed FacePEM model can automatically measure the emotional expressiveness of a facial motion sequence. We showed how the constructed FacePEM model can be effectively incorporated into various facial animation algorithms. For the sake of clear demonstrations, we choose data-driven expressive speech animation generation and expressive facial motion editing as two concrete application examples. Through a comparative user study, we showed that comparing with the traditional facial animation algorithms, the introduced perceptually guided expressive facial animation algorithms can significantly increase the emotional expressiveness and perceptual believability of synthesized facial animations.
Pose-Space Animation and Transfer of Facial Details
(The Eurographics Association, 2008) Bickel, Bernd; Lang, Manuel; Botsch, Mario; Otaduy, Miguel A.; Gross, Markus; Markus Gross and Doug James
This paper presents a novel method for real-time animation of highly-detailed facial expressions based on a multi-scale decomposition of facial geometry into large-scale motion and fine-scale details, such as expression wrinkles. Our hybrid animation is tailored to the specific characteristics of large- and fine-scale facial deformations: Large-scale deformations are computed with a fast linear shell model, which is intuitively and accurately controlled through a sparse set of motion-capture markers or user-defined handle points. Fine-scale facial details are incorporated using a novel pose-space deformation technique, which learns the correspondence of sparse measurements of skin strain to wrinkle formation from a small set of example poses. Our hybrid method features real-time animation of highly-detailed faces with realistic wrinkle formation, and allows both large-scale deformations and fine-scale wrinkles to be edited intuitively. Furthermore, our pose-space representation enables the transfer of facial details to novel expressions or other facial models.
Puppet Master: Designing Reactive Character Behavior by Demonstration
(The Eurographics Association, 2008) Young, James E.; Igarashi, Takeo; Sharlin, Ehud; Markus Gross and Doug James
Puppet Master is a system that enables designers to rapidly create interactive and autonomous animated character behaviors that react to a main character controlled by an end-user. The behavior is designed by demonstration, allowing non-technical artists to intuitively design the style, personality, and emotion of the character, traits which are very difficult to design using conventional programming. During training, designers demonstrate paired behavior between the main and reacting characters. During run time, the end user controls the main character and the system synthesizes the motion of the reacting character using the given training data. The algorithm is an extension of Image Analogies, modified to synthesize dynamic character behavior instead of an image. We introduce non-trivial extensions to the algorithm such as our selection of features, dynamic balancing between similarity metrics, and separate treatment of path trajectory and high-frequency motion texture. We implemented a prototype system using physical pucks tracked by a motion-capture system and conducted a user study demonstrating that novice users can easily and successfully design character personality and emotion using our system and that the resulting behaviors are meaningful and engaging
Real-Time Planning for Parameterized Human Motion
(The Eurographics Association, 2008) Lo, Wan-Yen; Zwicker, Matthias; Markus Gross and Doug James
We present a novel approach to learn motion controllers for real-time character animation based on motion capture data. We employ a tree-based regression algorithm for reinforcement learning, which enables us to generate motions that require planning. This approach is more flexible and more robust than previous strategies. We also extend the learning framework to include parameterized motions and interpolation. This enables us to control the character more precisely with a small amount of motion data. Finally, we present results of our algorithm for three different types of controllers.

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