SCA 10: Eurographics/SIGGRAPH Symposium on Computer Animation

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https://diglib.eg.org/handle/10.2312/426

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Linear-Time Dynamics for Multibody Systems with General Joint Models
(The Eurographics Association, 2010) Si, Weiguang; Guenter, Brian; MZoran Popovic and Miguel Otaduy
Most current linear-time forward dynamics algorithms support only simple types of joints due to difficulties in computing derivatives of joint transformations up to order two.We apply the D* symbolic differentiation algorithm to a recursive formulation of forward dynamics to get a highly efficient linear-time forward dynamics algorithm supporting multibody systems with general scleronomic joints. With this new algorithm we can easily build a treetopology multibody system with complex joint models and perform forward dynamics efficiently. The source code for the algorithm is freely available for non-commercial use.
A Real-time Cinematography System for Interactive 3D Environments
(The Eurographics Association, 2010) Lino, Christophe; Christie, Marc; Lamarche, Fabrice; Schofield, Guy; Olivier, Patrick; MZoran Popovic and Miguel Otaduy
Developers of interactive 3D applications, such as computer games, are expending increasing levels of effort on the challenge of creating more narrative experiences in virtual worlds. As a result, there is a pressing requirement to automate an essential component of a narrative - the cinematography - and develop camera control techniques that can be utilized within the context of interactive environments in which actions are not known in advance. Such camera control algorithms should be capable of enforcing both low-level geometric constraints, such as the visibility of key subjects, and more elaborate properties related to cinematic conventions such as characteristic viewpoints and continuity editing. In this paper, we present a fully automated real-time cinematography system that constructs a movie from a sequence of low-level narrative elements (events, key subjects actions and key subject motions). Our system computes appropriate viewpoints on these narrative elements, plans paths between viewpoints and performs cuts following cinematic conventions. Additionally, it offers an expressive framework which delivers notable variations in directorial style. Our process relies on a viewpoint space partitioning technique in 2D that identifies characteristic viewpoints of relevant actions for which we compute the partial and full visibility. These partitions, to which we refer as Director Volumes, provide a full characterization over the space of viewpoints. We build upon this spatial characterization to select the most appropriate director volumes, reason over the volumes to perform appropriate camera cuts and rely on traditional path-planning techniques to perform transitions. Our system represents a novel and expressive approach to cinematic camera control which stands in contrast to existing techniques that are mostly procedural, only concentrate on isolated aspects (visibility, transitions, editing, framing) or do not encounter for variations in directorial style.
Control Systems for Human Running using an Inverted Pendulum Model and a Reference Motion Capture Sequence
(The Eurographics Association, 2010) Kwon, Taesoo; Hodgins, Jessica; MZoran Popovic and Miguel Otaduy
Physical simulation is often proposed as a way to generate motion for interactive characters. A simulated character has the potential to adapt to changing terrain and disturbances in a realistic and robust manner. In this paper, we present a balancing control algorithm based on a simplified dynamic model, an inverted pendulum on a cart. The simplified model lacks the degrees of freedom found in a full human model, so we analyze a captured reference motion in a preprocessing step and use that information about human running patterns to supplement the balance algorithms provided by the inverted pendulum controller. At run-time, the controller plans a desired motion at every frame based on the current estimate of the pendulum state and a predicted pendulum trajectory. By tracking this time-varying trajectory, our controller creates a running character that dynamically balances, changes speed and makes turns. The initial controller can be optimized to further improve the motion quality with an objective function that minimizes the difference between a planned desired motion and a simulated motion. We demonstrate the power of this approach by generating running motions at a variety of speeds (3m/s to 5m/s), following a curved path, and in the presence of disturbance forces and a skipping motion.
BoLeRO: A Principled Technique for Including Bone Length Constraints in Motion Capture Occlusion Filling
(The Eurographics Association, 2010) Li, Lei; McCann, James; Pollard, Nancy; Faloutsos, Christos; MZoran Popovic and Miguel Otaduy
Given a motion capture sequence with occlusions, how can we recover the missing values, respecting bone-length constraints? Recent past work uses Linear Dynamical Systems (LDS), which work well, except for occasionally violating such constraints, and thus lead to unrealistic results. Our main contribution is a principled approach for preserving such distances. Specifically (a) we show how to formulate the problem as a constrained optimization problem, using two variations: hard constraints, and soft constraints; (b) we show how to efficiently solve both variations; (c) we demonstrate the realism of our approaches against competitors, on real motion capture data, illustrating that our 'soft constraints' version eventually produces more realistic results
Enhancing Fluid Animation with Adaptive, Controllable and Intermittent Turbulence
(The Eurographics Association, 2010) Zhao, Ye; Yuan, Zhi; Chen, Fan; MZoran Popovic and Miguel Otaduy
This paper proposes a new scheme for enhancing fluid animation with controllable turbulence. An existing fluid simulation from ordinary fluid solvers is fluctuated by turbulent variation modeled as a random process of forcing. The variation is precomputed as a sequence of solenoidal noise vector fields directly in the spectral domain, which is fast and easy to implement. The spectral generation enables flexible vortex scale and spectrum control following a user prescribed energy spectrum, e.g. Kolmogorov's cascade theory, so that the fields provide fluctuations in subgrid scales and/or in preferred large octaves. The vector fields are employed as turbulence forces to agitate the existing flow, where they act as a stimulus of turbulence inside the framework of the Navier-Stokes equations, leading to natural integration and temporal consistency. The scheme also facilitates adaptive turbulent enhancement steered by various physical or user-defined properties, such as strain rate, vorticity, distance to objects and scalar density, in critical local regions. Furthermore, an important feature of turbulent fluid, intermittency, is created by applying turbulence control during randomly selected temporal periods.
Reconstructing Surfaces of Particle-Based Fluids Using Anisotropic Kernels
(The Eurographics Association, 2010) Yu, Jihun; Turk, Greg; MZoran Popovic and Miguel Otaduy
In this paper we present a novel surface reconstruction method for particle-based fluid simulators such as Smoothed Particle Hydrodynamics. In particle-based simulations, fluid surfaces are usually defined as a level set of an implicit function. We formulate the implicit function as a sum of anisotropic smoothing kernels, and the direction of anisotropy at a particle is determined by performing Principal Component Analysis (PCA) over the neighboring particles. In addition, we perform a smoothing step that re-positions the centers of these smoothing kernels. Since these anisotropic smoothing kernels capture the local particle distributions more accurately, our method has advantages over existing methods in representing smooth surfaces, thin streams and sharp features of fluids. Our method is fast, easy to implement, and our results demonstrate a significant improvement in the quality of reconstructed surfaces as compared to existing methods.
Editing Dynamic Human Motions via Momentum and Force
(The Eurographics Association, 2010) Sok, Kwang Won; Yamane, Katsu; Lee, Jehee; Hodgins, Jessica K.; MZoran Popovic and Miguel Otaduy
We present an integrated framework for interactive editing of the momentum and external forces in a motion capture sequence. Allowing user control of the momentum and forces provides a powerful and intuitive editing tool for dynamic motions. To make a higher jump, for example, the user simply increases the linear momentum in the vertical direction, while our system automatically calculates a motion that maintains both the same landing position and physical plausibility. Our key insight is using trajectory optimization based on normalized dynamics to simultaneously propagate momentum and force space changes. We demonstrate our approach with edits of long sequences of dynamic actions, including kicks, jumps, and spins.
A Bayesian Interactive Optimization Approach to Procedural Animation Design
(The Eurographics Association, 2010) Brochu, Eric; Brochu, Tyson; Freitas, Nando de; MZoran Popovic and Miguel Otaduy
eters. In many cases, the models are complex and the parameters unintuitive for non-experts. In this paper, we present an optimization method for setting parameters of a procedural fluid animation system by showing the user examples of different parametrized animations and asking for feedback. Our method employs the Bayesian technique of bringing in prior belief based on previous runs of the system and/or expert knowledge, to assist users in finding good parameter settings in as few steps as possible. To do this, we introduce novel extensions to Bayesian optimization, which permit effective learning for parameter-based procedural animation applications. We show that even when users are trying to find a variety of different target animations, the system can learn and improve. We demonstrate the effectiveness of our method compared to related active learning methods. We also present a working application for assisting animators in the challenging task of designing curl-based velocity fields, even with minimal domain knowledge other than identifying when a simulation looks right .
FASTCD: Fracturing-Aware Stable Collision Detection
(The Eurographics Association, 2010) Heo, Jae-Pil; Seong, Joon-Kyung; Kim, DukSu; Otaduy, Miguel A.; Hong, Jeong-Mo; Tang, Min; Yoon, Sung-Eui; MZoran Popovic and Miguel Otaduy
We present a collision detection (CD) method for complex and large-scale fracturing models that have geometric and topological changes. We first propose a novel dual-cone culling method to improve the performance of CD, especially self-collision detection among fracturing models. Our dual-cone culling method has a small computational overhead and a conservative algorithm. Combined with bounding volume hierarchies (BVHs), our dual-cone culling method becomes approximate. However, we found that our method does not miss any collisions in the tested benchmarks. We also propose a novel, selective restructuring method that improves the overall performance of CD and reduces performance degradations at fracturing events. Our restructuring method is based on a culling efficiency metric that measures the expected number of overlap tests of a BVH. To further reduce the performance degradations at fracturing events, we also propose a novel, fast BVH construction method that builds multiple levels of the hierarchy in one iteration using a grid and hashing. We test our method with four different large-scale deforming benchmarks. Compared to the state-of-the-art methods, our method shows a more stable performance for CD by improving the performance by a factor of up to two orders of magnitude at frames when deforming models change their mesh topologies
Constraint-Based Simulation of Adhesive Contact
(The Eurographics Association, 2010) Gascon, Jorge; Zurdo, Javier S.; Otaduy, Miguel A.; MZoran Popovic and Miguel Otaduy
Dynamics with contact are often formulated as a constrained optimization problem. This approach allows handling in an integrated manner both non-penetration and frictional constraints. Following developments in the computational mechanics field, we have designed an algorithm for adding the simulation of adhesive contact constraints in the context of state-of-the-art constraint-based contact solvers. We show that implicit adhesion constraints can be handled with minor changes to existing solvers, and we demonstrate our algorithm on a diverse range of objects, including mass-spring cloth, volumetric finite-element models, and rigid bodies.
Goal-Directed Stepping with Momentum Control
(The Eurographics Association, 2010) Wu, Chun-Chih; Zordan, Victor; MZoran Popovic and Miguel Otaduy
This paper proposes a technique for animating simulated characters to perform controlled steps. The desired step is controlled by high-level goals, namely step position and step duration. These stepping goals guide the desired time-varying values for the center of mass and the stepping foot which in turn lead to objectives dictating the desired changes in momentum and joint angles over the duration of the step. Our approach employs a multiobjective optimization to solve for joint accelerations from the objectives and uses inverse dynamics to compute joint torques. Our approach can guide a character with purposeful, directable steps for controlling careful navigation of the character's position and orientation. In addition, the same system can be used to create protective steps to prevent falling as a reaction to a disturbance. A novel supervisory routine automatically chooses when and where to step based on an analysis of the momentum conditions for the character. We contrast this approach to previous methods for step recovery using the inverted pendulum.
Performance Capture with Physical Interaction
(The Eurographics Association, 2010) Nguyeny, Nam; Wheatland, Nkenge; Brown, David; Parise, Brian; Liu, C. Karen; Zordan, Victor; MZoran Popovic and Miguel Otaduy
This paper introduces a technique for combining performance-based animation with a physical model in order to synthesize complex interactions in an animated scene. The approach is to previsualize interaction of final integrated scene, online, while the performance is being recorded. To accomplish this goal, we propose a framework which unifies kinematic playback of motion capture and dynamic motion synthesis. The proposed method augments a real-time recording of a human actor with dynamics-based response in order to modify motion data based on the conditions of the character. The system unifies kinematic and dynamic aspects of the final motion while allowing user control over the outcome both temporally and spatially across the character's body. Examples of complex interactions interleaved with intelligent response underscore the power of the technique along with multi-person captures in which remote users interact physically in a shared virtual world.
Animating Non-Humanoid Characters with Human Motion Data
(The Eurographics Association, 2010) Yamane, Katsu; Ariki, Yuka; Hodgins, Jessica; MZoran Popovic and Miguel Otaduy
This paper presents a method for generating animations of non-humanoid characters from human motion capture data. Characters considered in this work have proportion and/or topology significantly different from humans, but are expected to convey expressions and emotions through body language that are understandable to human viewers. Keyframing is most commonly used to animate such characters. Our method provides an alternative for animating non-humanoid characters that leverages motion data from a human subject performing in the style of the target character. The method consists of a statistical mapping function learned from a small set of corresponding key poses, and a physics-based optimization process to improve the physical realism.We demonstrate our approach on three characters and a variety of motions with emotional expressions
Wrinkle Meshes
(The Eurographics Association, 2010) Müller, Matthias; Chentanez, Nuttapong; MZoran Popovic and Miguel Otaduy
We present a simple and fast method to add wrinkles to dynamic meshes such as simulated cloth or the skin of an animated character. To get the desired surface details, we attach a higher resolution wrinkle mesh to the coarse base mesh allowing the wrinkle vertices to deviate from their attachment positions within a limited range. The shape of the wrinkle mesh is determined by a static solver which runs in parallel to the motion of the base mesh. Our method can be used to automatically enhance a purely animated skin mesh with wrinkles which would be a tedious task to do by hand. The fact that the tessellation of the wrinkle mesh can be chosen independently of the structure of the base mesh can be used to control the look of the wrinkles. The locations of wrinkle formation can be defined by painting the maximum distance the wrinkle mesh is allowed to deviate from the base mesh. The second important application of wrinkle meshes is to add detail to simulated meshes such as cloth. Our method allows one to reduce the resolution of the simulation mesh without losing interesting surface detail. This speeds up the simulation, collision detection and handling and it reduces stretchiness. We show the efficiency and visual quality of the approach in a real-time setting.
Augmenting Hand Animation with Three-dimensional Secondary Motion
(The Eurographics Association, 2010) Jain, Eakta; Sheikh, Yaser; Mahler, Moshe; Hodgins, Jessica; MZoran Popovic and Miguel Otaduy
Secondary motion, or the motion of objects in response to that of the primary character, is widely used to amplify the audience's response to the character's motion and to provide a connection to the environment. These threedimensional (3D) effects are largely passive and tend to be time consuming to animate by hand, yet most are very effectively simulated in current animation software. In this paper, we present a technique for augmenting hand-drawn animation of human characters with 3D physical effects to create secondary motion. In particular, we create animations in which hand-drawn characters interact with cloth and clothing, dynamically simulated balls and particles, and a simple fluid simulation. The driving points or volumes for the secondary motion are tracked in two dimensions, reconstructed into three dimensions, and used to drive and collide with the simulated objects. Our technique employs user interaction that can be reasonably integrated into the traditional animation pipeline of drawing, cleanup, inbetweening, and coloring.
Interactive SPH Simulation and Rendering on the GPU
(The Eurographics Association, 2010) Goswami, Prashant; Schlegel, Philipp; Solenthaler, Barbara; Pajarola, Renato; MZoran Popovic and Miguel Otaduy
In this paper we introduce a novel parallel and interactive SPH simulation and rendering method on the GPU using CUDA which allows for high quality visualization. The crucial particle neighborhood search is based on Z-indexing and parallel sorting which eliminates GPU memory overhead due to grid or hierarchical data structures. Furthermore, it overcomes limitations imposed by shading languages allowing it to be very flexible and approaching the practical limits of modern graphics hardware. For visualizing the SPH simulation we introduce a new rendering pipeline. In the first step, all surface particles are efficiently extracted from the SPH particle cloud exploiting the simulation data. Subsequently, a partial and therefore fast distance field volume is rasterized from the surface particles. In the last step, the distance field volume is directly rendered using state-of-the-art GPU raycasting. This rendering pipeline allows for high quality visualization at very high frame rates.
PLEdestrians: A Least-Effort Approach to Crowd Simulation
(The Eurographics Association, 2010) Guy, Stephen J.; Chhugani, Jatin; Curtis, Sean; Dubey, Pradeep; Lin, Ming; Manocha, Dinesh; MZoran Popovic and Miguel Otaduy
We present a new algorithm for simulating large-scale crowds at interactive rates based on the Principle of Least Effort. Our approach uses an optimization method to compute a biomechanically energy-efficient, collision-free trajectory that minimizes the amount of effort for each heterogeneous agent in a large crowd. Moreover, the algorithm can automatically generate many emergent phenomena such as lane formation, crowd compression, edge and wake effects ant others. We compare the results from our simulations to data collected from prior studies in pedestrian and crowd dynamics, and provide visual comparisons with real-world video. In practice, our approach can interactively simulate large crowds with thousands of agents on a desktop PC and naturally generates a diverse set of emergent behaviors
Fast Local and Global Similarity Searches in Large Motion Capture Databases
(The Eurographics Association, 2010) Krueger, Bjoern; Tautges, Jochen; Weber, Andreas; Zinke, Arno; MZoran Popovic and Miguel Otaduy
Fast searching of content in large motion databases is essential for efficient motion analysis and synthesis. In this work we demonstrate that identifying locally similar regions in human motion data can be practical even for huge databases, if medium-dimensional (15 - 90 dimensional) feature sets are used for kd-tree-based nearest-neighborsearches. On the basis of kd-tree-based local neighborhood searches we devise a novel fast method for global similarity searches. We show that knn-searches can be used efficiently within the problems of (a) "numerical and logical similarity searches", (b) reconstruction of motions from sparse marker sets, and (c) building so called "fat graphs", tasks for which previously algorithms with preprocessing time quadratic in the size of the database and thus only applicable to small collections of motions had been presented. We test our techniques on the two largest freely available motion capture databases, the CMU and HDM05 motion databases comprising more than 750 min of motion capture data proving that our approach is not only theoretically applicable but also solves the problem of fast similarity searches in huge motion databases in practice.
Real-time Simulation of Large Bodies of Water with Small Scale Details
(The Eurographics Association, 2010) Chentanez, Nuttapong; Mueller, Matthias; MZoran Popovic and Miguel Otaduy
We present a hybrid water simulation method that combines grid based and particles based approaches. Our specialized shallow water solver can handle arbitrary underlying terrain slopes, arbitrary water depth and supports wet-dry regions tracking. To treat open water scenes we introduce a method for handling non-reflecting boundary conditions. Regions of liquid that cannot be represented by the height field including breaking waves, water falls and splashing due to rigid and soft bodies interaction are automatically turned into spray, splash and foam particles. The particles are treated as simple non-interacting point masses and they exchange mass and momentum with the height field fluid. We also present a method for procedurally adding small scale waves that are advected with the water flow. We demonstrate the effectiveness of our method in various test scene including a large flowing river along a valley with beaches, big rocks, steep cliffs and waterfalls. Both the grid and the particles simulations are implemented in CUDA. We achieve real-time performance on modern GPUs in all the examples.
Point Cloud Glue: Constraining Simulations Using the Procrustes Transform
(The Eurographics Association, 2010) Twigg, Christopher D.; Kacic-Alesic, Zoran; MZoran Popovic and Miguel Otaduy
In physical simulation, it is frequently useful to define constraints between deformable objects, ensuring that one object follows another. Existing techniques for enforcing these constraints define the relationship between the objects using barycentric coordinates, a linear combination of vertices. While simple to implement and understand, barycentric coordinates have one important drawback: for stability, weights must be non-negative, which limits the types of constraints that can be defined. We introduce the Point Cloud Glue, which uses the nearest fit rigid rotation (the Procrustes transform) to the deformable object's particles. Our key contribution is to demonstrate that we can differentiate through this minimization in a numerically stable manner, allowing our method to be used in many constrained dynamics systems including those based on bindings/embeddings and those based on Lagrange multipliers. We demonstrate the flexibility of our method through several examples.

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