SCA 04: Eurographics/SIGGRAPH Symposium on Computer Animation
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Item Finding Paths for Coherent Groups using Clearance(The Eurographics Association, 2004) Kamphuis, Arno; Overmars, Mark H.; R. Boulic and D. K. PaiVirtual environment are often populated with moving units and the paths for these units should be planned. When multiple units need to exhibit coherent behavior in a cluttered environment, current techniques often fail, i.e. the resulting paths for the units in the group lack the coherence required. In this paper, we propose a novel approach to motion planning for coherent groups of units. The method presented uses a path for a single unit, called the backbone path, which can be generated by any motion planner. This backbone path is extended to a corridor using the clearance along the path. The units can move freely inside this corridor. By limiting the width of this corridor, and the extent along the corridor where the units can move to, the approach guarantees coherence of the group. Experiments show that the generated paths exhibit group coherence as required, like passing on the same side of obstacles and waiting for fellow group mates to catch up. Performance measurements show that the approach is capable of generating the paths in real-time. In our implementation, the method requires just a few percent of the processor time for groups consisting of up to 100 units.Item Synchronization for dynamic blending of motions(The Eurographics Association, 2004) Ménardais, Stéphane; Kulpa, Richard; Multon, Franck; Arnaldi, Bruno; R. Boulic and D. K. PaiIn this paper we present a new real-time synchronization algorithm. In dynamic environments, motions need to be continuously adapted to obtain realistic animations. We propose an advanced time warping algorithm to synchronize such motions. This algorithm uses the sequence of support phases of the motions. It also takes into account the priority associated to each motion. It is based on an algebraic relation to detect incompatible motions and to select elements of the sequence to be enlarged. The resulting time warping function can be non-derivable so it is corrected by using a cardinal spline interpolation. In this paper, we demonstrate that our algorithm always finds at least one solution. This synchronization module is part of a complete animation engine called MKM already used in production.Item Better with Bubbles: Enhancing the Visual Realism of Simulated Fluid(The Eurographics Association, 2004) Greenwood, Shannon T.; House, Donald H.; R. Boulic and D. K. PaiWe present a method for including the visual effect of bubbles in a computer graphics fluid simulation, thus enhancing the illusion of realism for a splashing fluid. Previous fluid simulation methods have not included bubbles. Bubble creation is integrated into the particle level-set fluid simulation algorithm. Individual bubbles are approximated by spheres, which form more complex shapes where they intersect. The rendering of bubbles and fluid are integrated to create the appearance of one continuous surface. At the fluid-air boundary, we integrate bubbles whenever level-set marker particles pass from from the outside to the inside of the fluid. Thus, these particles represent air that has become trapped within the fluid surface. In addition, we detect empty pockets within the fluid, that are often formed due to turbulence, and create bubbles within this space. This is an inexpensive way of giving the impression that the air trapped in air pockets has become bubbles. Photo-realistic images of simulation results are rendered with a raytracer that has been enhanced to include caustics, and to handle bubble-bubble interfaces. Comparison of these images with images rendered without bubbles supports our position that the simple addition of bubbles to a fluid simulation greatly enhances visual realism.Item Precomputing Avatar Behavior from Human Motion Data(The Eurographics Association, 2004) Lee, Jehee; Lee, Kang Hoon; R. Boulic and D. K. PaiCreating controllable, responsive avatars is an important problem in computer games and virtual environments. Recently, large collections of motion capture data have been exploited for increased realism in avatar animation and control. Large motion sets have the advantage of accommodating a broad variety of natural human motion. However, when a motion set is large, the time required to identify an appropriate sequence of motions is the bottleneck for achieving interactive avatar control. In this paper, we present a novel method of precomputing avatar behavior from unlabelled motion data in order to animate and control avatars at minimal runtime cost. Based on dynamic programming, our method finds a control policy that indicates how the avatar should act in any given situation. We demonstrate the effectiveness of our approach through examples that include avatars interacting with each other and with the user.Item Performance Timing for Keyframe Animation(The Eurographics Association, 2004) Terra, Silvio C. L.; Metoyer, Ronald A.; R. Boulic and D. K. PaiKeyframing is a standard technique for generating computer animation that typically requires artistic ability and a set of skills for the software package being used. We are interested in addressing the needs of the novice animator who is not necessarily artistically skilled or familiar with keyframing interfaces. From our experience observing novice animators, it is clear that setting keyframe values is straightforward while specifying the keyframe timing is difficult and often time consuming. We present a novel method for novice users to time keyframes using gestures without changing the motion itself. The key to our approach is the separation of specification of keyframe values from the specification of keyframe timing. Our approach allows the user to 'act-out' the timing information using a simple 2D input device such as a mouse or pen-tablet. The user's input is analyzed and features of the user's input are mapped to features of the keyframed motion. The keyframes are then distributed in time according to the timing of the user's input path. We have implemented the approach as a plugin to the AliasWavefront Maya modeling and animation package. We demonstrate the approach on several example scenes and discuss its strengths and limitations.Item Momentum-based Parameterization of Dynamic Character Motion(The Eurographics Association, 2004) Abe, Yeuhi; Liu, C. Karen; Popovic, Zoran; R. Boulic and D. K. PaiThis paper presents a system for rapid editing of highly dynamic motion capture data. At the heart of this system is an optimization algorithm that can transform the captured motion so that it satisfies high-level user constraints while enforcing that the linear and angular momentum of the motion remain physically plausible. Unlike most previous approaches to motion editing, our algorithm does not require pose specification or model reduction, and the user only need specify high-level changes to the input motion. To preserve the dynamic behavior of the input motion, we introduce a spline-based parameterization that matches the linear and angular momentum patterns of the motion capture data. Because our algorithm enables rapid convergence by presenting a good initial state of the optimization, the user can efficiently generate a large number of realistic motions from a single input motion. The algorithm can then populate the dynamic space of motions by simple interpolation, effectively parameterizing the space of realistic motions. We show how this framework can be used to produce an effective interface for rapid creation of dynamic animations, as well as to drive the dynamic motion of a character in real-time.Item Real-time Speech Motion Synthesis from Recorded Motions(The Eurographics Association, 2004) Cao, Yong; Faloutsos, Petros; Kohler, Eddie; Pighin, Frédéric; R. Boulic and D. K. PaiData-driven approaches have been successfully used for realistic visual speech synthesis. However, little effort has been devoted to real-time lip-synching for interactive applications. In particular, algorithms that are based on a graph of motions are notorious for their exponential complexity. In this paper, we present a greedy graph search algorithm that yields vastly superior performance and allows real-time motion synthesis from a large database of motions. The time complexity of the algorithm is linear with respect to the size of an input utterance. In our experiments, the synthesis time for an input sentence of average length is under a second.Item Animation of Reactive Gaseous Fluids through Chemical Kinetics(The Eurographics Association, 2004) Ihm, Insung; Kang, Byungkwon; Cha, Deukhyun; R. Boulic and D. K. PaiAlthough chemically reactive fluids may be used effectively to increase the reality of visual effects, little work has been done with the general modeling of chemical reactions in computer animation. In this paper, we attempt to extend an established, physically based fluid simulation technique to handle reactive gaseous fluids. The proposed technique exploits the theory of chemical kinetics to account for a variety of chemical reactions that are frequently found in everyday life. In extending the existing fluid simulation method, we introduce a new set of physically motivated control parameters that allow an animator to control intuitively the behavior of reactive fluids. Our method is straightforward to implement, and is flexible enough to create various interesting visual effects including explosions and catalysis. We demonstrate the effectiveness of our new simulation technique by generating several animation examples with user control.Item Extended Galilean Invariance for Adaptive Fluid Simulation(The Eurographics Association, 2004) Shah, Maurya; Cohen, Jonathan M.; Patel, Sanjit; Lee, Penne; Pighin, Frédéric; R. Boulic and D. K. PaiIn an unbounded physical domain, simulating a turbulent fluid on an Eulerian grid is rather tricky. Since it is difficult to predict the motion of the fluid, it is also difficult to guess which computational domain would allow the simulation of the fluid without crossing the computational boundaries. To address this dilemma, we have developed a novel adaptive framework where the simulation grid follows the motion of the flow. Our technique is based on the principle of Galilean Invariance and the culling of simulation cells using a metric derived from continuative boundary conditions. We describe our framework and showcase its advantages over traditional techniques. Timing results and visual comparisons are presented.Item Interactive Motion Deformation with Prioritized Constraints(The Eurographics Association, 2004) Callennec, Benoît Le; Boulic, Ronan; R. Boulic and D. K. PaiIn this paper, we present an interactive motion deformation method to modify animations so that they satisfy a set of prioritized constraints. Our approach successfully handles the problem of retargetting, adjusting a motion, as well as adding significant changes to preexisting animations. We introduce the concept of prioritized constraints to avoid tweaking issues for competing constraints. Each frame is individually and smoothly adjusted to enforce a set of prioritized constraints. The iterative construction of the solution channels the convergence through intermediate solutions, enforcing the highest prioritized constraints first. In addition, we propose a new, simple formulation to control the position of the center of mass so that the resulting motions are physically plausible. Finally, we demonstrate that our method can address a wide range of motion editing problems.Item Layered Dynamic Control for Interactive Character Swimming(The Eurographics Association, 2004) Yang, Po-Feng; Laszlo, Joe; Singh, Karan; R. Boulic and D. K. PaiThis paper proposes a layered strategy for controlling character motion in a dynamically varying environment. We illustrate this approach in the context of a physically simulated human swimmer. The swimmer attempts to follow a dynamic target by augmenting cyclic stroke control with a set of pre-specified variations, based on the current state of the character and its environment. Control of a given swim stroke is decomposed into three layers: a basic stroke sequence, a set of per-stroke control variations, and a set of continuously applied control variations. Interactive control of the swimmer is possible as a result of an efficient physical simulation using a simplified fluid model. Our results show layered dynamic control to be an effective adaptive control technique in well conditioned physical simulations such as swimming, where simulation states resulting from control errors are recoverable.Item Enriching a Motion Collection by Transplanting Limbs(The Eurographics Association, 2004) Ikemoto, Leslie; Forsyth, David A.; R. Boulic and D. K. PaiThis paper describes a method that can significantly increase the size of a collection of motion observations by cutting limbs from one motion sequence and attaching them to another. Not all such transplants are successful, because correlations across the body are a significant feature of human motion. The method uses randomized search based around a set of rules to generate transplants that are (a) likely to be successful and (b) likely to enrich the existing motion collection. The resulting frames are annotated by a classifier to tell whether they look like human motion or not. We evaluate the method by obtaining motion demands from an application, synthesizing motions to meet those demands, and then scoring the synthesized motions. Motions synthesized using transplants are generally somewhat better than those synthesized without using transplants, because transplanting generates many frames quite close to the original frames, so that it is easier for the motion synthesis process to find a good path in the motion graph. Furthermore, we show classifier errors tend to have relatively little impact in practice. Finally, we show that transplanted motion data can be used to synthesize motions of a group coordinated in space and time without producing motions that share frames.Item Computing the Duration of Motion Transitions: An Empirical Approach(The Eurographics Association, 2004) Wang, Jing; Bodenheimer, Bobby; R. Boulic and D. K. PaiThis paper develops methods for determining a visually appealing length for a motion transition, i.e., a segue between two sequences of character animation. Motion transitions are an important component in generating compelling animation streams in virtual environments and computer games. For reasons of efficiency and speed, linear interpolation is often used as the transition method, where the motion is blended between specified start and end frames. The blend length of a transition using this technique is critical to the visual appearance of the motion. Two methods for determining an optimal blend length for such transitions are presented. These methods are suited to different types of motion. They are empirically evaluated through user studies. For the motions tested, we find (1) that visually pleasing transitions can be generated using our optimal blend lengths without further tuning of the blending parameters; and (2), that users prefer these methods over a generic fixed-length blend.Item Wavelet Compression of Parametrically Coherent Mesh Sequences(The Eurographics Association, 2004) Guskov, Igor; Khodakovsky, Andrei; R. Boulic and D. K. PaiWe introduce an efficient compression method for animated sequences of irregular meshes of the same connectivity. Our approach is to transform the original input meshes with an anisotropic wavelet transform running on top of a progressive mesh hierarchy, and progressively encode the resulting wavelet details. For temporally coherent mesh sequences we get additional improvement by encoding the differences of the wavelet coefficients. The resulting compression scheme is scalable, efficient, and significantly improves upon the current state of the art for the animated mesh compression.Item A Hybrid Algorithm for Modeling Ice Formation(The Eurographics Association, 2004) Kim, Theodore; Henson, Michael; Lin, Ming C.; R. Boulic and D. K. PaiWe present a novel algorithm that simulates ice formation. Motivated by the physical process of ice growth, we develop a novel hybrid algorithm by synthesizing three techniques: diffusion limited aggregation, phase field methods, and stable fluid solvers. Each technique maps to one of the three stages of solidification. The visual realism of the resulting algorithm appears to surpass that of each technique alone, particularly in animations of freezing. In addition, we present a faster, simplified phase field method, as well as a unified parameterization that enables artistic manipulation of the simulation. We illustrate the results on arbitrary 3D surfaces.Item Animation and Control of Breaking Waves(The Eurographics Association, 2004) Mihalef, Viorel; Metaxas, Dimitris; Sussman, Mark; R. Boulic and D. K. PaiControlling fluids is still an open and challenging problem in fluid animation. In this paper we develop a novel fluid animation control approach and we present its application to controlling breaking waves. In our Slice Method framework an animator defines the shape of a breaking wave at a desired moment in its evolution based on a library of breaking waves. Our system computes then the subsequent dynamics with the aid of a 3D Navier-Stokes solver. The wave dynamics previous to the moment the animator exerts control can also be generated based on the wave library. The animator is thus enabled to obtain a full animation of a breaking wave while controlling the shape and the timing of the breaking. An additional advantage of the method is that it provides a significantly faster method for obtaining the full 3D breaking wave evolution compared to starting the simulation at an early stage and using solely the 3D Navier-Stokes equations. We present a series of 2D and 3D breaking wave animations to demonstrate the power of the method.Item Cartoon Textures(The Eurographics Association, 2004) Juan, Christina de; Bodenheimer, Bobby; R. Boulic and D. K. PaiIn this paper we present a method for creating novel animations from a library of existing two-dimensional cartoon data. Drawing inspiration from the idea of video textures, sequences of similar-looking cartoon data are combined into a user-directed sequence. Starting with a small amount of cartoon data, we employ a method of nonlinear dimensionality reduction to discover a lower-dimensional structure of the data. The user selects a start and end frame and the system traverses this lower-dimensional manifold to re-sequence the data into a new animation. The system can automatically detect when a new sequence has visual discontinuities and may require additional source material.Item Example-Based Control of Human Motion(The Eurographics Association, 2004) Hsu, Eugene; Gentry, Sommer; Popovic, Jovan; R. Boulic and D. K. PaiIn human motion control applications, the mapping between a control specification and an appropriate target motion often defies an explicit encoding.We present a method that allows such a mapping to be defined by example, given that the control specification is recorded motion. Our method begins by building a database of semantically meaningful instances of the mapping, each of which is represented by synchronized segments of control and target motion. A dynamic programming algorithm can then be used to interpret an input control specification in terms of mapping instances. This interpretation induces a sequence of target segments from the database, which is concatenated to create the appropriate target motion. We evaluate our method on two examples of indirect control. In the first, we synthesize a walking human character that follows a sampled trajectory. In the second, we generate a synthetic partner for a dancer whose motion is acquired through motion capture.Item Autonomous Behaviors for Interactive Vehicle Animations(The Eurographics Association, 2004) Go, Jared; Vu, Thuc; Kuffner, James; R. Boulic and D. K. PaiWe present a method for synthesizing animations of autonomous space, water, and land-based vehicles in games or other interactive simulations. Controlling the motion of such vehicles to achieve a desirable behavior is difficult due to the constraints imposed by the system dynamics. We combine real-time path planning and a simplified physics model to automatically compute control actions to drive a vehicle from an input state to desirable output states based on a behavior cost function. Both offline trajectory preprocessing and online search are used to build an animation framework suitable for interactive vehicle simulations. We demonstrate synthesized animations of spacecraft performing a variety of autonomous behaviors, including Seek, Pursue, Avoid, Avoid Collision, and Flee. We also explore several enhancements to the basic planning algorithm and examine the resulting tradeoffs in runtime performance and quality of the generated motion.Item Methods for Exploring Expressive Stance(The Eurographics Association, 2004) Neff, Michael; Fiume, Eugene; R. Boulic and D. K. PaiThe postures a character adopts over time are a key expressive aspect of her movement. While IK tools help a character achieve positioning constraints, there are few tools that help an animator with the expressive aspects of a character s poses. Three aspects are combined in good pose design: achieving a set of world space constraints, finding a body shape that reflects the character s inner state and personality, and making adjustments to balance that act to strengthen the pose and also maintain realism. This is routinely done in the performing arts, but is uncommon in computer graphics. Our system combines all three components within a single body shape solver. The system combines feedback based balance control with a hybrid IK system that utilizes optimization and analytic IK components. The IK system has been carefully designed to allow direct control over various aesthetically important aspects of body shape, such as the type of curve in the spine and the relationship between the collar bones. The system allows for both low-level control and for higher level shape sets to be defined and used. Shape sets allow an animator to use a single scalar to vary a character s pose within a specified shape class, providing an intuitive parameterization of a posture. Changing shape sets allows an animator to quickly experiment with different posture options for a movement sequence, supporting rapid exploration of the aesthetic space.