SCA 04: Eurographics/SIGGRAPH Symposium on Computer Animation

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https://diglib.eg.org/handle/10.2312/432

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Layered Dynamic Control for Interactive Character Swimming
(The Eurographics Association, 2004) Yang, Po-Feng; Laszlo, Joe; Singh, Karan; R. Boulic and D. K. Pai
This 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.
Autonomous Behaviors for Interactive Vehicle Animations
(The Eurographics Association, 2004) Go, Jared; Vu, Thuc; Kuffner, James; R. Boulic and D. K. Pai
We 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.
Methods for Exploring Expressive Stance
(The Eurographics Association, 2004) Neff, Michael; Fiume, Eugene; R. Boulic and D. K. Pai
The 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.
Collision Between Deformable Objects Using Fast-Marching on Tetrahedral Models
(The Eurographics Association, 2004) Marchal, Damien; Aubert, Fabrice; Chaillou, Christophe; R. Boulic and D. K. Pai
This paper presents an approach to handling collision between deformable objects using tetrahedral decomposition. The tetrahedral volumetric model is often used to simulate deformable objects that handle cuts and splits. Interaction between such objects in a complex environment is still an open problem in interactive simulation. This paper is mainly focused on obtaining a fast computation of a reliable penalty response. The method consists in using an approximated distance map to compute a penalty based response. We propose to compute the distances to the boundary using a modified 'Closest Point' algorithm derived from Fast Marching. The presented algorithm, inspired by the [FL01], has the advantage of computing rapidly the 'Closest Point' in the volumetric tetrahedral mesh without any use of an additional computation grid. From the resulting distance map a response is computed using a new "segment-in-object" response that offers more reliable results than the "point-in-object" generally used in previous works. Using this collision model, simulation at interactive rate can be considered in an environment composed of objects that can be deformed and cut.
Animation and Control of Breaking Waves
(The Eurographics Association, 2004) Mihalef, Viorel; Metaxas, Dimitris; Sussman, Mark; R. Boulic and D. K. Pai
Controlling 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.
Evaluating Motion Graphs for Character Navigation
(The Eurographics Association, 2004) Reitsma, Paul S. A.; Pollard, Nancy S.; R. Boulic and D. K. Pai
Realistic and directable humanlike characters are an ongoing goal in animation. Motion graph data structures hold much promise for achieving this goal. However, the quality of the results obtained from a motion graph may not be easy to predict from the input motion segments. This paper introduces the idea of assessing a data structure such as a motion graph for its utility in a particular application. We focus on navigation tasks and define metrics for evaluating expected path quality and coverage for a given environment. One key to evaluating a motion graph for navigation tasks is to first embed it into the environment in a way that captures all possible paths that might result from 'playing back' the motion graph within that environment. This paper describes an algorithm for accomplishing this embedding that preserves the flexibility of the original motion graph. We use the metrics defined in this paper to compare motion datasets and to highlight areas where these datasets could be improved.
Crowdbrush: Interactive Authoring of Real-time Crowd Scenes
(The Eurographics Association, 2004) Ulicny, Branislav; Ciechomski, Pablo de Heras; Thalmann, Daniel; R. Boulic and D. K. Pai
Recent advances in computer graphics techniques and increasing power of graphics hardware made it possible to display and animate large crowds in real-time. Most of the research efforts have been directed towards improving rendering or behavior control; the question how to author crowd scenes in an efficient way is usually not addressed. We introduce a novel approach to create complex scenes involving thousands of animated individuals in a simple and intuitive way. By employing a brush metaphor, analogous to the tools used in image manipulation programs, we can distribute, modify and control crowd members in real-time with immediate visual feedback. We define concepts of operators and instance properties that allow to create and manage variety in populations of virtual humans. An efficient technique allowing to render up to several thousands of fully three-dimensional polygonal characters with keyframed animations at interactive framerates is presented. The potential of our approach is demonstrated by authoring a scenario of a virtual audience in a theater and a scenario of a pedestrian crowd in a city.
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. Pai
In 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.
Finding Paths for Coherent Groups using Clearance
(The Eurographics Association, 2004) Kamphuis, Arno; Overmars, Mark H.; R. Boulic and D. K. Pai
Virtual 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.
Example-Based Control of Human Motion
(The Eurographics Association, 2004) Hsu, Eugene; Gentry, Sommer; Popovic, Jovan; R. Boulic and D. K. Pai
In 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.
Interactive Motion Deformation with Prioritized Constraints
(The Eurographics Association, 2004) Callennec, Benoît Le; Boulic, Ronan; R. Boulic and D. K. Pai
In 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.
Animation of Reactive Gaseous Fluids through Chemical Kinetics
(The Eurographics Association, 2004) Ihm, Insung; Kang, Byungkwon; Cha, Deukhyun; R. Boulic and D. K. Pai
Although 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.
Momentum-based Parameterization of Dynamic Character Motion
(The Eurographics Association, 2004) Abe, Yeuhi; Liu, C. Karen; Popovic, Zoran; R. Boulic and D. K. Pai
This 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.
Enriching a Motion Collection by Transplanting Limbs
(The Eurographics Association, 2004) Ikemoto, Leslie; Forsyth, David A.; R. Boulic and D. K. Pai
This 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.
Computing the Duration of Motion Transitions: An Empirical Approach
(The Eurographics Association, 2004) Wang, Jing; Bodenheimer, Bobby; R. Boulic and D. K. Pai
This 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.
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. Pai
Data-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.
Cartoon Textures
(The Eurographics Association, 2004) Juan, Christina de; Bodenheimer, Bobby; R. Boulic and D. K. Pai
In 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.
Performance Timing for Keyframe Animation
(The Eurographics Association, 2004) Terra, Silvio C. L.; Metoyer, Ronald A.; R. Boulic and D. K. Pai
Keyframing 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.
Breathe Easy: Model and control of simulated respiration for animation
(The Eurographics Association, 2004) Zordan, Victor B.; Celly, Bhrigu; Chiu, Bill; DiLorenzo, Paul C.; R. Boulic and D. K. Pai
Animation of the breath has been largely ignored by the graphics community, even though it is a signature movement of the human body and an indicator for lifelike motion. In this paper, we present an anatomically inspired, physically based model of the human torso for the visual simulation of espiration using a mixed system of rigid and deformable parts. This novel composition of anatomical components is necessary to capture the key characteristics of breathing motion visible in the human trunk because the movement is generated fundamentally through the combination of both rigid bone and soft tissue. We propose a simple anatomically meaningful muscle element based on springs, which is used throughout both actively to drive the motion of the ribs and diaphragm and passively for other muscles like those of the abdomen. In addition, we introduce a straightforward method for preserving incompressible volume in deformable bodies to use in approximating the motion of the gut related to breath. Through the careful construction of this anatomically based torso, control for respiration becomes the generation of periodic contraction signals for a minimal set of two muscle groups. We show the flexibility of ourapproach through the animation of several breathing styles using our system and we verify our results through video and analytical comparisons.
Flow Tiles
(The Eurographics Association, 2004) Chenney, Stephen; R. Boulic and D. K. Pai
We present flow tiles, a novel technique for representing and designing velocity fields. Unlike existing procedural flow generators, tiling offers a natural user interface for field design. Tilings can be constructed to meet a wide variety of external and internal boundary conditions, making them suitable for inclusion in larger environments. Tiles offer memory savings through the re-use of prototypical elements. Each flow tile contains a small field and many tiles can be combined to produce large flows. The corners and edges of tiles are constructed to ensure continuity across boundaries between tiles. In addition, all our tiles and the resulting tiling are divergence-free and hence suitable for representing a range of effects. We discuss issues that arise in designing flow tiles, algorithms for creating tilings, and three applications: a crowd on city streets, a river flowing between banks, and swirling fog. The first two applications use stationary fields, while the latter demonstrates a dynamic field.

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