Skip navigation.

Learning Silhouette Features for Control of Human Motion

Liu Ren, Gregory Shakhnarovich, Jessica K. Hodgins, Hanspeter Pfister, and Paul Viola

Abstract

We present a vision-based performance interface for controlling animated human characters. The system combines information about the user's motion contained in silhouettes from several viewpoints with domain knowledge contained in a motion capture database to interactively produce a high quality animation. Such an interactive system will be useful for authoring, teleconferencing, or as a control interface for a character in a game. In our system, the user performs in front of three video cameras; the resulting silhouettes are used to estimate his or her orientation and body configuration based on a set of discriminative local features. Those features are selected by a machine learning algorithm during a preprocessing step. Sequences of motions that approximate the user's actions are extracted from the motion database and scaled in time to match the speed of the user's motion. We use swing dancing, an example of complex human motion, to demonstrate the effectiveness of our approach and compare the results obtained with discriminative local features to those obtained with global features, Hu moments, and to ground truth measurement from a motion capture system.

Citation

Liu Ren, Gregory Shakhnarovich, Jessica K. Hodgins, Hanspeter Pfister, and Paul Viola. Learning silhouette features for control of human motion. ACM Transactions on Graphics, 24(4), October 2005. [BiBTeX]

Links

Oral Fixations

James Duesing, Jessica K. Hodgins, Bum Lee, Moshe Mahler, Jay Oberski, Sang Il Park, and David Tinapple

Abstract

Oral Fixations is a single channel video installation that evolves over a seven hour time period. The project is a darkly humorous look at a habit of endless consumption and the resulting accumulation of waste. A narrative gradually emerges from the on-screen action that depicts a large mouthed character who dances while flossing its one protruding tooth. A conveyor belt regularly delivers factory-farm fresh hams that the character delights in taking one large bite from and then tossing aside. Over the duration of the piece the hams begin to pile up in the room until at the end of seven hours the room is filled with the refuse of this gluttony. The viewer is encouraged to revisit the piece periodically throughout the day and see how the discarded hams build an oddly humorous environment of waste around the character.

Citation

James Duesing, Jessica K. Hodgins, Bum Lee, Moshe Mahler, Jay Oberski, Sang Il Park, and David Tinapple. Oral fixations. In SIGGRAPH '05: ACM SIGGRAPH 2005 Electronic Art and Animation Catalog, pages 118–119, New York, NY, USA, 2005. ACM Press. [BiBTeX]

Links

A Data-Driven Approach to Quantifying Natural Human Motion

Liu Ren, Alton Patrick, Alexei A. Efros, Jessica K. Hodgins, and James M. Rehg

Abstract

In this paper, we investigate whether it is possible to develop a measure that quantifies the naturalness of human motion (as defined by a large database). Such a measure might prove useful in verifying that a motion editing operation had not destroyed the naturalness of a motion capture clip or that a synthetic motion transition was within the space of those seen in natural human motion. We present an ensemble approach to attack this problem. In our approach, we first hierarchically decompose human motion into its constituent parts (individual joints, limbs, and full body) and build a statistical model of each one using existing machine learning techniques such as mixture of Gaussians (MoG), hidden Markov models (HMM), and switching linear dynamic systems (SLDS). We then combine these models into an ensemble model for classification of the motion as natural or unnatural. We also implement a Naive Bayes (NB) model for a baseline comparison. We test these techniques on motion capture data held out from a database, keyframed motions, edited motions, motions with noise added, and synthetic motion transitions. We present the results as receiver operating characteristic (ROC) curves and compare the results to the judgments made by subjects in a user study.

Citation

Liu Ren, Alton Patrick, Alexei A. Efros, Jessica K. Hodgins, and James M. Rehg. A data-driven approach to quantifying natural human motion. ACM Transactions on Graphics (SIGGRAPH 2005), 24(3), August 2005. [BiBTeX]

Links

A Practical Analytic Single Scattering Model for Real Time Rendering

Bo Sun, Ravi Ramamoorthi, Srinivasa Narasimhan, and Shree K. Nayar

Abstract

We consider real-time rendering of scenes in participating media, capturing the effects of light scattering in fog, mist and haze. While a number of sophisticated approaches based on Monte Carlo and finite element simulation have been developed, those methods do not work at interactive rates. The most common real-time methods are essentially simple variants of the OpenGL fog model. While easy to use and specify, that model excludes many important qualitative effects like glows around light sources, the impact of volumetric scattering on the appearance of surfaces such as the diffusing of glossy highlights, and the appearance under complex lighting such as environment maps. In this paper, we present an alternative physically based approach that captures these effects while maintaining real-time performance and the ease-of-use of the OpenGL fog model. Our method is based on an explicit analytic integration of the single scattering light transport equations for an isotropic point light source in a homogeneous participating medium. We can implement the model in modern programmable graphics hardware using a few small numerical lookup tables stored as texture maps. Our model can also be easily adapted to generate the appearances of materials with arbitrary BRDFs, environment map lighting, and precomputed radiance transfer methods, in the presence of participating media. Hence, our techniques can be widely used in real-time rendering applications.

Citation

Bo Sun, Ravi Ramamoorthi, Srinivasa Narasimhan, and Shree K. Nayar. A practical analytic single scattering model for real time rendering. ACM Transactions on Graphics (SIGGRAPH 2005), 24(3), August 2005. [BiBTeX]

Links

Analyzing the Physical Correctness of Interpolated Human Motion

Alla Safonova and Jessica K. Hodgins

Abstract

Two human motions can be linearly interpolated to produce a new motion, giving the animator control over the length of a jump, the speed of walking, or the height of a kick. Over the past ten years, this simple technique has been shown to produce surprisingly natural looking results. In this paper, we analyze the motions produced by this technique for physical correctness and suggest small modifications to the standard interpolation technique that in some circumstances will produce significantly more natural looking motion.

Citation

Alla Safonova and Jessica K. Hodgins. Analyzing the physical correctness of interpolated human motion. In 2005 ACM SIGGRAPH / Eurographics Symposium on Computer Animation, August 2005. [BiBTeX]

Links

Automatic Photo Pop-up

Derek Hoiem, Alexei A. Efros, and Martial Hebert

Abstract

Our system automatically constructs simple "pop-up" 3D models, like those one would find in a children's book, out of a single outdoor image. The labels each region of an outdoor image as "ground", "vertical", or "sky". Line segments fitted to the ground-vertical boundary in the image and an estimate of the horizon's position provide the necessary information to determine where to "cut" and "fold" in the image. After cutting and folding, the model is popped up, and the image is texture mapped onto the model.

Citation

Derek Hoiem, Alexei A. Efros, and Martial Hebert. Automatic photo pop-up. ACM Transactions on Graphics (SIGGRAPH 2005), 24(3), August 2005. [BiBTeX]

Links

Behavior Planning for Character Animation

Manfred Lau and James J. Kuffner

Abstract

This paper explores a behavior planning approach to automatically generate realistic motions for animated characters. Motion clips are abstracted as high-level behaviors and associated with a behavior finite-state machine (FSM) that defines the movement capabilities of a virtual character. During runtime, motion is generated automatically by a planning algorithm that performs a global search of the FSM and computes a sequence of behaviors for the character to reach a user-designated goal position. Our technique can generate interesting animations using a relatively small amount of data, making it attractive for resource-limited game platforms. It also scales efficiently to large motion databases, because the search performance is primarily dependent on the complexity of the behavior FSM rather than on the amount of data. Heuristic cost functions that the planner uses to evaluate candidate motions provide a flexible framework from which an animator can control character preferences for certain types of behavior. We show results of synthesized animations involving up to one hundred human and animal characters planning simultaneously in both static and dynamic environments.

Citation

Manfred Lau and James J. Kuffner. Behavior planning for character animation. In 2005 ACM SIGGRAPH / Eurographics Symposium on Computer Animation, pages 271–280, August 2005. [BiBTeX]

Links

Performance Animation from Low-dimensional Control Signals

Jinxiang Chai and Jessica K. Hodgins

Abstract

The ability to accurately reconstruct a user's motion in real time would allow the intuitive control of characters in computer games, the control of avatars for virtual reality or electronically mediated communication, and the rapid prototyping of character animations. This project introduces an approach to performance animation that employs video cameras and a small set of retro-reflective markers to create a low-cost, easy-to-use system that might someday be practical for home use. The low-dimensional control signals from the user's performance are supplemented by a database of pre-recorded human motion. At run time, the system automatically learns a series of local models from a set of motion capture examples that are a close match to the marker locations captured by the cameras. These local models are then used to reconstruct the motion of the user as a full-body animation. We demonstrate the power and flexibility of this approach by having users control six behaviors in real time without significant latency: walking, running, hopping, jumping, boxing, and Kendo (Japanese sword art). The reconstructed motion is based on a single large human motion database. Our experiments indicate that this approach scales well with the size and heterogeneity of the database and is robust to variations in kinematics between users. The resulting animation also captures the individual style of the user's motion through spatial-temporal interpolation of the data. Finally, we assess the quality of the reconstructed motion by comparing against ground truth data simultaneously captured with a full marker set in a commercial motion capture system.

Citation

Jinxiang Chai and Jessica K. Hodgins. Performance animation from low-dimensional control signals. ACM Transactions on Graphics (SIGGRAPH 2005), 24(3), August 2005. [BiBTeX]

Links

Physically Based Grasping Control from Example

Nancy S. Pollard and Victor B. Zordan

Abstract

Animated human characters in everyday scenarios must interact with the environment using their hands. Captured human motion can provide a database of realistic examples. However, examples involving contact are difficult to edit and retarget; realism can suffer when a grasp does not appear secure or when an apparent impact does not disturb the hand or the object. Physically based simulations can preserve plausibility through simulating interaction forces. However, such physical models must be driven by a controller, and creating effective controllers for new motion tasks remains a challenge. In this project, we present a controller for physically based grasping that draws from motion capture data. Our controller explicitly includes passive and active components to uphold compliant yet controllable motion, and it adds compensation for movement of the arm and for gravity to make the behavior of passive and active components less dependent on the dynamics of arm motion. Given a set of motion capture grasp examples, our system solves for all but a small set of parameters for this controller automatically. We demonstrate results for tasks including grasping and two-hand interaction and show that a controller derived from a single motion capture example can be used to form grasps of different object geometries.

Citation

Nancy S. Pollard and Victor B. Zordan. Physically based grasping control from example. In 2005 ACM SIGGRAPH / Eurographics Symposium on Computer Animation, August 2005. [BiBTeX]

Links

Real-Time Subspace Integration for St. Venant-Kirchhoff Deformable Models

Jernej Barbič and Doug L. James

Abstract

In this paper, we present an approach for fast subspace integration of reduced-coordinate nonlinear deformable models that is suitable for interactive applications in computer graphics and haptics. Our approach exploits dimensional model reduction to build reduced-coordinate deformable models for objects with complex geometry. We exploit the fact that model reduction on large deformation models with linear materials (as commonly used in graphics) result in internal force models that are simply cubic polynomials in reduced coordinates. Coefficients of these polynomials can be precomputed, for efficient runtime evaluation. This allows simulation of nonlinear dynamics using fast implicit Newmark subspace integrators, with subspace integration costs independent of geometric complexity. We present two useful approaches for generating low-dimensional subspace bases: modal derivatives and an interactive sketching technique. Mass-scaled principal component analysis (mass-PCA) is suggested for dimensionality reduction. Finally, several examples are given from computer animation to illustrate high performance, including force-feedback haptic rendering of a complicated object undergoing large deformations.

Citation

Jernej Barbič and Doug L. James. Real-time subspace integration for St. Venant-Kirchhoff deformable models. ACM Transactions on Graphics (SIGGRAPH 2005), 24(3):982–990, August 2005. [BiBTeX]

Links

Skinning Mesh Animations

Doug L. James and Christopher D. Twigg

Abstract

We extend approaches for skinning characters to the general setting of skinning deformable mesh animations. We provide an automatic algorithm for generating progressive skinning approximations, that is particularly efficient for pseudo-articulated motions. Our contributions include the use of nonparametric mean shift clustering of high-dimensional mesh rotation sequences to automatically identify statistically relevant bones, and robust least squares methods to determine bone transformations, bone-vertex influence sets, and vertex weight values. We use a low-rank data reduction model defined in the undeformed mesh configuration to provide progressive convergence with a fixed number of bones. We show that the resulting skinned animations enable efficient hardware rendering, rest pose editing, and deformable collision detection. Finally, we present numerous examples where skins were automatically generated using a single set of parameter values.

Citation

Doug L. James and Christopher D. Twigg. Skinning mesh animations. ACM Transactions on Graphics (SIGGRAPH 2005), 24(3), August 2005. [BiBTeX]

Links

Vortex Fluid for Gaseous Phenomena

Sang Il Park and Myoung Jun Kim

Abstract

In this paper, we present a method for visual simulation of gaseous phenomena based on the vortex method. Using a localized vortex flow as a basic building block, the whole flow field is described. As a result, we generate more dynamic swirling fluid flows while achieving computational efficiency by concentrating only on the localized vorticity region during the simulation. Based on the lagrangian framework, we consider various boundary conditions. By exploiting the panel method, we satisfy the no-through boundary condition in a Lagrangian way. A simple and effective way of handling the no-slip boundary condition is also presented. In treating the no-slip boundary condition, we allow a user to assign the roughness of the boundary surface, which further improves visual realism.

Citation

Sang Il Park and Myoung Jun Kim. Vortex fluid for gaseous phenomena. In 2005 ACM SIGGRAPH / Eurographics Symposium on Computer Animation, August 2005. [BiBTeX]

Links