Paul G. Kry
Assistant Professor

Contact
 
School of Computer Science, McGill University
3480 University Street
McConnell Building, Room 318
Montréal, QC, H3A 0E9
Canada
office: MC113N
phone: +1 514 398 2577
fax: +1 514 398 3883
email:
kry@ cs. mcgill.ca
Teaching

Current Courses

+ Previous Courses 

Research Interests and Information

My research interests include computer graphics, physically based animation, skin deformations of articulated characters, motion capture, interaction, and physically based modeling of humans and animals. I am specifically interested in human and animal motor control (e.g., locomotion, grasping, manipulation) in combination with natural phenomena such as the physics of rigid objects, deformation, and contact. Example application areas include computer animation for video games and movies, training simulations, ergonomics, and biologically inspired robotics and programming by demonstration. An important aspect of my work is the combination of real world measurements, approximate models, and physically based simulation. I am also interested in machine learning, numerical methods, and audio.


+ Program Committees 

+ Biographical Information 

Research Projects and Publications  
 



Interaction Capture and Synthesis
P. G. Kry and D. K. Pai, ACM Transactions on Graphics, 25:3 (SIGGRAPH), 2006

Modifying motion capture to satisfy the constraints of new animation is difficult when contact is involved, and a critical problem for animation of hands. The compliance with which a character makes contact also reveals important aspects of the movement’s purpose. We present a new technique called interaction capture, for capturing these contact phenomena. We capture contact forces at the same time as motion, at a high rate, and use both to estimate a nominal reference trajectory and joint compliance. Unlike traditional methods, our method estimates joint compliance without the need for motorized perturbation devices. New interactions can then be synthesized by physically based simulation. We describe a novel position-based linear complementarity problem formulation that includes friction, breaking contact, and the compliant coupling between contacts at different fingers. The technique is validated using data from previous work and our own perturbation-based estimates.

PDF (8MB)
BIBTEX
MOVIE [WMV] (7MB)
PROJECT PAGE





Interaction Capture and Synthesis of Human Hands
P. G. Kry, PhD Thesis, 2005

This thesis addresses several issues in modelling interaction with human hands in computer graphics and animation. Modifying motion capture to satisfy the constraints of new animation is difficult when contact is involved because physical interaction involves energy or power transfer between the system of interest and the environment, and is a critical problem for computer animation of hands. Although contact force measurements provide a means of monitoring this transfer, motion capture as currently used for creating animation has largely ignored contact forces. We present a system of capturing synchronized motion and contact forces, called interaction capture. We transform interactions such as grasping into joint compliances and a nominal reference trajectory in an approach inspired by the equilibrium point hypothesis of human motor control. New interactions are synthesized through simulation of a quasi-static compliant articulated model in a dynamic environment that includes friction. This uses a novel position-based linear complementarity problem formulation that includes friction, breaking contact, and coupled compliance between contacts at different fingers. We present methods for reliable interaction capture, addressing calibration, force estimation, and synchronization. Additionally, although joint compliances are traditionally estimated with perturbation-based methods, we introduce a technique that instead produces estimates without perturbation. We validate our results with data from previous work and our own perturbation-based estimates. A complementary goal of this work is hand-based interaction in virtual environments. We present techniques for whole-hand interaction using the Tango, a novel sensor that performs interaction capture by measuring pressure images and accelerations. We approximate grasp hand-shapes from previously observed data through rotationally invariant comparison of pressure measurements. We also introduce methods involving heuristics and thresholds that make reliable drift-free navigation possible with the Tango. Lastly, rendering the skin deformations of articulated characters is a fundamental problem for computer animation of hands. We present a deformation model, called EigenSkin, which provides a means of rendering physically- or example-based deformation models at interactive rates on graphics hardware.

PDF PRINT QUALITY (37.2MB) PDF WEB (7.1MB) BIBTEX MOVIE EXAMPES [WMV] (8MB) MOVIE CAPTURED GRASP [WMV] (1.7MB) MOVIE GRASP ON APPLE [WMV] (1.0MB) MOVIE GRASP ON BULB [WMV] (0.7MB) MOVIE BULB MANIPULATION [WMV] (7.4MB)





The Tango: a tangible tangoreceptive whole-hand human interface
D. K. Pai, E. W. VanDerLoo, S. Sadhukhan, and P. G. Kry, World Haptics Symposium, 2005

We describe the Tango, a new passive haptic interface for whole-hand interaction with 3D objects. The Tango is shaped like a ball and can be grasped comfortably in one hand. Its pressure sensitive skin measures the contact pressures exerted by the user's hand, and accelerometers within the device measure its motion and attitude. This information can be used for novel modes of interaction with three dimensional objects. We describe the design of the device, and the software for interpreting the sensor data for user interaction.

PDF (1.2MB) BIBTEX





Continuous Contact Simulation for Smooth Surfaces
P. G. Kry, and D. K. Pai, ACM Transactions on Graphics, 22:1, 2003

Dynamics simulation of smooth surfaced rigid bodies in contact is a critical problem in physically based animation and interactive virtual environments. We describe a technique which uses reduced coordinates to evolve a single continuous contact between smooth piece-wise parametric surfaces. The incorporation of friction into our algorithm is straightforward. The dynamics equations, though slightly more complex due to the reduced coordinate formulation, can be integrated easily using explicit integrators without the need for constraint stabilization. Because the reduced coordinates confine integration errors within the constraint manifold, a large choice of step sizes are possible with visually acceptable results. We demonstrate these results using Loop Subdivision surfaces with parametric evaluation.

PDF (1.8MB) BIBTEX MOVIE [AVI-MPG4] (10.5MB)





EigenSkin: Real Time Large Deformation Character Skinning in Graphics Hardware
P. G. Kry, D. L. James, and D. K. Pai, ACM SIGGRAPH Symposium on Computer Animation, 2002

We present a technique which allows subtle nonlinear quasistatic deformations of articulated characters to be compactly approximated by data-dependent eigenbases which are optimized for real time rendering on commodity graphics hardware. The method extends the common Skeletal-Subspace Deformation (SSD) technique to provide efficient approximations of the complex deformation behaviors exhibited in simulated, measured, and artist-drawn characters. Instead of storing displacements for key poses (which may be numerous), we precompute principal components of the deformation influences for individual kinematic joints, and so construct error-optimal eigenbases describing each joint's deformation subspace. Pose-dependent deformations are then expressed in terms of these reduced eigenbases, allowing precomputed coefficients of the eigenbasis to be interpolated at run time. Vertex program hardware can then efficiently render nonlinear skin deformations using a small number of eigendisplacements stored in graphics hardware. We refer to the final resulting character skinning construct as the model's EigenSkin. Animation results are presented for a very large nonlinear finite element model of a human hand rendered in real time at minimal cost to the main CPU.

PDF(5.5MB) BIBTEX MOVIE [AVI-MPG4] (18.4MB) SHADOW PUPPET MOVIE [AVI-MPG4] (23MB) PROJECT PAGE





FoleyAutomatic: Physically-based Sound Effects for Interactive Simulation and Animation
K. van den Doel, P. G. Kry, and D. K. Pai, SIGGRAPH 2001

We describe algorithms for real-time synthesis of realistic sound effects for interactive simulations (e.g., games) and animation. These sound effects are produced automatically, from 3D models using dynamic simulation and user interaction. We develop algorithms that are efficient, physically-based, and can be controlled by users in natural ways. We develop effective techniques for producing high quality continuous contact sounds from dynamic simulations running at video rates which are slow relative to audio synthesis. We accomplish this using modal models driven by contact forces modeled at audio rates, which are much higher than the graphics frame rate. The contact forces can be computed from simulations or can be custom designed. We demonstrate the effectiveness with complex realistic simulations.

PDF (1.1MB) BIBTEX MOVIE [MPG] (12.7MB)





Fast Contact Evolution for Piecewise Smooth Surfaces
P. G. Kry, Master's Thesis, 2000

Dynamics simulation of smooth bodies in contact is a critical problem in physically based animation and interactive virtual environments. We describe a technique which uses reduced coordinates to evolve a single continuous contact between Loop subdivision surfaces. The incorporation of both slip and no-slip friction into our algorithm is straightforward. The dynamics equations, though slightly more complex due to the reduced coordinate formulation, can be integrated easily using explicit integrators without the need for constraint stabilization. The use of reduced coordinates also confines integration errors to lie within the constraint manifold which is preferable for visualization. Our algorithm is suitable for piecewise parametric or parameterizable surfaces with polygonal domain boundaries. Because a contact will not always remain in the same patch, we demonstrate how a contact can be evolved across patch boundaries. We also address the issue of non-regular parameterizations occurring in Loop subdivision surfaces through surface replacement with n sided S-patch surfaces. Three simulations show our results. We partially verify our technique first with a frictionless system and then with a blob sliding and rolling inside a bowl. Our third simulation shows that our formulation correctly predicts the spin reversal of a rattleback top. We also present timings of the various components of the algorithm.

SEE TOG PAPER RATTLEBACK MOVIE [MPG] (1.2MB)





Forward Dynamics Algorithms for Multibody Chains and Contact
D. K. Pai, U. M. Ascher, and P. G. Kry, ICRA 2000

We describe a framework for derivation of several forward dynamics algorithms used in robotics. The framework is based on formulating an augmented system and performing block matrix elimination on this system. Several popular algorithms such as the O(N) Articulated Body method, and Composite Rigid Body method can be easily derived. We also derive an algorithm for simulation of contact between smooth bodies of arbitrary shape, in contact coordinates. Finally, we discuss some potential numerical difficulties that could arise and their solution.

PDF (0.2MB) BIBTEX