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Paul
G. Kry Assistant Professor |
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Contact
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School
of Computer Science,
McGill University
3480 University Street McConnell Engineering Building, Room 318 Montréal, QC, H3A 2A7 Canada |
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Teaching
Current Courses
COMP 599 Winter 2010, Fundamentals of Computer Animation
Previous Courses
COMP 557 Fall 2009, Fundamentals of Computer Graphics
COMP 599 Winter 2009, Fundamentals of Computer Animation
COMP 202 Fall 2008, Introduction to Computing 1
COMP 764B Winter 2008, Computer Graphics and Animation.
Research Interests
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.
Biographical Information
I grew up in Ottawa and moved to Waterloo in 1992 to attend the University of Waterloo where I obtained a B.Math. in computer science with electrical engineering electives (1997). During my undergraduate studies, I went on a one year exchange to Compiègne, France which included a six month work term in Paris. After graduating, I worked for a little over a year at Televitesse (now known as March Networks) before moving to Vancouver for graduate studies at the University of British Columbia. There, I obtained my M.Sc. (2000) and Ph.D. (2005) in computer science with Dinesh K. Pai. While my Ph.D. is from UBC, I followed Dinesh to New Jersey in 2002 to complete my research at Rutgers University. After finishing I moved directly to Grenoble for postdoctoral work with the EVASION group at INRIA Rhône Alpes and the LNRS at Université René Descartes. I arrived at McGill University as an Assistant Professor in January 2008.
Research Projects and Publications
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A Robust and Multi-scale Modal Analysis for Sound Synthesis C. Picard, F. Faure, G. Drettakis, P. G. Kry, DAFX, 2009 This paper presents a new approach to modal synthesis for rendering sounds of virtual objects. We propose a generic method for modal analysis that preserves sound variety across the surface of an object, at different scales of resolution and for a variety of complex geometries. The technique performs automatic voxelization of a surface model and automatic tuning of the parameters of hexahedral finite elements, based on the distribution of material in each cell. The voxelization is performed using a sparse regular grid embedding of the object, which easily permits the construction of plausible lower resolution approximations of the modal model. With our approach, we can compute the audible impulse response of a variety of objects. Our solution is robust and can handle non-manifold geometries that include both volumetric and surface parts, such as those used in games, training simulations, and other interactive virtual environment. PDF (2.7 MB) MOVIE |
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Composite Elements on the iPhone M. Williams, P. G. Kry, SIGGRAPH Demo, 2009 Use your fingers to squish a 3D model, and let gravity make the model tumble against the walls of its environment. C-FEM is an interactive demonstration that runs on the iPhone and iPod touch. The application is a proof of concept which includes examples demonstrating piecewise interpolation and non homogeneous elastic properties. The demo was made available as a free application in late July 2009. PROJECT PAGE C-FEM DOWNLOAD |
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Preserving Topology and Elasticity for Embedded Deformable Models M. Nesme, P. G. Kry, L. Jeřábková, F. Faure, SIGGRAPH, 2009 In this paper we introduce a new approach for the embedding of linear elastic deformable models. Our technique results in significant improvements in the efficient physically based simulation of highly detailed objects. First, our embedding takes into account topological details, that is, disconnected parts that fall into the same coarse element are simulated independently. Second, we account for the varying material properties by computing stiffness and interpolation functions for coarse elements which accurately approximate the behavior of the embedded material. Finally, we also take into account empty space in the coarse embeddings, which provides a better simulation of the boundary. The result is a straightforward approach to simulating complex deformable models with the ease and speed associated with a coarse regular embedding, and with a quality of detail that would only be possible at much finer resolution. PROJECT PAGE PDF (5 MB) MOVIE [DivX AVI] (29.2 MB) |
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Modal Locomotion: Animating Virtual Characters with Natural Vibrations P. G. Kry, L. Reveret, F. Faure, and M.-P.Cani, Eurographics, 2009 We present a general method to intuitively create a wide range of locomotion controllers for 3D legged characters. The key of our approach is the assumption that efficient locomotion can exploit the natural vibration modes of the body, where these modes are related to morphological parameters such as the shape, size, mass, and joint stiffness. The vibration modes are computed for a mechanical model of any 3D character with rigid bones, elastic joints, and additional constraints as desired. A small number of vibration modes can be selected with respect to their relevance to locomotion patterns and combined into a compact controller driven by very few parameters. We show that these controllers can be used in dynamic simulations of simple creatures, and for kinematic animations of more complex creatures of a variety of shapes and sizes. PDF (0.7MB) MOVIE [XVID AVI] (17MB) DOG TROT [WMV] (1.7MB) DOG GALLOP [WMV] (1.1MB) |
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A Multi-modal Floor-space for Experiencing Material Deformation Underfoot in Virtual Reality A. W. Law, B. V. Peck, Y. Visell, P. G. Kry, and J. R. Cooperstock, IEEE International Workshop on Haptic Audio Visual Environments and Games, 2008 We present a floor-space design that provides the impression of walking on various terrains by rendering graphical, audio and haptic stimuli synchronously with low latency. Currently, interactive floors tend to focus on visual and auditory feedback but have neglected to explore the role of haptics. Our design recreates these three modalities in a direct manner where the sensors and reactive cues are located in the same area. The goal of this project is the creation of a realistic and dynamic area of ground that allows multiple, untethered users to engage in intuitive interaction via locomotion in a virtual or augmented reality environment. PDF (0.8MB) |
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HandNavigator:
Hands-on Interaction for Desktop Virtual Reality P. G. Kry, A. Pihuit, A. Bernhert, and M.-P. Cani, VRST, 2008 This paper presents a novel interaction system, aimed at hands-on manipulation of digital models through natural hand gestures. Our system is composed of a new physical interaction device coupled with a simulated compliant virtual hand model. The physical interface consists of a SpaceNavigator, augmented with pressure sensors to detect directional forces applied by the user's fingertips. This information controls the position, orientation, and posture of the virtual hand in the same way that the SpaceNavigator (an isometric input device) uses measured forces to animate a virtual frame. In this manner, user control does not involve fatigue due to reaching gestures or holding a desired hand shape. During contact, the user has a realistic visual feedback in the form of plausible interactions between the virtual hand and its environment, while our device provides some passive tactile feedback. Our device is well suited to any situation where hand gesture, contact, or manipulation tasks need to be performed in virtual. We demonstrate the device in several simple virtual worlds and evaluate it through a series of user studies. PDF (4MB) MOVIE [MOV] (7MB) |
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Animating Virtual
Character Locomotion and Other Oscillatory Motions P. G. Kry, L. Reveret, F. Faure, M.-P.Cani Cognitive Animation Workshop 2008 / SIGGRAPH Sketches 2007 We present a method for animating locomotion of physically based virtual characters. The key to our approach is based on the observation that efficient locomotion should exploit the natural passive response of the character's dynamical system. We specifically focus on the natural vibration modes, which are affected by parameters such as shape, size, mass, and joint stiffness. From these modal vibrations we can extract the most promising modes with respect to locomotion, and combine them with different amplitudes, phases, and frequencies to animate various gaits. To create locomotion controllers, the search for control parameters is reduced since we only need to consider a small number of modes rather than a large number of degrees of freedom. This can be done by optimization, guided by captured motion analysis, but is also easy enough to do by hand. Mode shapes are also useful as a low dimensional basis for interactive puppetry, and may lead to simplifications in the higher level control of other movements. PDF (1MB) |
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Hands-on Virtual Clay / La sculpture virtuelle à portée de main A. Pihuit, P. G. Kry, M.-P.Cani SMI 2008 / AFRV 2007 This project concerns a new interaction system designed for hands-on 3D shape modeling and deformation through natural hand gestures. Our system is made of a Phantom haptic device coupled with a deformable foam ball that supports pressure sensors. These sensors detect forces exerted by the user’s fingertips, and are used to control the configuration of a compliant virtual hand that is modeling soft virtual clay. During interaction, the user is provided both passive tactile feedback through the foam ball, and realistic visual feedback since the virtual hand deforms due to its interaction in the virtual environment. The combination of all these feedbacks provides the artist with a good immersion allowing for effective sculpting in a virtual world. PDF (0.7MB) |
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Grasp Recognition and
Manipulation with the Tango P. G. Kry and D. K. Pai, ISER, 2006 We describe a novel user interface for natural, whole hand interaction with 3D environments. Our interface uses a graspable device called the Tango, which looks like a ball but measures contact pressures on its surface at 256 tactual elements (taxels) at a high rate (100 Hz). The acceleration of the device is also measured. The key idea is to use this information to recognize the shape and movement of the user’s hand grasping the object. This allows the user to interact with 3D virtual objects using a hand avatar. The interface provides passive force feedback, and is easier to use than interfaces that require wearing gloves or other sensors on the hand. We describe a rotationally invariant matching algorithm for recognizing the hand shape from examples of previous interaction collected with motion capture. We also describe examples of 3D interaction using our system. PDF (2.5MB) MOVIE [WMV] (12MB) |
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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 |
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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) |
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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 |
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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) |
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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 |
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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) |
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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. RATTLEBACK MOVIE [MPG] (1.2MB) |
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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 |