Dinesh K. Pai's research while affiliated with University of British Columbia and other places
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Publications (248)
We describe a multiresolution representation of parametric curves that is well suited to haptic interaction. Our approach has been implemented on an embedded controller for the Pantograph haptic device, and has been used for haptic exploration of boundary curves obtained from images. The resolution of the representation can be dynamically adapted t...
Dynamics simulation with frictional contacts is important for a wide range of applications, from cloth simulation to object manipulation. Recent methods using smoothed friction forces have enabled robust and differentiable simulation of elastodynamics with friction. However, the resulting frictional behaviors can be qualitatively inaccurate and may...
In situ measurement of frictional properties, particularly of human skin, is challenging. A major challenge is that the measured surface may be curved and oriented arbitrarily relative to the measurement device, making it difficult to obtain accurate and reliable friction estimates. Here we propose a method to simultaneously estimate the orientatio...
Simulation of human soft tissues in contact with their environment is essential in many fields, including visual effects and apparel design. Biological tissues are nearly incompressible. However, standard methods employ compressible elasticity models and achieve incompressibility indirectly by setting Poisson's ratio to be close to 0.5. This approa...
Simulation of human soft tissues in contact with their environment is essential in many fields, including visual effects and apparel design. Biological tissues are nearly incompressible. However, standard methods employ compressible elasticity models and achieve incompressibility indirectly by setting Poisson's ratio to be close to 0.5. This approa...
Frictional contact between deformable elastic objects remains a difficult simulation problem in computer graphics. Traditionally, contact has been resolved using sophisticated collision detection schemes and methods that build on the assumption that contact happens between polygons. While polygonal surfaces are an efficient representation for solid...
We examine a variety of numerical methods that arise when considering dynamical systems in the context of physics-based simulations of deformable objects. Such problems arise in various applications, including animation, robotics, control and fabrication. The goals and merits of suitable numerical algorithms for these applications are different fro...
We examine a variety of numerical methods that arise when considering dynamical systems in the context of physics-based simulations of deformable objects. Such problems arise in various applications, including animation, robotics, control and fabrication. The goals and merits of suitable numerical algorithms for these applications are different fro...
Physics-based simulation methods for deformable objects suffer limitations due to the conflicting requirements that are placed on them. The work horse semi-implicit (SI) backward Euler method is very stable and inexpensive, but it is also a blunt instrument. It applies heavy damping, which depends on the timestep, to all solution modes and not just...
We present two practical methods for measurement of spectral skin reflectance suited for live subjects, and drive a spectral BSSRDF model with appropriate complexity to match skin appearance in photographs, including human faces. Our primary measurement method employs illuminating a subject with two complementary uniform spectral illumination condi...
Elastodynamic system simulation is a key procedure in computer graphics and robotics applications. To enable these simulations, the governing differential system is discretized in space (employing FEM) and then in time. For many simulation-based applications keeping the spatial resolution of the computational mesh effectively coarse is crucial for...
Human motion capture using video-based or sensor-based methods gives animators the capability to directly translate complex human motions to create lifelike character animations. Advances in motion capture algorithms have improved their accuracy for estimating human generalized motion coordinates (joint angles and body positions). However, the trad...
Improper activation of the quadriceps muscles vastus medialis (VM) and vastus lateralis (VL) has been implicated in the development of patellofemoral pain (PFP). This explanation of PFP assumes that VM and VL produce opposing mediolateral forces on the patella. Although studies have provided evidence for opposing actions of VM and VL on the patella...
Realistic appearance modeling of human skin is an important research topic with a variety of application in computer graphics. Various diffusion based BSSRDF models [Jensen et al. 2001, Donner and Jensen 2005, Donner and Jensen 2006] have been introduced in graphics to efficiently simulate subsurface scattering in skin including modeling its layere...
Motion in depth is commonly misperceived in Virtual Reality (VR), making it difficult to intercept moving objects, for example, in games. We investigate whether motion cues could be modified to improve these interactions in VR. We developed a time-to-contact estimation task, in which observers (
$n=18$
) had to indicate by button press when a loom...
Simulating how the human body deforms in contact with external objects, tight clothing, or other humans is of central importance to many fields. Despite great advances in numerical methods, the material properties required to accurately simulate the body of a real human have been sorely lacking. Here we show that mechanical properties of the human...
Subject-specific musculoskeletal models are increasingly used in biomedical applications to predict endpoint forces due to muscle activation, matching predicted forces to experimentally observed forces at a specific limb configuration. However, it is difficult to precisely measure the limb configuration at which these forces are observed. The conse...
This paper presents a mirror-like augmented reality (AR) system to display the internal anatomy of the current user. Using a single Microsoft V2.0 Kinect (later on referenced as the Kinect), we animate in real-time a user-specific model of internal anatomy according to the user’s motion and we superimpose it onto the user’s color map. Users can vis...
High quality simulations of the dynamics of soft flexible objects can be rather costly, because the assembly of internal forces through an often nonlinear stiffness at each time step is expensive. Many standard implicit integrators introduce significant, time-step dependent artificial damping. Here we propose and demonstrate the effectiveness of an...
We present a novel method to enrich standard rigid-body impact models with a spatially varying coefficient of restitution map, or Bounce Map. Even state-of-the art methods in computer graphics assume that for a single rigid body, post- and pre-impact dynamics are related with a single global, constant, namely the coefficient of restitution. We firs...
Purpose:
The neural control of pursuit eye movements to visual textures that simultaneously translate and rotate has largely been neglected. Here we propose that pursuit of such targets-texture pursuit-is a fully three-dimensional task that utilizes all three degrees of freedom of the eye, including torsion.
Methods:
Head-fixed healthy human adu...
Eye movements aid visual perception and guide actions such as reaching or grasping. Most previous work on eyehand coordination has focused on saccadic eye movements. Here we show that smooth pursuit eye movement accuracy strongly predicts both interception accuracy and the strategy used to intercept a moving object. We developed a naturalistic task...
We propose a system for real-time animation of eyes that can be interactively controlled in a WebGL enabled device using a small number of animation parameters, including gaze. These animation parameters can be obtained using traditional keyframed animation curves, measured from an actor's performance using off-the-shelf eye tracking methods, or es...
Humans show effortless dexterity while manipulating objects using their own hands. However, specifying the motion of a virtual character's hand or of a robotic manipulator remains a difficult task that requires animation expertise or extensive periods of offline motion capture. We present Hands On: a real-time, adaptive animation interface, driven...
We present a new, Eulerian-on-Lagrangian approach for modeling cloth. When a cloth modeled using the traditional Lagrangian approach is moved around an object with sharp corners, such as the edge of a table, the cloth cannot always bend smoothly around the object because it can bend only at its nodes. With our method, these constraints are built in...
Three-dimensional fingertip trajectory was examined under different force levels of the lumbrical muscle to clarify the function of the lumbrical muscle in free index finger motion. The metacarpophalangeal joint balancing effect of the lumbrical muscle in the thumb-up position was also examined. The motions of the finger bones were recorded during...
The tendons of the hand and other biomechanical systems form a complex network of sheaths, pulleys, and branches. By modeling these anatomical structures, we obtain realistic simulations of coordination and dynamics that were previously not possible. First, we introduce Eulerian-on-Lagrangian discretization of tendon strands, with a new selective q...
We present a data driven model of eye movement, that includes movement of the globes, the periorbital soft tissues and eyelids and also the formation of wrinkles in the tissues. We describe a pipeline for measurement and estimation of tissue movement around the eyes using monocular high speed video capture. We use dense optical flow techniques to s...
A method for simulating movement of a skin associated with a body comprises maintaining, by a computer, a representation of the skin comprising vertices, each vertex of the representation corresponding to a material point of the skin and comprising associated data, the associated data for each vertex comprising: constant body coordinates which repr...
We describe a system for active stabilization of cameras mounted on highly dynamic robots. To focus on careful performance evaluation of the stabilization algorithm, we use a camera mounted on a robotic test platform that can have unknown perturbations in the horizontal plane, a commonly occurring scenario in mobile robotics. We show that the camer...
We introduce a new framework for simulating the dynamics of musculoskeletal systems, with volumetric muscles in close contact and a novel data-driven muscle activation model. Muscles are simulated using an Eulerian-on-Lagrangian discretization that handles volume preservation, large deformation, and close contact between adjacent tissues. Volume pr...
This course focuses on the use of Eulerian solids to simulate muscles, tendons and other highly constrained mechanical and biomechanical systems. We aim to give the reader both a theoretical and practical introduction to the Eulerian solids methodology by focusing on relevant applications in physically based animation. Eulerian methods have many ad...
We present a new technique for simulating retinal image formation by tracing a large number of rays from objects in three dimensions as they pass through the optic apparatus of the eye to objects. Simulating human optics is useful for understanding basic questions of vision science and for studying vision defects and their corrections. Because of t...
Robots and simulations provide complementary approaches for exploring different aspects of hand modeling. Robotic systems have the advantage in that all physical interactions, such as contact between tendons and bones, are automatically and correctly taken into account. Conversely, software simulations can more easily incorporate different material...
This is a short survey of musculoskeletal hand models in computer graphics. We review a number of simulation models under six categories: example-based, passive muscles, joint torques, moment arms, dynamic strands, and volumetric.
To generate a force at the hand in a given spatial direction and with a given magnitude the central nervous system (CNS) has to coordinate the recruitment of many muscles. Because of the redundancy in the musculoskeletal system, the CNS can choose one of infinitely many possible muscle activation patterns which generate the same force. What strateg...
During visuomotor adaptation a novel mapping between visual targets and motor commands is gradually acquired. How muscle activation patterns are affected by this process is an open question. We tested whether the structure of muscle synergies is preserved during adaptation to a visuomotor rotation. Eight subjects applied targeted isometric forces o...
Introduction: We frequently observe horizontal and vertical movements of the eyes, but what is less often noted is the fact that the eye can also rotate about the line of sight, resulting in torsional eye movements. Torsion may serve to compensate for rotations of the head, but its exact function is unclear. Here we examine the functional role of t...
Whether the nervous system relies on modularity to simplify acquisition and control of complex motor skills remains controversial. To date, evidence for modularity has been indirect, based on statistical regularities in the motor commands captured by muscle synergies. Here we provide direct evidence by testing the prediction that in a truly modular...
We present a novel approach for simulating thin hyperelastic skin. Real human skin is only a few millimeters thick. It can stretch and slide over underlying body structures such as muscles, bones, and tendons, revealing rich details of a moving character. Simulating such skin is challenging because it is in close contact with the body and shares it...
We describe an Eulerian-on-Lagrangian solid simulator that reduces or eliminates many of the problems experienced by fully Eulerian methods but retains their advantages. Our method does not require the construction of an explicit object discretization and the fixed nature of the simulation mesh avoids tangling during large deformations. By introduc...
Purpose:
Humans make smooth pursuit eye movements to bring the image of a moving object onto the fovea. Although pursuit accuracy is critical to prevent motion blur, the eye often falls behind the target. Previous studies suggest that pursuit accuracy differs between motion directions. Here, we systematically assess asymmetries in smooth pursuit....
In this study, we investigate the pattern of coordination between motions of two joints generated by intrinsic and extrinsic muscles. The joint motions were created by excursion controller that pulled seven muscle tendons independently according to manually designed four excursion patterns. The result of the experiment clearly showed linear relatio...
We present a new type of deformable model which combines the realism of physically based continuum mechanics models and the usability of frame-based skinning methods, allowing the interactive simulation of objects with heterogeneous material properties and complex geometries. The degrees of freedom are coordinate frames. In contrast with traditiona...
In order to guide and improve rehabilitation interventions for grip function after spinal cord injury (SCI), it is important to have a detailed understanding of the motor control strategies that the central nervous system uses to control the hand. We examined whether changes in the motor control of the hand after SCI are manifested in the form of c...
We present a novel framework for animating human characters performing fast visually guided tasks, such as catching a ball. The main idea is to consider the coordinated dynamics of sensing and movement. Based on experimental evidence about such behaviors, we propose a generative model that constructs interception behavior online, using discrete sub...
A method for image-based contact detection and modeling, with guaranteed precision on the intersection volume, is presented. Unlike previous image-based methods, our method optimizes a nonuniform ray sampling resolution and allows precise control of the volume error. By cumulatively projecting all mesh edges into a generalized 2D texture, we constr...
Because of the redundancy in the musculoskeletal system, to generate a force at the hand in a given direction and with a given magnitude the central nervous system (CNS) has to select one of infinitely many possible muscle activation patterns. What strategies and constraints underlie such selection is an open and debated issue. The CNS might select...
The contribution of lumbrical muscles for the articulated finger motion is yet to be clarified. In this paper we analyze the role of the lumbrical muscle by using excursion controller and cadaver specimens. The result of the experiment clearly shows that the activation of the lumbrical muscle enlarges the arc of the fingertip trajectory except in t...
We present a new apparatus, excursion controller, to measure and analyze musculoskeletal motion of cadaveric fingers. The result of the preliminary experiment clearly shows that the range of excursion of the interosseous for abduction-adduction and that of the extensors for hyperextension were estimated to be 5 to 8mm, while that of the flexors for...
Passive properties of muscles and tendons, including their elasticity, have been suggested to influence motor control. We examine here the potential role of passive elastic muscle properties at the rat ankle joint, focusing on their potential to specify an equilibrium position of the ankle. We measured the position-dependent passive torques at the...
Standard approaches to mapping force to displacement or force to velocity with multi-DoF isometric haptic devices typically ignore the directional variability in a user's feasible wrench. We demonstrate that such a directionally-uniform mapping tends to either over-sensitize the interaction in some directions or under-utilize the user's operational...
Although musculoskeletal models are commonly used, validating the muscle actions predicted by such models is often difficult. In situ isometric measurements are a possible solution. The base of the skeleton is immobilized and the endpoint of the limb is rigidly attached to a 6-axis force transducer. Individual muscles are stimulated and the resulti...
Training simulators have proven their worth in a variety of fields, from piloting to air-traffic control to nuclear power station monitoring. Designing surgical simulators, however, poses the challenge of creating trainers that effectively instill not only high-level understanding of the steps to be taken in a given situation, but also the low-leve...
We examine some basic assumptions underlying current musculoskeletal simulators and suggest alternative solutions. First, we discuss the role of muscle mass and show that current simulators may miss a fundamental feature of musculoskeletal dynamics. Second, we discuss the importance of efficient and versatile constraint handling. Even though the co...
Static optimization approaches to estimating muscle tensions rely on the assumption that the muscle activity pattern is in some sense optimal. However, in the case of individuals with a neuromuscular impairment, this assumption is likely not to hold true. We present an approach to muscle tension estimation that does not rely on any optimality assum...
Recent studies have demonstrated a role for the elastic protein titin in active muscle, but the mechanisms by which titin plays this role remain to be elucidated. In active muscle, Ca(2+)-binding has been shown to increase titin stiffness, but the observed increase is too small to explain the increased stiffness of parallel elastic elements upon mu...
A significant challenge in applications of computer animation is the simulation of ropes, cables, and other highly constrained strandlike physical curves. Such scenarios occur frequently, for instance, when a strand wraps around rigid bodies or passes through narrow sheaths. Purely Lagrangian methods designed for less constrained applications such...
A new method to simulate deformable objects with heterogeneous material properties and complex geometries is presented. Given a volumetric map of the material properties and an arbitrary number of control nodes, a distribution of the nodes is computed automatically, as well as the associated shape functions. Reference frames attached to the nodes a...
Simulating viscoelastic solids undergoing large, nonlinear deformations in close contact is challenging. In addition to inter-object contact, methods relying on Lagrangian discretizations must handle degenerate cases by explicitly remeshing or resampling the object. Eulerian methods, which discretize space itself, provide an interesting alternative...
We describe and test a non-linear control algorithm inspired by the behavior of motor neurons in humans and other animals during extremely fast saccadic eye movements. The algorithm is implemented on a robotic eye, which includes a stiff camera cable, similar to the optic nerve, which adds a complicated non-linear stiffness to the plant. For high s...
We have developed a framework for simulating muscles and tendons using a dynamic modeling primitive, a “strand, ” based on cubic spline curves with inertia [Sueda et al. 2008]. With this framework, we do not require moment-arm data; the tendons are simulated
We present a new type of deformable model which combines the realism of physically-based continuum mechanics models and the usability of framebased skinning methods. The degrees of freedom are coordinate frames. In contrast with traditional skinning, frame positions are not scripted but move in reaction to internal body forces. The displacement fie...
Musculoskeletal models are often created by making detailed anatomical measurements of muscle properties. These measurements can then be used to determine the parameters of canonical models of muscle action. We describe here a complementary approach for developing and validating muscle models, using in situ measurements of muscle actions. We charac...
Diffusion Tensor Imaging (DTI) allows the non-invasive study of muscle fiber architecture but musculoskeletal DTI suffers from low signal-to-noise ratio. Noise in the computed tensor fields can lead to poorly reconstructed muscle fiber fields. This paper describes an algorithm for producing denoised muscle fiber fields from noisy diffusion tensor d...
We describe an efficient method for reconstructing the activity in human muscles from an array of voltage sensors on the skin surface. MRI is used to obtain morphometric data which are segmented into muscle tissue, fat, bone and skin, from which a finite element model for volume conduction is constructed. The inverse problem of finding the current...
In developing a quantitative understanding of bullfrog motor control, we are building an accurate computer model of the underlying musculoskeletal plant. We began by creating a high-resolution virtual model of the bullfrog skeleton. First, a template model of the skeleton was developed using a GE lightspeed VCT scanner. This model was low resolutio...
Dynamic simulation of human eye movements, with realistic physical models of extraocular muscles (EOMs), may greatly advance our understanding of the complexities of the oculomotor system and aid in treatment of visuomotor disorders. In this paper we describe the first three dimensional (3D) biomechanical model which can simulate the dynamics of oc...
Creating and animating subject-specific anatomical models is traditionally a difficult process involving medical image segmentation, geometric corrections and the manual definition of kinematic parameters. In this paper, we introduce a novel template morphing algorithm that facilitates three-dimensional modelling and parameterization of skeletons....
By combining a few modules, the CNS may learn new control policies quickly and efficiently. Support for a modular organization of the motor system has recently come from the observation of low dimensionality in the motor commands. However, stronger evidence would come from testing the predictions on the effect of an intervention on the mechanisms r...
Most current models of musculoskeletal dynamics lump a muscle's mass with its body segment, and then simulate the dynamics of these body segments connected by joints. As shown here, this popular approach leads to errors in the system's inertia matrix and hence in all aspects of the dynamics. Two simplified mathematical models were created to captur...
We present research examining the function of complex muscles in the rat hindlimb. Two related sets of experiments are described. In the first, we examine the degree of specificity in spinal pattern generators, assessing whether the pattern generators at birth are capable of differentially activating intramuscular subdivisions in the complex hindli...
Based on recent developments in computer simulation, medical imaging, and neurophysiology, we can now construct detailed functional models of the human biomechanical plant, its neural control, and its physical interaction with the environment. These new developments will have a profound impact on both how computer animation will be done in the futu...
We consider the geometric numerical integration of Hamiltonian systems
subject to both equality and "hard" inequality constraints. As in the standard
geometric integration setting, we target long-term structure preservation. We
additionally, however, also consider invariant preservation over persistent,
simultaneous and/or frequent boundary interac...
Understanding the neural control and mechanics of human eye movement has signicant implications for treating vision disorders. A computational model incorporating physiological properties and non- linear kinematics of the oculomotor plant's geometry and extraocular muscle (EOM) mechanics is desirable for scientic studies and clinical applications....
When you hear a salient sound, it is natural to look at it to find out what is happening. Orienting the eyes to look at sounds is essential to our ability to identify and understand the events occurring in our environment. This behavior involves both sensorimotor and multisensory integration: a sound elicits a movement of the visual sense organ, th...
In this paper, we present a simple approach for estimating the average longitudinal strains from models reconstructed from medical images. It can be used for many incompressible generalized cylindrical tissues, such as tendons, ligaments, and fusiform muscles; the major deformation directions of these soft tissues are along the longitudinal axes. T...
Understanding the mechanisms of eye movement is difficult without a realistic biomechanical model. We present an efficient and robust computational framework for building subject-specific models of the orbit from magnetic resonance images (MRIs). We reconstruct three-dimensional geometric models of the major structures of the orbit (six extraocular...
We approach the problem of determining a general method for augmenting haptic simulators to amplify the perceptually salient aspects of the interaction that induce effective skill transfer. Using such a method, we seek to simplify the design of haptic simulators that can improve training effectiveness without requiring expensive improvements in the...
We present a new discrete velocity-level formulation of frictional contact dynamics that reduces to a pair of coupled projections and introduce a simple fixed-point property of this coupled system. This allows us to construct a novel algorithm for accurate frictional contact resolution based on a simple staggered sequence of projections. The algori...
We describe a methodology called computed myography to qualitatively and quantitatively determine the activation level of individual muscles by voltage measurements from an array of voltage sensors on the skin surface. A finite element model for electrostatics simulation is constructed from morphometric data. For the inverse problem, we utilize a g...
We describe an automatic technique for generating the motion of tendons and muscles under the skin of a traditionally animated character. This is achieved by integrating the traditional animation pipeline with a novel biomechanical simulator capable of dynamic simulation with complex routing constraints on muscles and ten- dons. We also describe an...
In this paper we introduce a new model of the mechanics of a finger pad that is efficient to s