Tolga G. Goktekin

Adaptive Nonlinear Finite Elements for Deformable Body Simulation Using Dynamic Progressive Meshes

Realistic behavior of deformable objects is essential for many applications such as simulation for surgical training. Existing techniques of deformable modeling for real time simulation have either used approximate methods that are not physically accurate or linear methods that do not produce reasonable global behavior. Nonlinear finite element methods (FEM) are globally accurate, but conventional FEM is not real time. In this paper, we apply nonlinear FEM using mass lumping to produce a diagonal mass matrix that allows real time computation. Adaptive meshing is necessary to provide sufficient detail where required while minimizing unnecessary computation. We propose a scheme for mesh adaptation based on an extension of the progressive mesh concept, which we call dynamic progressive meshes.

A method for animating viscoelastic fluids

This paper describes a technique for animating the behavior of viscoelastic fluids, such as mucus, liquid soap, pudding, toothpaste, or clay, that exhibit a combination of both fluid and solid characteristics. The technique builds upon prior Eulerian methods for animating incompressible fluids with free surfaces by including additional elastic terms in the basic Navier-Stokes equations. The elastic terms are computed by integrating and advecting strain-rate throughout the fluid. Transition from elastic resistance to viscous flow is controlled by von Misess yield condition, and subsequent behavior is then governed by a quasi-linear plasticity model.

A Virtual Environment Testbed for Training Laparoscopic Surgical Skills

With the introduction of minimally invasive techniques, surgeons must learn skills and procedures that are radically different from traditional open surgery. Traditional methods of surgical training that were adequate when techniques and instrumentation changed relatively slowly may not be as efficient or effective in training substantially new procedures. Virtual environments are a promising new medium for training. This paper describes a testbed developed at the San Francisco, Berkeley, and Santa Barbara campuses of the University of California for research in understanding, assessing, and training surgical skills. The testbed includes virtual environments for training perceptual motor skills, spatial skills, and critical steps of surgical procedures. Novel technical elements of the testbed include a four-DOF haptic interface, a fast collision detection algorithm for detecting contact between rigid and deformable objects, and parallel processing of physical modeling and rendering. The major technical challenge in surgical simulation to be investigated using the testbed is the development of accurate, real-time methods for modeling deformable tissue behavior. Several simulations have been implemented in the testbed, including environments for assessing performance of basic perceptual motor skills, training the use of an angled laparoscope, and teaching critical steps of the cholecystectomy, a common laparoscopic procedure. The major challenges of extending and integrating these tools for training are discussed.

A semi-Lagrangian contouring method for fluid simulation

In this article, we present a semi-Lagrangian surface tracking method for use with fluid simulations. Our method maintains an explicit polygonal mesh that defines the surface, and an octree data structure that provides both a spatial index for the mesh and a means for efficiently approximating the signed distance to the surface. At each timestep, a new surface is constructed by extracting the zero set of an advected signed-distance function. Semi-Lagrangian backward path tracing is used to advect the signed-distance function. One of the primary advantages of this formulation is that it enables tracking of surface characteristics, such as color or texture coordinates, at negligible additional cost. We include several examples demonstrating that the method can be effectively used as part of a fluid simulation to animate complex and interesting fluid behaviors.

Fluids in deforming meshes

This paper describes a simple modification to an Eulerian fluid simulation that permits the underlying mesh to deform independent of the simulated fluids motion. The modification consists of a straightforward adaptation of the commonly used semi-Lagrangian advection method to account for the meshs motion. Because the method does not require more interpolation steps than standard semi-Lagrangian integration, it does not suffer from additional smoothing and requires only the added cost of updating the mesh. By specifying appropriate boundary conditions, mesh boundaries can behave like moving obstacles that act on the fluid resulting in a number of interesting effects. The paper includes several examples that have been computed on moving tetrahedral meshes.

GiPSi: a framework for open source/open architecture software development for organ-level surgical simulation

This paper presents the architectural details of an evolving open source/open architecture software framework for developing organ-level surgical simulations. Our goal is to facilitate shared development of reusable models, to accommodate heterogeneous models of computation, and to provide a framework for interfacing multiple heterogeneous models. The framework provides an application programming interface for interfacing dynamic models defined over spatial domains. It is specifically designed to be independent of the specifics of the modeling methods used, and therefore facilitates seamless integration of heterogeneous models and processes. Furthermore, each model has separate geometries for visualization, simulation, and interfacing, allowing the model developer to choose the most natural geometric representation for each case. Input/output interfaces for visualization and haptics for real-time interactive applications have also been provided

GiPSi: An Open Source/Open Architecture Software Development Framework for Surgical Simulation

In this paper we propose an open source/open architecture framework for developing organ level surgical simulations. Our goal is to facilitate shared development of reusable models, to accommodate heterogeneous models of computation, and to provide a framework for interfacing multiple heterogeneous models. The framework provides an intuitive API for interfacing models with spatial relationships. It is specifically designed to be independent of the specifics of the modeling methods used and therefore facilitates seamless integration of heterogeneous models and processes. Furthermore, each model has separate geometries for visualization, simulation, and interfacing, allowing the modeler choose the most natural geometric representation for each case.

A method for cartoon-style rendering of liquid animations

In this paper we present a visually compelling and informative cartoon rendering style for liquid animations. Our style is inspired by animations such as Futurama,1 The Little Mermaid,2 and Bambi2. We take as input a liquid surface obtained from a three-dimensional physically based liquid simulation system and output animations that evoke a cartoon style and convey liquid movement. Our method is based on four cues that emphasize properties of the liquids shape and motion. We use bold outlines to emphasize depth discontinuities, patches of constant color to highlight near-silhouettes and areas of thinness, and, optionally place temporally coherent oriented textures on the liquid surface to help convey motion.

Simulating whitewater rapids in Ratatouille

In Pixar’s Ratatouille, a key story point involves a rat being swept through the sewers of Paris, plummeting down waterfalls and along steeply sloping tunnels, through a series of high-speed S-bends which cause the torrent of water to bank up sharply on each turn. Bringing the director’s vision of this wild and perilous rapids sequence to the screen required us to use a wide variety of water effects techniques to give the appearance of rushing water, spray, foam and bubbles. One of the greatest challenges was to pull these diverse techniques together into a seamless sequence.

An Interactive Parallel Multigrid FEM Simulator

Interactively simulating nonlinear deformable human organs for surgical training and planning purposes demands high computational power which lacks in single processor machine. We build an interactive deformable objects simulator on a highly scalable computer cluster using nonlinear FEM and the novel multigrid explicit ODE solver which is stabler than single level schemes. The system consists of a graphical front end client on a workstation connected to a parallel simulation server that runs on a Linux cluster. After discussing the methodology in detail, the analysis of the speedup and preliminary results are presented.