Derek Wills

The development of an arthroscopic surgical simulator with haptic feedback

Interest in improved training methods for minimally invasive surgery (MIS) has led to the development of a number of computer graphics based surgical simulators. These provide a safe environment for training in surgical procedures and offer potential advantages over conventional techniques. This paper starts with a brief review of existing work in the area, then goes on to describe a surgical simulator for Knee arthroscopy developed at the University of Hull. This provides real time simulation of the arthroscopic view, collision detection between instruments and objects within the joint, interaction with deformable objects and limited haptic feedback through the instruments.

Implicit Fitting Using Radial Basis Functions with Ellipsoid Constraint

Implicit planar curve and surface fitting to a set of scattered points plays an important role in solving a wide variety of problems occurring in computer graphics modelling, computer graphics animation, and computer assisted surgery. The fitted implicit surfaces can be either algebraic or non‐algebraic. The main problem with most algebraic surface fitting algorithms is that the surface fitted to a given data set is often unbounded, multiple sheeted, and disconnected when a high degree polynomial is used, whereas a low degree polynomial is too simple to represent general shapes. Recently, there has been increasing interest in non‐algebraic implicit surface fitting. In these techniques, one popular way of representing an implicit surface has been the use of radial basis functions. This type of implicit surface can represent various shapes to a high level of accuracy. In this paper, we present an implicit surface fitting algorithm using radial basis functions with an ellipsoid constraint. This method does not need to build interior and exterior layers for the given data set or to use information on surface normal but still can fit the data accurately. Furthermore, the fitted shape can still capture the main features of the object when the data sets are extremely sparse. The algorithm involves solving a simple general eigen‐system and a computation of the inverse or psedo‐inverse of a matrix, which is straightforward to implement.

Visualizing underwater environments using multifrequency sonar

This article introduces seabed visualization by describing three case studies that use a high-speed, multifrequency, continuous scan sonar called the Seabed Visualization System. The case studies involve: modeling a harbour wall in Holland, permitting a virtual inspection of the harbour environment; visualizing a sunken military vessel, the SS Richard Montgomery; and visualizing underwater pipelines in Easington, England.

VoxColliDe: Voxel collision detection for virtual environments

Collision detection is fundamental in achievingnatural dynamics in virtual environments, but current algorithms are too slow, causing a major bottleneck in processing and hindering the building of interactive simulation environments. This paper provides an overview of the collision detection problem and current attempted solutions. A voxel-based approach to rigid-body collision detection is presented, with its potential high performance explained.Voxel collision detection takes place on a pair-wise basis, involving two additional representations of a polygonal object, a Voxmap and a Point Shell. These are constructed in a pre-processing step and allow fast collision detection through a simple look-up reference of points into voxels. Collision performance depends upon the number of points in the shell, and can trade accuracy for speed. A range ofpruning techniques, needed to cut down the number of objects undergoing collision testing, are reviewed and implemented. These allow most effective use of the voxel collision detection algorithm in multi-body simulations, such as virtual environments.Performance evaluations demonstrate the voxel collision detection algorithms ability to achieve interactive rates (above 20 Hz) for both high precision pair-wise collision tests, and for large numbers of objects in multi-body environments. The voxel collision detection algorithm is suitable for parallel, hardware implementation. This provides the potential for great enhancements to already extremely high performance, rendering the voxel-based approach to collision detection all the more promising.

A Data Structure for Artificial Terrain Generation

This paper addresses the problems of maintaining the consistency of spatial data under recursive subdivision algorithms for artificial terrain generation. Fractal modelling techniques are outlined, with particular reference to those based on fractional Brownian motion, and included is a brief history of recursive subdivision techniques. Existing polyhedral data structures are reviewed and shown to be inadequate for maintenance of consistency as polygons are subdivided. A new edge‐based data structure is presented which provides for the efficient use and consistent storage of spatial data under these conditions. The data structure is applicable to polygons of any order and allows neighbouring polygons of different order to be stored and subdivided. Artificial terrain is demonstrated from subdivision of the hexagon, of the quadrilateral and of a combination of hexagons, quadrilaterals and triangles.

Artificial planets with fractal feature specification

We describe the generation of artificial planets using a regular polyhedron as an initial mesh. The specification of arbitrary terrain features based on a quaternary triangular mesh in conjunction with fractal subdivision helps generate a potentially infinite variety of planets with 3D terrain and optional ocean surfaces. A method for invariant point identification overcomes the problem of internal consistency. Only polygons within a clipping window are subdivided and rendered, which controls the growth in the number of polygons as resolution increases and the surface is approached. The extension to a WWW-based collaborative planetary generation system for virtual worlds is also prototyped.

Use of a modified kalman filter for a visually coupled system application

Spatial tracking devices are frequently used in virtual environments, such as in the case of helmet mounted displays, to dynamically determine the users viewpoint and line of sight. Temporal distortion effects are perceived by the user as a result of the lag between head movement and visual feedback. Measurements of phase lag have been made and to help alleviate these problems, predictive filtering techniques are frequently used. We report on studies that have been made in the use of a modified Kalman filter algorithm. The implementation provides favourable results in terms of reduction on the effect of phase lag.

Towards Context- Dependent Interpolation of Digital Elevation Models

The fractional Brownian motion (fBm) model has become increasingly popular in recent years as a mechanism for the synthesis of many natural phenomena. Unfortunately, the required interdependence of neighbouring points over an unbounded area (context dependence) renders valid approximations to fBm extremely inefficient. This paper describes a technique for the interpolation of digital elevation models to an arbitrarily high spatial resolution. This is achieved using an efficient recursive subdivision algorithm which has the ability to permit some degree of context dependence within, and across the boundaries of interpolation areas. The application of this technique to flight simulation is described and the technique is evaluated based on its ability to reduce run‐time storage requirements.

Manipulation of terrain data for a real-time display application

There is a need for a graphics display for data which is stored as a terrain data base. Due to space and cost limitations of the application, a system has been developed to work on a microcomputer system which includes transputers to allow parallel processing. This paper describes the data representation and manipulation which has been taken in order to provide a real-time graphical display. This data representation is suitable for use in a parallel processing environment. — From the Abstract

An overview of physically-based modelling techniques for virtual environments

A class of virtual environments is concerned with the representation of behaviour that is apparent in the real world. In order to model this behaviour, sophisticated physical models are required. The development of these models, classed asphysically-based modelling, is based upon the fundamental concepts of Newtonian dynamics. Considerable research into physically-based modelling has already been conducted by the computer graphics community, permitting realistic animation of object motion. The application of physical models to virtual environments poses further problems, not least that of real-time execution in a fully interactive environment. This paper gives an overview of the existing computer graphics research concerned with physically-based modelling, discussing the merits and problems of various techniques in terms of the requirements of virtual environment.