Welcome to my space on the web. Here you can find various things related
to my interests including my research publications, software I have developed,
and some of the photographs I have taken.


I am currently employed as a postdoctorate researcher in the Scientific Visualization and Computer Graphics group at the University of Groningen, The Netherlands. My past and present research interests include:

  • Voxel-based representation of terrain
  • 3D graphics and rendering
  • Visual analytics
  • GPU computing
  • Medical imaging and visualisation
  • Quantum computing

Below you can find details and downloads for most of my publications. Please feel free to contact me to discuss collaboration opportunities.

Accelerating Colonic Polyp Detection Using Commodity Graphics Hardware

Authors: David Williams, Valeriu Codreanu, Jos B.T.M. Roerdink, Po Yang, Baoquan Liu, Feng Dong and Alessandro Chiarini

Abstract: We present a parallel implementation of an algorithm for the detection of colonic polyps from CT data sets. This implementation is designed specifically to take advantage of the computational power available on modern Graphics Processing Units (GPUs), which significantly reduces the execution time to streamline the workflow of clinicians examining the data. We provide details about the changes which were made to the existing algorithm to suit the new target hardware, and perform tests which demonstrate that the results are a very close match to the reference implementation while being computed in a fraction of the time.

Published: International Conference on Computer Medical Applications

Year: 2013

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Volumetric Representation of Virtual Environments

Authors: David Williams

Abstract: The use of height maps as a mechanism for representing terrains is well established within computer graphics and gaming. Height maps are a conceptually simple representation, easy to visualize, and simple to create. Furthermore, there is a large body of research into the manipulation and rendering of such data. However, there are also serious limitations that result from this rather simplistic representation, such as the inability to support caves and overhangs.

In this article, we take the concept of two-dimensional height maps and show how they can be extended to fully three-dimensional volumes. This is a representation that naturally and consistently handles the kind of geological structures mentioned previously. It also allows easy real-time modification, and as such can be used to create powerful terrain editors or unique game play opportunities.

Published: Game Engine Gems

Year: 2010

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Volumetric Curved Planar Reformation for Virtual Endoscopy

Authors: David Williams, Soren Grimm, Ernesto Coto, Abdul Roudsari and Haralambos Hatzakis

Abstract: Curved Planar Reformation (CPR) has proved to be a practical and widely used tool for the visualization of curved tubular structures within the human body. It has been useful in medical procedures involving the examination of blood vessels and the spine. However, it is more difficult to use it for large, tubular, structures such as the trachea and the colon because abnormalities may be smaller relative to the size of the structure and may not have such distinct density and shape characteristics.

Our new approach improves on this situation by using volume rendering for hollow regions and standard CPR for the surrounding tissue. This effectively combines gray scale contextual information with detailed color information from the area of interest. The approach is successfully used with each of the standard CPR types and the resulting images are promising as an alternative to virtual endoscopy.

Because the CPR and the volume rendering are tightly coupled, the projection method used has a significant effect on properties of the volume renderer such as distortion and isometry. We describe and compare the different CPR projection methods and how they affect the volume rendering process.

A version of the algorithm is also presented which makes use of importance driven techniques; this ensures the users attention is always focused on the area of interest and also improves the speed of the algorithm.

Published: IEEE Transactions on Visualization and Computer Graphics

Year: 2007

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O-Buffer Based IFT Watershed from Markers for Large Medical Datasets

Authors: Ernesto Coto, Soren Grimm and David Williams

Abstract: The watershed transform from markers is a very popular image segmentation operator. The Image Foresting Transform (IFT) watershed is a common method to compute the watershed transform from markers using a priority queue,but which can consume too much memory when applied to three-dimensional medical datasets. This is a considerable limitation on the applicability of the IFT watershed,as the size of medical datasets keeps increasing at a faster pace than physical memory technologies develop. This paper presents the O-IFT watershed,a new type of IFT watershed based on the O-Buffer framework,an d introduces an efficient data representation which considerably reduces the memory consumption of the algorithm. In addition,th is paper introduces the O-Buckets, a new implementation of the priority queue which further reduces the memory consumption of the algorithm. The new O-IFT watershed with O-Buckets allows the application of the watershed transform from markers to large medical datasets.

Published: Computers & Graphics

Year: 2007

Download: Not available

Volumetric CPR as an Enhancement to Virtual Colonoscopy Systems

Authors: David Williams, Soren Grimm, Ernesto Coto, Abdul Roudsari and Haralambos Hatzakis

Abstract: Volumetric CPR is a new visualisation technique for curved tubular structures within the body. It works by splitting a structure lengthwise and presenting it to the user as two or more halves, each containing a detailed volume rendering of the inside of the structure and a greyscale rendering providing context information. In this work we look at the integration of the Volumetric CPR with an existing virtual colonoscopy system and the advantages which it can bring.

We demonstrate that the Volumetric CPR is a useful tool for displaying additional information not typically available during a flythrough, such as real-time surface coverage data or translucency rendering. We also show that, because the Volumetric CPR provides and alternative view on the colon, it increases surface coverage from 86.8% (for a flythrough in each direction) to 99.2%; significantly improving the chances of detecting abnormalities.

Published: Computer Assisted Radiology and Surgery

Year: 2007

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Visualisation of Curved Tubular Structures in Medical Datasets : An Application to Virtual Colonoscopy

Authors: David Williams

Abstract: Medical conditions affecting the colon are problematic to diagnose due to the difficulty in examining this particular internal organ. To date, the most widely used approach is to perform a colonoscopy; a procedure in which a small camera is inserted into the colon to examine its surface. This procedure is unpleasant and potentially dangerous for the patient, and is expensive and time consuming for the hospital. As a result, patients at risk of developing the conditions are not always screened as often as would be desirable.

Over the last few years a new approach known as virtual colonoscopy has been gaining popularity. The method uses information from a CT scan to reconstruct a 3D model of the colon which can then be examined without the patient needing to undergo a colonoscopy. This approach is now commonly used when screening for polyps (an indication of colon cancer) but can not be so easily used on conditions such as Inflammatory Bowel Disease (IBD) where information beyond the shape of the surface is required.

Abstract continues in thesis...

Published: This was my PhD thesis

Year: 2007

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Simulating and Compiling Code for the Sequential Quantum Random Access Machine

Authors: Rajagopal Nagarajan, Nikolaos Papanikolaou and David Williams

Abstract: We present the SQRAM architecture for quantum computing, which is based on Knill's QRAM model. We detail a suitable instruction set, which implements a universal set of quantum gates, and demonstrate the operation of the SQRAM with Deutsch's quantum algorithm. The compilation of high-level quantum programs for the SQRAM machine is considered; we present templates for quantum assembly code and a method for decomposing matrices for complex quantum operations. The SQRAM simulator and compiler are discussed, along with directions for future work.

Published: Electronic Notes in Theoretical Computer Science

Year: 2007

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Quantum Computer Architecture, Assembly Language, and Compilation

Authors: David Williams

Abstract: The design for a quantum computer based on the QRAM architecture is presented, detailing its method of operation when executing both classical and quantum algorithms. An assembly-like instructions set for this machine is also defined which allows it to be universal in the set of quantum algorithms it can represent.

Next a look at the issue of compiling a high-level quantum programming language into 'byte-code' for the machine designed previously is presented. The transformation of programming language constructs, decomposition of complex quantum operations, and some possible optimisation techniques are studied.

Lastly the notions of communication and concurrency are introduced to the quantum computer. The necessary instructions are added to it and demonstrations of quantum teleportation and the dense-coding protocol are provided. The then work finishes with a look at high-level languages allowing communication and discusses how they might be compiled for the new architecture.

Published: This was my Masters thesis

Year: 2004

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