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Dive into the research topics where Timothy A. Clarke is active.

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Featured researches published by Timothy A. Clarke.


Photonics for Industrial Applications | 1994

Comparison of some techniques for the subpixel location of discrete target images

Mark R. Shortis; Timothy A. Clarke; Tim Short

Signalizing points of interest on the object to be measured is a reliable and common method of achieving optimum target location accuracy for many high precision measurement tasks. In photogrammetric metrology, images of the targets originate from photographs and CCD cameras. Regardless of whether the photographs are scanned or the digital images are captured directly, the overall accuracy of the technique is partly dependent on the precise and accurate location of the target images. However, it is often not clear which technique to choose for a particular task, or what are the significant sources of error. The research described in this paper describes aspects of target recognition, thresholding, and location. The results of a series of simulation experiments are used to analyze the performance of subpixel target location techniques such as: centroiding; Gaussian shape fitting; and ellipse fitting, under varying conditions.


Proceedings of SPIE | 1995

Practical testing of the precision and accuracy of target image centering algorithms

Mark R. Shortis; Timothy A. Clarke; S Robson

Close range photogrammetry and vision metrology often use signalized points in the form of active or passive targets. Many theoretical and some practical tests of different target image centering algorithms have been carried out. This paper will describe the empirical testing of several such algorithms using real data acquired for industrial measurement projects and camera calibrations. The precision and accuracy of the centering algorithms will be characterized by analysis of self calibrating network solutions using multiple camera stations and a target array. Particular emphasis will be placed on the comparison between centroiding and ellipse fitting to locate target image center.


intelligent vehicles symposium | 1996

A semi-autonomous sewer surveillance and inspection vehicle

R.M. Gooch; Timothy A. Clarke; Tim Ellis

There are millions of kilometres of sewer pipes of varying size and condition. Often, the exact position and state of these sewers is unknown. In order to map the pipe assets accurately, to detect blockage, breakage, erosion, encrustation and in order to plan maintenance or to build fluid models for flow analysis it is necessary to inspect the sewer assets. Man-entry sewers are inspected by the direct intervention of a human operative. Non man-entry sewers are typically inspected using a video camera mounted on a motorised trolley.


Optical 3D Measurement Techniques II: Applications in Inspection, Quality Control, and Robotics | 1994

Estimator for the random error in subpixel target location and its use in the bundle adjustment

Timothy A. Clarke; M. A. R. Cooper; John G. Fryer

A CCD camera and frame store provide image data that are affected by a number of imperfect processes. However, most of these can be quantified or estimated. For instance, the quantization process of allocating integer values from 0 to 255 for the intensity level of each pixel has well-known statistical properties. This paper discusses how these can lead to an estimate of the precision of location of target image centroids. Two centroid algorithms are analyzed. The theory is tested against experimental and simulated data.


Optical Testing and Metrology III: Recent Advances in Industrial Optical Inspection | 1991

Laser-based triangulation techniques in optical inspection of industrial structures

Timothy A. Clarke; K.T.V. Grattan; N. E. Lindsey

An optical triangulation technique using a solid state laser diode sourceand detector, providing sub-pixel resolution on a CCD detector was developed and is discussed. A novel approach to the problem of non-linear accuracy is outlined. Results of extensive research into improving the accuracy of the device with particular reference to configuration, optical components, the laser source, linearity, resolution, calibration and interpolation are presented.


british machine vision conference | 1995

Robust tracking of circular features

Xiang Pan; Tim Ellis; Timothy A. Clarke

This paper considers the problem of internally inspecting underground pipes, such as sewers, in order to identify and locate major structural defects or damage to the pipes, and to identify potential defects which may lead to failure. It considers three aspects of the pipe inspection task; namely, the extraction of geometric primitives from the pipe image sequences, the tracking of these primitives over time, and the detection of (potential) structural collapse of the pipes. Knowledge of the environment (i.e. the reflectivity and geometry characteristics of the pipes) is exploited to develop appropriate methods for extracting the pipe joint features from edge information and to fit a pipe joint model (a circle, or set of connected arc segments) to the grouped features; and to track these features in sequential images. The paper describes a novel implementation of a Hough-based circle detection algorithm and compares its operation with a least-squares curve fitting algorithm for the detection of circular image features. Results of robust tracking of these pipe joints over a large sequence of image frames (some 1500 frames) are presented. Finally, these circle data are employed to aid the detection of partial pipe collapse.


Optical Tools for Manufacturing and Advanced Automation | 1993

Suitability of the Pulnix TM6CN CCD camera for photogrammetric measurement

S Robson; Timothy A. Clarke; Jin Chen

The Pulnix TM6CN CCD camera appears to be a suitable choice for many close range photogrammetric applications where the cost of the final system is a factor. The reasons for this are: its small size, low power consumption, pixel clock output, variable electronic shutter, and relatively high resolution. However, to have any confidence in such a camera a thorough examination is required to assess its characteristics. In this paper an investigation of three of these cameras is described, and their suitability for close range photogrammetry evaluated. The main factors assessed are system component influences, warm-up effects, line jitter, principal point location and lens calibration. The influence of the frame-store on the use of the camera is also estimated and where possible excluded. Results of using these cameras for close range measurement are given and analyzed. While many users will have or prefer to buy other cameras, the evaluation of this particular camera should give an understanding of the important features of such image sensors, their use in photogrammetric measuring systems and the processes of evaluating their physical properties.


Proceedings of SPIE | 1995

Analysis of subpixel target location accuracy using Fourier-transform-based models

Timothy A. Clarke; Xinchi Wang

The simulation of the performance of subpixel algorithms under known conditions is a valuable tool. For instance, the optimum size for a target can be determined as well as the influence of varying the threshold level or target size. Previous work looked at the effects of quantization and additive noise on the location of target images. More recently, work by Shortis et al, 1994 has tested more algorithms and looked at other effects such as those caused by saturation and DC offset. In this paper the electronic noise present in imagery from typical CCD cameras is both measured and used within a simulation of the subpixel location performance of the centroid and squared centroid method. The effect on target lcoation of uneven background characteristics are also analyzed. To achieve this the physical characteristics are modeled by using the Fourier transform of both background and target models and combining them in the frequency domain. By performing the inverse Fourier transform the resulting target image is used to locate the target. The simulation methodology will be explained and tests performed so that a better understanding of the factors that contribute to subpixel errors can be gained.


Photonics for Industrial Applications | 1994

Detection and tracking of pipe joints in noisy images

Xiang Pan; Timothy A. Clarke; Tim Ellis

The remote and automatic inspection of the inside of pipes and tunnels is an important industrial application area. The main characteristics of the environment found in commonly used pipes such as sewers are: limitations on the camera spatial position; a large variety of surface features; a wide range of surface reflectivity due to the orientation of parts of the pipe, e.g. the joints; and many disturbances to the environment due for example to: mist, water spray, or hanging debris. The objective of this research is defect detection and classification; however, a first stage is the construction of a model of the pipe structure by pipe joint tracking. This paper describes work to exploit the knowledge of the environment to: build a model of the defects, reflectivity characteristics and pipe characteristics; develop appropriate methods for grouping the pipe joint features within each image from edge information; fit a pipe joint model (a circle, or connected arcs) to the grouped features; and to track these features in sequential images. Each stage in these processes has been analyzed to optimize the performance in terms of reliability and speed of operation. The methods that have been developed are described and results of robust pipe joint tracking over a large sequence of images are presented. The paper also presents results of experiments of applying several common edge detectors to images which have been corrupted by JPEG encoding and spatial sub-sampling. The subsequent robustness of a Hough based method for the detection of circular images features is also reported.


Photonics for Industrial Applications | 1994

Optimized target-matching based on a 3D space intersection and a constrained search for multiple camera views

Jin Chen; Timothy A. Clarke; S Robson; K.T.V. Grattan

In this paper the problem of finding correspondences between target images in multiple views is studied. A 3D target matching algorithm is developed which can be used as a direct replacement for the epipolar constrained matching method without requiring precise camera parameters. Approximate camera parameters are iteratively refined by combining the matching procedure with the bundle adjustment method. Several techniques are discussed to improve the reliability and efficiency of the method using a 3D space constrained search technique for the matching of target images in multiple viewpoints. A globally consistent constrained search is developed in which pseudo target images are defined to overcome the problem of occluded targets. Hypothesis testing and heuristic method are also used to improve the efficiency and robustness of the matching process. An analysis of the methods used is given and a general algorithm is designed. The resulting algorithm is shown to successfully find the correspondence between targets using many viewpoints. Simulation trials and practical tests are performed to verify the reliability and efficiency of the algorithm.

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S Robson

University College London

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Jin Chen

City University London

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Xiang Pan

City University London

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Xinchi Wang

City University London

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R.M. Gooch

City University London

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