Showing papers by "John Porrill published in 1998"

A deformable model of the human iris for measuring small three-dimensional eye movements

Abstract: This paper describes a deformable model of the human iris which forms part of a system for accurate off-line measurement of binocular three-dimensional eye movements, particularly cyclotorsion (torsion), from video image sequences. At least two existing systems measure torsion from infrared video images by pupil tracking followed by cross correlation using arcs of bandpass-filtered iris texture. Unfortunately, pupil expansion and contraction reduces the accuracy of this method unless drugs are used to constrict the pupil, which causes temporary blurred vision. A five-parameter deformable model of the iris is therefore developed for analysing images obtained without the use of drugs. This model can translate (horizontal and vertical eye motion), rotate (torsion) and scale both uniformly and radially (pupil changes). Torsion measurements obtained with the model are repeatable and accurate to within 0.1°; this performance is illustrated by analysing binocular torsion during fixation on a stationary target.

Deformable model of the human iris for measuring ocular torsion from video images

Abstract: The authors describe a deformable model of the human iris, which forms part of a system for accurate offline measurement of binocular eye movements, particularly cyclotorsion (torsion), from video image sequences. At least two existing systems measure torsion from infrared video images by pupil tracking followed by cross-correlation of bandpass filtered iris sectors. Unfortunately, pupil expansion and contraction reduce the accuracy of this method. In addition, infrared iris images typically contain very little texture, so correlation can be unreliable. A five-parameter deformable model of the iris was therefore developed for analysing images taken in visible light. This model can translate (horizontal and vertical eye motion), rotate (torsion) and scale both uniformly and radially (pupil changes). A series of software simulations and hardware tests suggest that torsion measurements obtained with the model are repeatable and accurate to within 0.1°. This performance is illustrated by analysing binocular torsion during fixation on a static target; the results match previously published data from other equipment.

Kinematic coordination of reach and balance.

Abstract: In the present article, the authors review inverse kinematic research across a number of fields to highlight the similarities among modeling approaches to the problems of reaching and maintenance of posture. Two particular techniques are described: the Berkinblit algorithm, used to model the leg-wiping reflexes of the frog, and the parallel control scheme of Hinton, in which simple stick-man simulations maintain a balanced posture while reaching. Those 2 algorithms are shown to be equivalent to one another and are described in terms of transpose jacobian control. A controller based on those methods is compared with one based on resolved motion rate control or pseudoinverse control, commonly used in robotics applications. Both controllers are applied to the problem of coordinating reaching and balancing behavior in a simulated 3-dimensional anthropomorphic robot. It is demonstrated that both of those techniques can produce postures that are surprisingly natural.