Thomas F. Krile

Digital stereo image analyzer for generating automated 3-D measures of optic disc deformation in glaucoma

The major limitations of precise evaluation of retinal structures in present clinical situations are the lack of standardization, the inherent subjectivity involved in the interpretation of retinal images, and intra- as well as interobserver variability. While evaluating optic disc deformation in glaucoma, these limitations could be overcome by using advanced digital image analysis techniques to generate precise metrics; from stereo optic disc image pairs. A digital stereovision system for visualizing the topography of the optic nerve head from stereo optic disc images is presented. We have developed an algorithm, combining power cepstrum and zero-mean-normalized cross correlation techniques, which extracts depth information using coarse-to-fine disparity between corresponding windows in a stereo pair. The gray level encoded sparse disparity matrix is subjected to a cubic B-spline operation to generate smooth representations of the optic cup/disc surfaces and new three-dimensional (3-D) metrics from isodisparity contours. Despite the challenges involved in 3-D surface recovery, the robustness of our algorithm in finding disparities within the constraints used has been validated using stereo pairs with known disparities. In a preliminary longitudinal study of glaucoma patients, a strong correlation is found between the computer-generated quantitative cup/disc volume metrics and manual metrics commonly used in a clinic. The computer generated new metrics, however, eliminate the subjective variability and greatly reduce the time and cost involved in manual metric generation in follow-up studies of glaucoma.

Ambiguity function display: an improved coherent processor

A coherent optical processor for displaying a signals ambiguity function is described. The required time delay is realized by 45 degrees rotations of two identical input transparencies and the Doppler shift by a 1-D Fourier transformation. The entire ambiguity function is displayed in the output (Doppler shift-time delay) plane. Examples of the optically computed ambiguity function for single and double pulse signals are shown to be in excellent agreement with theory. Advantages of this approach over other schemes and possible extension to real time processing are also discussed.

Multiplex holography with chirp-modulated binary phase-coded reference-beam masks.

Certain binary codes developed for spread-spectrum communication applications can be used to construct families of pseudorandom diffuser masks suitable for multiplex holography. Binary codes are used so that fabrication of the diffuser masks can be relatively straightforward. A simple technique is described for chirp-modulating the binary diffuser masks to achieve the advantage of polyphase masks, i.e., improved correlation properties, without having to construct them. Numerical comparisons of the correlation properties of optimal binary codes with and without chirp-modulation are presented.

2-D Optical Multistage Interconnection Networks

This paper discusses the need for 2-D multistage optical interconnection networks in parallel computing systems and demonstrates the benefits obtained from a 2-D multi-stage perfect shuffle network.

Power cepstrum and spectrum techniques applied to image registration

The use of power cepstrum analysis in image registration is explored. Rotational shifts and translational shifts are corrected separately. The technique involves two main ideas. First, after preprocessing to remove extraneous information and information which could result in false registration parameters, a rotational shift is changed into a translational shift by using the shift-invariant property of the power spectrum. Second, power cepstrum analysis is used to correct the translational shift. Because of the introduction of these ideas, this new algorithm can work very fast and accurately compared to conventional correlation techniques. This registration technique is applied to sequential fundus images with potential application in detecting changes in fundus anomalies.

Holographic representations of space-variant systems using phase-coded reference beams

A new holographic implementation of a sampling technique permits, in principle, a straightforward representation of 2-D space-variant optical systems. The set of sample transfer functions required for the representation is recorded on a single holographic plate by utilizing phase coded reference beams. Because this approach does not depend on volume effects in the recording medium in an essential way, the holograms can be produced digitally, as well as optically. Basic concepts and preliminary experimental investigations related to this approach are presented and discussed.

Image compression in signal-dependent noise.

The performance of an image compression scheme is affected by the presence of noise, and the achievable compression may be reduced significantly. We investigated the effects of specific signal-dependent-noise (SDN) sources, such as film-grain and speckle noise, on image compression, using JPEG (Joint Photographic Experts Group) standard image compression. For the improvement of compression ratios noisy images are preprocessed for noise suppression before compression is applied. Two approaches are employed for noise suppression. In one approach an estimator designed specifically for the SDN model is used. In an alternate approach, the noise is first transformed into signal-independent noise (SIN) and then an estimator designed for SIN is employed. The performances of these two schemes are compared. The compression results achieved for noiseless, noisy, and restored images are also presented.

Space-variant processing of 1-D signals

Two general schemes for 1-D space-variant processing are proposed. The direct output display scheme gives the space-variant system output along a line in the processors output plane. The output spectrum display scheme directly computes the space-variant systems output spectrum. Both of these schemes utilize a 1-D input and a line spread function mask. Example applications and experimental results are also presented.

Estimation in signal-dependent film-grain noise

Optimal estimators are derived for a signal-dependent film-grain noise model, and the effect of signal-dependence on the estimatorss structures is investigated. Due to the mathematical complexity of these optimal estimators, various suboptimal estimators are proposed. Computer simulations are then presented which compare the optimal and suboptimal estimators with regard to mean square estimation error, sensitivity to signal-dependence, and robustness with respect to the a priori signal probability density function.

Determination of Hopfield associative memory characteristics using a simple parameter

Abstract A new statistical method is proposed for exploring the characteristics of the Hopfield associative memory (HAM). The existence of an average signal-to-noise ratio parameter (which we call C) has been successfully applied to derive equations that are capable of concisely estimating the storage capacity of (a) direct convergence nets in which the initial vector is required to precisely converge to the memorized vector in one iteration and (b) indirect convergence nets in which a specified error ϵ is allowed after multiple iterations. The close tie between the memory capacity and the required convergence probability of the HAM is described. The significance of the 1-to-1 relationship between the indirect convergence probability and the parameter ϵ/η (η = probability of a neuron state being an incorrect bit) is shown. The importance of the parameter ηN in determining the capacity of the direct convergence nets is also discussed. The theory not only provides the memory capacity for various versions of the HAM, but it gives an explicit solution for the radius of attraction parameter.