Computer-generated and phase-only synthetic discriminant function filters

Generalization of the Jared and Ennis method of complex transmittance objects for the generation of synthetic discriminant function filters

We present a simple method of constructing synthetic discriminant function filters optimized to take into account the modulation of liquid-crystal devices. This relaxation algorithm, a generalization of the Jared and Ennis method, is an iterative method that includes arbitrary modulations for both scene and filter, extending the problem to the complex plane. Simulated and experimental results obtained in a VanderLugt correlator are presented for a two-class recognition problem. The optimal number of images needed to describe an object in a filter generated in this way is discussed, and the influence of the spatial light modulation resolution on the correlation is studied.

Multireference binary phase-only filter optimized by regions of support

A new formulation of a multiplex filter for filtering-based optical processors, based on the VanderLugt architecture, is presented. The multireference binary phase-only filter (MBPOF), optimized by regions of support (ROS), constitutes a formal rewriting of some multiplex or composite filters including optimization functions, such as the distribution function and the selection function. The first function optimizes the multiplexing of references into the multireference filter. The second function defines the ROS of an object's Fourier spectrum and can be independently used to optimize the conventional binary phase-only filter. Both functions result from a segmentation of the Fourier plane. The MBPOF with ROS can be optically implemented in a filtering-based optical processor owing to a binary-phase spatial light modulator. Simulation and optical results are given for different examples of the BPOF and the MBPOF, both with ROS optimizing different criteria of performance, such as peak-to-correlation energy, discrimination capability, and distortion sensitivity.

High-diffraction-efficiency pseudorandom encoding

Pseudorandom encoding (PRE) is a statistics-based procedure in which a pure-phase spatial light modulator (SLM) can yield, on the average, the prescribed diffraction pattern specified by the user. We seek to combine PRE with the optimization of an aperture-based target function. The target function is a fully complex input transmittance, unrealizable by a phase-only SLM, that generates a prescribed light intensity. The optimization is done to increase the diffraction efficiency of the overall process. We compare three optimization methods-Monte Carlo simulation, a genetic algorithm, and a gradient search-for maximizing the diffraction efficiency of a spot-array generator. Calculated solutions are then encoded by PRE, and the resulting diffraction patterns are computer simulated. Details on the complexity of each procedure are furnished, as well as comparisons on the quality, such as uniformity of the output spot array.

Random phase encoding of composite fully complex filters

The mapping of complex-valued functions onto phase-only spatial light modulators is examined. Random phase encoding effectively adds amplitude control to the phase-only filter and can be used to trade off systematic errors of the phase-only filter for random errors. This is illustrated for the problem of recognizing a threedimensional object from arbitrary views. The complex-valued composite filters that constitute a filter bank design are encoded by phase-only and pseudorandom methods. The best recognition probabilities are achieved by blending the two methods so that only the smallest amplitudes are randomly encoded.

Multilevel phase- and amplitude-encoded modified-filter synthetic-discriminant-function filters

The performance of the modified-filter synthetic-discriminant-function (MfSDF) filter with multilevel phase and amplitude (MLAP) constraints is investigated with various in-plane rotated images from an in-class Bradley armored personnel carrier vehicle and an out-of-class Abram MI tank; this is of interest because of the commercial availability of liquid-crystal televisions, which are able to encode the gray-level amplitude and/or the discrete multilevel phase information. The evaluation is performed to explain better the image-distortion range that can be covered effectively by MLAP/MfSDF filters. The results show that the MLAP/MfSDF filter offers much-improved correlator system performance with a greater allowable image-distortion range while maintaining 100% discrimination between in-class and out-ofclass images; furthermore, it shows an improved ability to accommodate the input image noise when compared with the MfSDF filter with a binary phase-only constraint. Thus the MLAP/MfSDF can be implemented effectively by a hybrid optical/digital correlator system to track a vehicle or a tank dynamically as it moves along a random trajectory across the input field.

Computation of arbitrarily constrained synthetic discriminant functions

An algorithm for computing correlation filters based on synthetic discriminant functions that can be displayed on current spatial light modulators is presented. The procedure is nondivergent, computationally feasible, and capable of producing multiple solutions, thus overcoming some of the pitfalls of previous methods.

Modified filter synthetic discriminant functions for improved optical correlator performance

By the filter modulation operator N, the modified filter synthetic discriminant function permits advantageous preprocessing of individual training-set images that are used in a linear combination to construct the filter synthetic discriminant function, which applies a modulation operator M to the synthetic discriminant function. A relaxation algorithm is used to satisfy the equal correlation peaks rule in the correlator output plane. As the filter modulation operators M and N can be given any functional form, the modified filter synthetic discriminant function design proposed is sufficiently general to be described as a unified filter modulation synthetic discriminant function design.

Random mask encoding of multiplexed phase-only and binary phase-only filters

In the formation of multiplexed filters, the phase-only and binary phase-only operations result in spurious cross terms and a nonlinear scaling. We use complementary random binary patterns having different pattern densities to encode the various functions within the multiplexed function. The orthogonality of these complementary masks permits separability of the phase-only or binary phase-only operations. By changing the pixel density of the random pattern, we can easily change the relative weights of the functions that compose the multiplexed filter. Experimental results are shown that demonstrate the effectiveness of this approach.

Tutorial survey of composite filter designs for optical correlators

A tutorial survey is presented of the many composite filter designs proposed for distortion-invariant optical pattern recognition. Remarks are made throughout regarding areas for further investigation.

Digital phase-encoded inverse filter for optical pattern recognition

Optical pattern recognition can profit from the progress in coding theory and technology that has been made in digital holography. The calculation of a phase-encoded inverse filter is described. This filter allows compromises between discrimination capability and diffraction efficiency. Phase quantization facilitates the materialization of the filter.

Improved synthetic discriminant function performance using Fresnel lens-encoded binary phase-only filters

Synthetic discriminant functions (SDFs) made using Fresnel lens-encoded binary phase-only filters are shown to yield increased performance relative to SDFs constructed using conventional binary phase-only filters. We present both computer simulations and experimental results in which these SDFs are written onto the magnetooptic spatial light modulator.

Distortion range of filter synthetic discriminant function binary phase-only filters

This paper considers the performance of filter synthetic-discriminant-function binary-phase-only filters (fSDF-BPOFs) with images in the distortion range that were not members of the training set. This evaluation was performed to understand better the distortion range that can be effectively covered by fSDF-BPOFs. The peak correlation and peak clutter responses were measured over various distortion ranges for in-plane and out-of-plane rotations. The peak correlation initially decreased rapidly as the distortion range increased but decreased gradually as the distortion range became larger for the images used in this study. The distortion ranges that can be effectively covered by fSDF-BPOFs appear to be modest: 0-30 degrees for inplane rotation and 0-10 degrees for out-of-plane rotation. The results indicate that a temporal multiplexing scheme will be required to cover a large distortion range when BPOFs are used.

Inclusion of filter modulation in synthetic-discriminant-function construction

A technique in which the filter modulation is included in the synthesis of a synthetic-discriminant-function matched spatial filter (SDF MSF) is presented. In the filter synthesis, a system of simultaneous nonlinear equations is solved with an iteration procedure. A computer simulation of the new method using thresholded images of the Space Shuttle over a range of aspect angles was performed for phase-only filters (POFs) and binary-phase-only filters (BPOFs). The filters constructed are capable of obtaining the specified peak-correlation response to within 1% with a high signal-to-clutter-ratio for the one-class problem, the two-class problem, and the multilevel problem. In contrast, conventional projection SDF POFs and BPOFs are unable to produce the desired peak-correlation response.

Optimal binary phase-only matched filters

Many spatial light modulators and computer-generated hologram techniques can very efficiently implement binary phase-only filters. At present, almost all binary phase-only filters are designed by first designing a matched spatial filter and then binarizing it. There is no theoretical basis to expect that such a filter will be optimal. This paper first develops a theory for optimal binary phase-only filters and then presents a numerical algorithm which designs the optimal binary phase-only matched filter for a given image. The filter is optimal in the matched filter sense of maximizing SNR at the output origin. Characteristics of the optimal filter compared to conventional binary phase-only filters are discussed.

Transform-ratio ternary phase-amplitude filter formulation for improved correlation discrimination

A method of formulating ternary-valued (-1,0,1) correlation filters based on the ratio of spectral energies of target and nontarget patterns is proposed, and performance of such filters is investigated by computer simulations of correlation. The results confirm the intuitive expectation that such filters can enhance signalto- clutter and discrimination performance for target recognition in the presence of large amounts of input noise. These filters may be viewed as an extension of binary phase-only filters and similarly are motivated by the prospect of near-term real-time implementation.