Log-polar transform-based wavelet-modified maximum average correlation height filter for distortion invariance in a hybrid digital-optical correlator

We discuss and implement a log-polar transform-based distortion-invariant filter for automatic target recognition applications. The log-polar transform is a known space-invariant image representation used in several image vision systems to eliminate the effects of scale and rotation in an image. For in-plane rotation invariance and scale invariance, a log-polar transform-based filter was synthesized. In cases of in-plane rotation invariance, peaks shift horizontally, and in cases of scale invariance, peaks shift vertically. To achieve out-of-plane rotation invariance, log-polar images were used to train the wavelet-modified maximum average correlation height (WaveMACH) filter. The designed filters were implemented in the hybrid digital-optical correlation scheme. It was observed that, for a certain range of rotation and scale differences, the correlation signals merge with the strong dc. To solve this problem a shift was introduced in the log-polar image of the target. The use of a chirp function for dc removal has also been discussed. Correlation peak height and peak-to-sidelobe ratio have been calculated as metrics of goodness of the log-polar transform-based WaveMACH filter. Experimental results are presented.     

Theory and optical implementation of the geometrical approach of multiple circular harmonic filters

The circular harmonic filter contains only one component of the image. Its discrimination capability has been questionable. The geometrical approach of multiple circular harmonic filters uses relative locations of the correlation peaks as the rotation-, shift-, and intensity-invariant features for pattern recognition. Each feature depends on the entire image. This approach has a good discrimination capability. Optical real-time implementation of the on-axis continuous phase-only circular harmonic filters by the use of a commercial liquid-crystal television is shown. A harmonic analysis shows that the phase-mostly filter can tolerate coupled amplitude modulation at the acceptable expense of the output signal-to-noise ratio. An optical experiment of the geometrical approach of multiple circular harmonic filters for a multiple-image input is described. The cross-correlation peaks between the individual filters and the clutter are eliminated, because they are not in good locations.     

Fractional Wiener filter

The fractional correlation is a new operation that can easily be implemented by optical means. This operation might be useful for shift-variant pattern recognition and for image restoration. The Wiener filter is the optimal filter according to the minimal square-error criterion. For a given spectral noise that distorts the reference image, this filter is optimal for restoring an image in noise. A fractional Wiener filter is suggested for restoring reference objects in a fractional correlation system. The new filter sometimes performs better than the conventional Wiener filter.     

Shift and scale‐invariant correlation for pattern recognition based on the bidimensional empirical mode decomposition with noise robustness

Abstract

A novel filter is proposed to improve the noise robustness and discrimination capability for shift and scale invariant pattern recognition. This filter is a combination of mellin radial harmonic filter (RHF) and the bidimensional empirical mode decomposition. The basic principle of this method is to make use of partial reconstructions of the image by the relevant intrinsic mode functions corresponding to the most important structures of the image. A criterion is proposed to determine the proper number of intrinsic mode functions to be discarded for denoising by discussing the characteristic of the noise. The proposed filter provides a wider allowable scale change of the object. Within this range, the correlation peak intensity is relatively uniform even in the case of noise. This proposed filter has been tested experimentally to confirm the result from numerical simulations for cases with and without additive white Gaussian noise.     

Two class minimax distance transform correlation filter

A new correlation filter formulation (that we refer to as the minimax distance transform correlation filter (MDTCF) is presented that minimizes the average squared distance from the filtered desired (or true-) class training images to a filtered reference image while maximizing the mean squared distance of the filtered undesired (or false-) class training images to this filtered reference image. This approach increases the separation between the false-class correlation outputs and the true-class correlation outputs. Classification can be performned using the squared distance of a filtered test image to the chosen filtered reference image. We show that the previously introduced distance classifier correlation filter (DCCF) is similar to a special case of MDTCF. We also examine the differences between the DCCF and the MDTCF, and show that MDTCF can offer increased discrimination performance. Also, MDTCF performance is evaluated on two different face databases.     

Reference phase-encoded fringe-adjusted joint transform correlation

A novel reference phase-encoded joint transform correlation technique is proposed for efficient multiple-target detection. The proposed method employs phase encoding for the reference image and nonlinear Fourier plane apodization to optimize the detection performance. Existing joint transform correlators (JTC's) require multistep on-line processing to eliminate the false alarms. The proposed reference phase-encoded JTC overcomes false-target detection by eliminating the false correlation peaks while alleviating the effects of noise and other artifacts in just one step, thus ensuring higher processing speed. This technique yields only one peak per target instead of a pair of peaks produced by alternate JTC's. An all-optical implementation for the reference phase-encoded JTC technique is proposed, and computer simulation results are presented.     

Adaptive binary joint transform correlator for image recognition

Our research has shown that the autocorrelation peaks of a binary joint transform correlator are affected by input scenes' backgrounds. An adaptive method is proposed to overcome this problem. The image of interest is first extracted from the background based on the position of the highest correlation peak of the input and reference images. The extracted image is then correlated with the reference to obtain the final correlation peak. Numerical simulations showed that the final autocorrelation peak is the maximum constant for a specified reference image.     

Optical threshold-decomposed Boolean-based gray-scale filter

Because of the advances made in optical technology, an extension can be made from the well-known stack filter to a new kind of gray-scale filter by using arbitrary Boolean functions instead of positive Boolean functions in the second step of a three-step process. Applications of these threshold-decomposed Boolean-based gray-scale filters in image processing include image smoothing and feature detection. A general optical implementation approach of nonlinear threshold-decomposed Boolean-based gray-scale filters is also described that utilizes area coding, gray scale correlation and lookup table thresholding techniques. Some experimental results are demonstrated.     

Joint transform correlator subtracting a modified Fourier spectrum

The optically generated joint Fourier transform (JFT) of a test image and a reference image is processed using a new method: the JFT is recorded twice. In the second recording the reference image is phase shifted by π with respect to the first recording. The two JFT's are subtracted and binarized with a threshold of zero. Strong correlation peaks are obtained, and correlations within the test image are suppressed. Some results of optical implementation are presented, using a ferroelectric liquid crystal display with 128 × 128 pixels for data input. The phase shift of the reference was implemented by the contrast-inverted reference input on the binary light-modulating device. Processing of the JFT is done by a CCD camera, a frame grabber, and a personal computer.     

Fresnel transform-based correlator

We present a new technique for information processing using Fresnel transform-based correlation. The main emphasis is on the design of a correlator involving the reference object and its near-field diffraction pattern at an optimized distance. The input-scene image and its diffraction pattern constitute the input pattern of the new correlator. The new technique shows a significant increase in discrimination ability and optical efficiency. Moreover, different encoding methods, such as the phase-only filter or the matched filter, can be used in conjunction with this method. A theoretical analysis as well as examples are given.     

Invariant optical-pattern recognition based on a contour bank

We propose a new optical pattern-recognition method that is invariant to shift, rotation, and scale changes. By means of a zero-order circular-harmonic filter the input object can be transformed into a correlation peak whose outline retains a shape similar to the outline of the input object. After multilevel thresholding and binarization, a bank composed of many contours with different sizes is formed. The invariant recognition can be obtained by a small contour line being taking as a reference and its second-order circular-harmonic component being taken as a filter. In addition to the scale invariance the correlation peak is superior to the classical circular-harmonic filter and the phase-only circular-harmonic filter in terms of peak intensity, half-maximum width, and peak-to-sidelobe ratio.     

An Approximate Method to Obtain the Amplitude Distribution on the Exit Reference Sphere Due to a Non-uniform Transmission Filter

Abstract

A method is proposed to calculate the amplitude on the reference sphere due to a non-uniform transmission filter. The method may be divided into two stages: imaging of the filter plane, and projection of the filter image plane on the reference sphere. In this way we deduce some expressions which relate the coordinates on the filter plane to those on the reference sphere. Although the method is approximate, it has the advantage of saving computing time, since it is only necessary to trace two rays for each considered field. The accuracy of the method is studied by applying it to different real systems.     

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.     

Characterization and mapping of carotenoids in the algae Dunaliella and Phaeodactylum using Raman and target orthogonal partial least squares

A method was developed for the characterisation of carotenoid pigments in algal species using Raman spectroscopy in combination with multivariate hyperspectral analysis. Target orthogonal partial least squares (T-OPLS) operates by designating one known reference spectrum as the target. The target spectrum is put as the single y column in an OPLS regression model where the X matrix consists of the unfolded image spectra as variables in its columns. The spectral shape of the OPLS first orthogonal target score enabled us to verify the peak positions of the standard, and detect new peaks, not present in the reference standard. It was shown that the mixture of carotenoids present in the algae did not fully match the reference spectrum, however, the method provided enough information to make an analysis possible also in this case. The image results were constructed from the OPLS loading vectors that were showing a correlation map for the reference spectrum from the predictive loadings and maps of the occurrence of deviations from the orthogonal loadings.     

匹配滤波器中参考目标位置对相关峰位置的影响

针对利用光学相关器识别运动多目标的精确定位问题,提出了参考图像目标置中的匹配滤波器设计方法.该方法根据场景图像中目标位置、匹配滤波器中参考目标位置和相关峰位置之间的关系,采用将目标放置在参考图像中心以克服单一目标和运动多目标识别过程中相关峰的不确定偏移.实验结果表明,在范德卢格特型光电混合相关器中,采用该方法设计匹配滤波器可以使相关峰位置与目标位置一一对应,并且能够直观的实时显示出被识别目标在场景图像中的位置,提高了运动目标的定位准确性和跟踪稳定性.     

Implementation of arbitrary real-valued correlation filters for the shadow-casting incoherent correlator

We describe an incoherent correlator, based on the shadow-casting principle, that is able to implement any real-valued linear correlation filter. The correlation filter and the input image are displayed on commercial liquid-crystal television (LCTV) panels. Although it cannot handle high-resolution images, the incoherent correlator is lensless, compact, low cost, and uses a white-light source. A bipolar technique is devised to represent any linear filter, computed from a single reference image or composite, in the correlator. We demonstrate experimentally the efficiency of the design in the case of optimal trade-off (OT) filters and optimal trade-off synthetic discriminant function (OT-SDF) filters.     

Nonlinear filter for pattern recognition invariant to illumination and to out-of-plane rotations

Automatic target recognition in uncontrolled conditions is a difficult task because many parametersare involved. This study deals with the recognition of targets under limited out-of-plane rotations while maintaining invariance to ambient light illumination. Contrast invariance is achieved by using the recently developed locally adaptive contrast-invariant filter, a method that yields correlation peaks whose values are invariant under any linear transformation of intensity. To reduce the sensitivity to the orientation of the object we replace the reference in the nonlinear filter by a synthetic discriminant filter. The range used for out-of-plane rotations was 40 degrees with a depression angle of 20 degrees. We present results for unsegmented targets on complex backgrounds with the presence of false targets.     

Hybrid digital-optical correlation employing a chirp-encoded simulated-annealing-based rotation-invariant and distortion-tolerant filter

The simulated annealing (SA) algorithm based on entropy optimization is a technique of synthesizing distortion-invariant matched filters capable of discriminating very similar images. The synthesis of rotation-invariant filters using modified SA-based filter equations and their tolerance to distortions are studied. The filters are trained with true class images rotated in-plane at 3 degrees intervals between 0 degrees and 360 degrees . A total of seven filters are required over the whole range for both CCD or thermal images. Optical correlation in a hybrid digital-optical correlator results in an unwanted zero-order dc along with two first-order (+/-1) correlation peaks. A chirp function multiplied with the filter separates out the three peaks to three different planes, and only one peak in focus is captured in a camera. The performance of the modified SA-based filter has been studied in comparison to the conventional SA filter as well as with other filters.     

Optical angular mapping for pattern recognition using self-pumped phase conjugation

An optical angular feature mapping of an object for pattern recognition is presented that uses self-pumped phase conjugation followed by a vertical projection. In this method the vertical contour features of an object are most enhanced and vertically projected at each angle as the object is rotated. The vertical enhancement of the contour and the projection are performed optically with self-pumped phase conjugation and a cylindrical lens, respectively. The peaks in the resulting projection at the angle are located and integrated to produce an angular signature function. The signature function is then energy normalized for scale invariance, and pattern recognition is performed by calculation of a cross correlation between two signature functions. The method exhibits invariance to shift, rotation, and variations in scale. Effects of a spatial filter and incident-beam conditions used for phase conjugation are also discussed along with experimental results.     

The measurement of magnetic microfields

A new transmission electron microscope method of measuring fine scale magnetic flux distributions, and in particular the fields associated with magnetic recording heads, is described. An island structured metallic foil or an amorphous film acts as a diffuse scatterer of transmitted electrons. A magnetic field in the region below the scatterer deflects the electron trajectories and as a result distorts the image of the scatterer. The local shearing of the image is compared to a reference zero field image by a double exposure of a photographic plate. Optical Fraunhofer diffraction by transmission through the developed plate shows Young's fringes which reveal the direction and magnitude of the local image shift which is related to the magnetic field component perpendicular to the electron trajectories. In some applications the method is potentially several orders of magnitude more sensitive than other electron beam methods.