Image contrast enhancement using adjacent-blocks-based modification for local histogram equalization

Infrared images usually have some non-ideal characteristics such as weak target-to-background contrast and strong noise. Because of these characteristics, it is necessary to apply the contrast enhancement algorithm to improve the visual quality of infrared images. Histogram equalization (HE) algorithm is a widely used contrast enhancement algorithm due to its effectiveness and simple implementation. But a drawback of HE algorithm is that the local contrast of an image cannot be equally enhanced. Local histogram equalization algorithms are proved to be the effective techniques for local image contrast enhancement. However, over-enhancement of noise and artifacts can be easily found in the local histogram equalization enhanced images. In this paper, a new contrast enhancement technique based on local histogram equalization algorithm is proposed to overcome the drawbacks mentioned above. The input images are segmented into three kinds of overlapped sub-blocks using the gradients of them. To overcome the over-enhancement effect, the histograms of these sub-blocks are then modified by adjacent sub-blocks. We pay more attention to improve the contrast of detail information while the brightness of the flat region in these sub-blocks is well preserved. It will be shown that the proposed algorithm outperforms other related algorithms by enhancing the local contrast without introducing over-enhancement effects and additional noise.     

光学干涉图像处理中基于质量权值的离散余弦变换解包裹相位

为减少噪声对相位恢复过程的影响,快速得到正确的解包裹相位,提出了一种改进的相位解包裹方法——加权离散余弦变换解包裹算法。该方法把离散余弦变换和标识相位数据好坏的质量权值结合起来,兼有速度快和可靠度高的优势。为验证此算法,对模拟和实验得到的包裹相位图添加随机噪声和散粒噪声,同时采用加权与非加权离散余弦变换算法进行处理,所得到的解包裹结果与未加噪声的解包裹相位值进行比较,结果表明,通过加权离散余弦变换算法恢复的相位图比非加权离散余弦变换算法所恢复的相位图更接近于理想值,而且两种算法的运行速度基本相同,这证明提出的算法不仅保证了效率高的优点,而且所恢复的解包裹相位具有较好的噪声免疫能力和可靠度。     

基于分支设置的质量导引相位展开算法

二维相位展开方法是近年来较为活跃、引起关注的一个研究课题,它在许多测量应用中有着重要的作用。尽管掩膜阻断算法在多个领域都有成功应用实例,该算法存在着固有的缺陷。为了克服掩膜阻断算法的缺陷,综合分支阻断方法和质量导引方法的优点,提出一种基于分支设置的质量导引相位展开新算法。它先以一个初始质量图来引导分支的设置,然后把分支对应的相位质量设置为最低,从而产生一个新的质量图,最后按新质量图来引导相位展开,并使用几个包裹相位图来验证此方法的有效性。计算机模拟相位图和实际相位图的相位展开结果表明,在存在复杂轮廓不连续和高噪声的情况下,该算法优于模板阻断算法,能得到较好的相位展开结果。     

A comparison of iterative algorithms and a mixed approach for in-line X-ray phase retrieval

Previous studies have shown that iterative in-line X-ray phase retrieval algorithms may have higher precision than direct retrieval algorithms. This communication compares three iterative phase retrieval algorithms in terms of accuracy and efficiency using computer simulations. We found the Fourier transformation based algorithm (FT) is of the fastest convergence, while the Poisson-solver based algorithm (PS) has higher precision. The traditional Gerchberg-Saxton algorithm (GS) is very slow and sometimes does not converge in our tests. Then a mixed FT-PS algorithm is presented to achieve both high efficiency and high accuracy. The mixed algorithm is tested using simulated images with different noise level and experimentally obtained images of a piece of chicken breast muscle.     

基于Rician分布的散斑噪声的超声图像模拟算法*

为了衡量各种去噪算法的性能,在干净无噪声的图像上添加接近真实且可控的散斑噪声是非常重要的,提出一种基于Rician分布的不完全发育的斑点噪声的超声图像模拟算法。该算法考虑到了声波区域中包含孤立的强散射体的情况,同时结合了超声成像的扫描过程。以合成图像和肾脏图像为体模进行了模拟实验,并对最终生成的伪超声图像进行了噪声分布统计及拟合检验。实验结果表明该算法生成的伪超声图像在视觉上和理论上都接近真实的超声图像。     

便携式X光机图像降噪复合算法研究

便携式X光机应用越来越广,因而对其所成图像的降噪也显得尤为重要.本文首先讨论和分析了便携式X光机成像系统噪声的来源及其特性,针对传统单一降噪算法存在的不足,提出了一种新的复合式降噪算法.该复合算法不仅对高斯噪声和脉冲噪声有很好的滤除效果,同时也能有效地保护图像的细节部分.仿真实验证明,该算法切实可行.     

A statistical method for characterizing the noise in nonlinearly reconstructed images from undersampled MR data: The POCS example

The projection-onto-convex-sets (POCS) algorithm is a powerful tool for reconstructing high-resolution images from undersampled k-space data. It is a nonlinear iterative method that attempts to estimate values for missing data. The convergence of the algorithm and its other deterministic properties are well established, but relatively little is known about how noise in the source data influences noise in the final reconstructed image. In this paper, we present an experimental treatment of the statistical properties in POCS and investigate 12 stochastic models for its noise distribution beside its nonlinear point spread functions. Statistical results show that as the ratio of the missing k-space data increases, the noise distribution in POCS images is no longer Rayleigh as with conventional linear Fourier reconstruction. Instead, the probability density function for the noise is well approximated by a lognormal distribution. For small missing data ratios, however, the noise remains Rayleigh distributed. Preliminary results show that in the presence of noise, POCS images are often dominated by POCS-enhanced noise rather than POCS-induced artifacts. Implicit in this work is the presentation of a general statistical method that can be used to assess the noise properties in other nonlinear reconstruction algorithms.     

Improving performance of FxRLS algorithm for active noise control of impulsive noise

Active noise control (ANC) systems employing adaptive filters suffer from stability issues in the presence of impulsive noise. New impulsive noise control algorithms based on filtered-x recursive least square (FxRLS) algorithm are presented. The FxRLS algorithm gives better convergence than the filtered-x least mean square (FxLMS) algorithm and its variants but lacks robustness in the presence of high impulsive noise. In order to improve the robustness of FxRLS algorithm for ANC of impulsive noise, two modifications are suggested. First proposed modification clips the reference and error signals while, the second modification incorporates energy of the error signal in the gain of FxRLS (MGFxRLS) algorithm. The results demonstrate improved stability and robustness of proposed modifications in the FxRLS algorithm. However, another limitation associated with the FxRLS algorithm is its computationally complex nature. In order to reduce the computational load, a hybrid algorithm based on proposed MGFxRLS and normalized step size FxLMS (NSS-FXLMS) is also developed in this paper. The proposed hybrid algorithm combines the stability of NSS-FxLMS algorithm with the fast convergence speed of the proposed MGFxRLS algorithm. The results of the proposed hybrid algorithm prove that its convergence speed is faster than that of NSS-FxLMS algorithm with computational complexity lesser than that of FxRLS algorithm.     

A novel feedback active noise control for broadband chaotic noise and random noise

The feedback active noise control (ANC) can be seen as a predictor, the conventional method based on filtered-x least mean square (FXLMS) algorithm can only be useful for linear and tonal noise, but for nonlinear and broadband noise, it is useless. The feedback ANC using functional link artificial neural networks (FLANN) based on filtered-s least mean square (FSLMS) algorithm can reduce some nonlinear noise such as chaotic noise, but the noise cancellation performance is not very well, at the same time, it is not useful to random noise. To solve the problem above, a new feedback ANC using wavelet packet FXLMS (WPFXLMS) algorithm is proposed in this paper. By decomposing the broadband noise into several band-limited parts which are predictable and each part is controlled independently, the proposed algorithm can not only suppress the chaotic noise, but also mitigate the random noise. Compared with FXLMS and FSLMS algorithms, proposed WPFXLMS algorithm also holds the best performance on noise cancellation. Numerous simulations are conducted to demonstrate the effectiveness of the proposed WPFXLMS algorithm.     

一种自适应的自动聚焦算法

自动聚焦是图像测量和计算机视觉中的重要技术。研究发现,以前的一些自动聚焦算法,在没有噪声的影响下,效果较好,但一旦有噪声的影响,算法可能会失效,因此,提出了一种自适应的自动聚焦方法。首先通过一个基于局部小窗口的自适应门限将边缘像素和非边缘像素区别开来,然后再加上一个全局门限来进一步滤除噪声的影响,最后用一些自动聚焦算法来计算经过自适应处理后的图像,画出它们的聚焦曲线。经过大量的实验,结果表明,该算法不仅具有单峰性强,无偏性好,灵敏度高等特点,而且还具有很好的抗噪声性能。     

A generalized temporal phase unwrapping algorithm for three-dimensional profilometry

A generalized temporal phase unwrapping algorithm is proposed for absolute phase measurement of object surfaces with complex topography. We first make comments on error analysis of various temporal phase unwrapping (TPU) algorithms and point out problems existing in these techniques. We present a generalized TPU algorithm (GTPU), which eliminates some limitations imposed on the fringe sequence for the classical TPU algorithms. So the desired number of fringes can be determined according to practical situation, leading to a flexible method for phase reconstruction. In addition, compared with the classical TPU algorithms, GTPU has better noise immunity and less computational complexity. Theoretical analysis and experiment results are presented to show the validity of the proposed algorithm.     

一种抗噪声的图像自动对焦算法

传统的图像自动对焦算法易受环境噪声影响,导致焦距评价函数的单峰性、无偏性和灵敏度降低.提出了一种基于噪声抑制的图像自动对焦算法,采用Sobel算子加权滤波抑制噪声,再利用全局门限分割边缘像素,最后用四种算子作为对焦评价函数,得出对焦曲线.实验表明,该算法具有较好的抗噪声性能.     

结构光测量中快速相位解包裹算法的讨论

对Schofield等提出的快速傅里叶变换（采用4次傅里叶变换和4次逆傅里叶）解包裹算法和Volkov等提出的相位重建算法（仅采用两次傅里叶变换和一次逆傅里叶变换）进行了讨论,提出基于离散余弦变换的快速相位解包裹算法.指出前两种算法在处理一般带噪音的模拟相位图时具有明显的优势,其处理结果非常接近理想值,而在处理相位变化剧烈或不连续区域的实验相位图（采用四步相移法测量雕刻佛像的三维面形）时出现较大误差,甚至无法处理,而基于离散余弦变换算法能很好的解决这个问题,并且拥有比前两种算法更快的运算速度.在实际应用中,针对不同的相位图把Schofield所提出的算法与基于离散余弦变换的算法结合起来,可以解决大部分相位解包裹问题.     

Circle detection using scan lines and histograms

Circle detection is significant in image processing and pattern recognition. We present a new algorithm for detecting circles, which is based on the global geometric symmetry of circles. First, the horizontal and vertical midpoint histograms of the edge image are obtained by using scan lines. Then, we apply the peak-finding algorithm to the midpoint histograms to look for the center of the circle. The normalized radius histogram is finally used to verify the existence of the circle and extract its radius. Synthetic images with different levels of pepper noise and real images containing several circles have been taken to test the performance. Experimental results demonstrate that the proposed algorithm has the advantage of computational efficiency as compared with the randomized Hough transform and some other algorithms.     

Phase-Measuring Algorithms to Suppress Spatially Nonuniform Phase Modulation in a Two-Beam Interferometer

Phase modulation of presently used phase-shifting interferometers is assumed to be spatially uniform across the observing aperture. However, calibration errors or the configuration of an interferometer can cause a spatial nonuniformity in the phase modulation. Spatial nonuniformity causes a significant error in the measured phase when the phase modulator has nonlinear sensitivity. An even-order nonlinearity in the phase modulation in particular contributes to the errors. Lowest-order errors can be suppressed by adding a new symmetry to the sampling functions of the phase-shifting algorithm, however the algorithm suffers from large random noise. The random noise is shown to be decreased substantially by applying one more sampled frame to the algorithm. We derive new seven-sample and eight-sample algorithms that can compensate for a nonuniform phase shift and has much less random noise than the previous algorithm we proposed.     

An efficient algorithm used for full-field optical coherence tomography

An efficient algorithm for computing optical coherence tomography is presented, which is based on the discrete differences of a time/depth sequence of interferograms. The existing multiple-step phase-shift algorithms, including those solved from Carré and Hariharan equations as well as circular-step algorithm proposed by Dubois et al., are analyzed and compared with the proposed algorithm. The analytical and experimental results show the computational efficiency of the new algorithm outperforms others. The simulations also demonstrate that the algorithm based on derivatives of four phase-shifted images is less sensitive to the phase-shift noise comparing to traditional 4-step phase-shift-based algorithm.     

去除乘性噪音的主成分分析算法

雷达成像系统的进一步应用依赖于对图像中噪音的有效抑制.在目前现有消除噪音方法的基础上,基于图像的局部相似性,结合主成分分析法,提出一种新的有效去除乘性噪音的滤波算法.乘性噪音经对数变换后可转化为加性噪音处理.分析了对数域中噪音的类型.首先在图像的对数域,通过非局部方法选取局部相似块作为训练样本,利用主成分分析法提取出信号的主要特征.然后基于统计理论中最小均方误差估计法给出了一种适用于图像信息的阈值原则.最后分析了变换过程引起的偏差,由对数域的偏估计得到滤波图像.数值实验验证了新算法的有效性.对比于目前提出的变分方法,新算法处理后的图像有更高的信噪比和更好的视觉效果,且具有一定的实用性.     

Algorithms for suppressing ultrasonic backscattering from material structure

In pulse-echo ultrasonic inspection the backscattering from the material structure appears in the received ultrasonic images as clutter, often referred to as grain noise, which impairs the inspection results. A toolbox including algorithms for suppressing ultrasonic clutter is presented in the paper. Several processing algorithms capable of suppressing grain noise have been proposed, of which the split spectrum processing (SSP) probably is the most renowned. The classical SSP technique applies a filter bank to some frequency band that has to be precisely known in advance, to obtain a set of narrow-band signals that are tested for mutual correlation using some statistical operation. A number of SSP algorithms with different statistical operations are included in the toolbox. A completely different approach is to use explicit statistical models of grain noise and defects and to design an optimal filter based on those models. A simple such algorithm, based on noncoherent detection (NCD) known from communications, is also included in the toolbox. The toolbox, implemented in Matlab, is provided with a user-friendly graphical interface facilitating comparison of the algorithms.     

基于数学形态学的数字全息再现像融合方法

针对数字全息中不同再现距离获得的携带不同聚焦信息的再现像, 提出了一种基于数学形态学的多聚焦再现像融合方法, 以有效扩展成像景深。首先通过小波-Controulet变换获得源图像的高频和低频分量;然后, 针对数字全息中含散斑噪声的特点, 对高频分量采用基于数学形态学区域能量的方法进行融合, 对低频分量采用加权对比度法进行融合;最后, 将融合系数反变换得到融合图像。通过对算法的有效性分析和实验验证, 将本文提出的方法与不加入数学形态学的融合方法进行了对比研究。结果表明, 基于数学形态学的融合方法能充分抑制散斑噪声的影响, 保留更多细节信息, 有效扩展了成像景深范围达11.5 cm。其中, 对于表面粗糙且信息量较少的骰子, 基于数学形态学方法的空间梯度算子提高了11.8%, 熵值提高了2.7%;对于表面光滑且信息量较多的硬币, 其空间梯度算子提高了13.6%, 熵值提高了2.8%。     

Color image encryption and decryption for twin images in fractional Fourier domain

We propose a method for the encryption of twin color images using fractional Fourier transform (FRT). The color images to be encrypted are converted into the indexed image formats before being processed through twin image encryption algorithm based on the FRT. The proposed algorithm uses one random code in the image domain and one random phase code in the FRT domain to perform double image encryption. The conversion of both the input RGB images into their indexed formats facilitates single-channel processing for each image, and is more compact and robust as compared to multichannel techniques. Different fractional orders, the random masks in image- and FRT domain are the keys to enhance the security of the proposed system. The algorithms to implement the proposed encryption and decryption schemes are discussed, and results of digital simulation are presented. We examine sensitivity of the proposed scheme against the use of unauthorized keys (e.g. incorrect fractional orders, incorrect random phase mask etc.). Robustness of the method against occlusion and noise has also been discussed.