一种改进的Chirp Scaling成像算法

常规CS算法在算法推导过程中存在两个近似,即二维频率域泰勒展开忽略了距离频率的高次项以及二次距离压缩没有考虑调频斜率随距离的变化。随着SAR分辨率的进一步提高,由这些近似而引入的误差已不能忽略,不进行补偿成像效果将明显下降。该文提出了一种改进的CS算法,通过补偿距离频率的三次项,明显减小了算法的相位误差,使成像效果有明显改善,仿真结果验证了该方法的有效性。     

基于分数阶傅里叶变换的两步运动补偿CS算法

针对传统的基于傅里叶变换的两步运动补偿SAR成像算法在处理非平稳运动误差时效果不显著的问题,提出了基于分数阶傅里叶变换(Fr FT)的两步运动补偿CS算法,以期消除距离向运动误差的影响,从而获得高质量的SAR图像。仿真结果和基于实测SAR数据的成像结果都表明,所提算法能很好地消除距离向运动误差的影响。     

条带式SAR常用成像算法的误差及计算效率分析

针对条带式SAR系统设计中,二次相位误差的大小和计算效率是进行算法选择的重要依据,而一般文献只做定性分析的问题,该文对距离多普勒(近似)、距离多普勒(精确)、Chirp Scaling、ωKA(近似)和ωKA(精确)5种常用算法,推导了二次相位误差和计算量的公式,对条带式常用成像算法的二次相位误差及计算量进行了详细的定量分析。编写了一个带图形操作界面的计算程序,可供用户计算条带式模式下5种常用算法的二次相位误差及计算量。通过用户界面,可以方便地得到5种成像算法的相位误差及计算量,以及计算量和相位误差随入射角的变化,从而为用户成像算法的选择提供一定依据。     

基于四阶传递函数大斜视角SAR成像算法

针对研究SAR大斜视工作时,为增加天线实时侦察效果,对低阶ETF(Exact transfor function)算法处理方位向相位的近似误差进行改进,在传统ETF成像算法基础上研究了四阶传递函数算法.采用四阶距离模型,分析了距离向聚焦深度为分块数据处理提供了依据,同时在距离多普勒域对非参考距离处目标进行了相位补偿.通过场景目标仿真对算法有效性进行了验证.仿真结果表明,算法可有效补偿大斜视角时方位向近似误差,提高了精度,满足大斜视时成像要求,同时避免插值处理,计算量小.便于实时成像.     

大斜视双基地SAR的二维可分离成像算法

提出了大斜视双基地SAR的二维可分离成像算法.首先建立了大斜视双基地SAR回波信号模型,并对其距离徙动和方位调频率的变化特性进行了分析;在此基础上,将二维可分离成像算法推广应用到了双基地SAR领域,解决了大斜视双基地SAR回波信号线性距离徙动补偿后方位调频率在方位向出现的空变性问题.算法采用了由平移不变双基地SAR几何关系所确定的新匹配函数,并考虑了3次相位项对成像效果的影响,改善了成像的效果.最后通过点目标仿真验证了算法的有效性.     

双参数估计的改进自聚焦算法

传统自聚焦算法的多普勒调频率拟合模型中当最近斜距存在误差时,由于只考虑载机地速对多普勒调频率的影响,导致多普勒调频率拟合后会引入沿距离向变化的二次相位误差,该二次相位误差会严重影响高分辨率合成孔径雷达(SAR)成像的质量。为解决此问题,通过分析载机地速和最近斜距两个系统参数存在误差时对多频率调频率拟合的影响,提出了基于自聚焦算法结合最小二乘方法联合估计最近斜距和载机地速的双参数估计算法。该算法采用的估计模型相比于传统单参数估计算法更加符合真实的方位多普勒模型,在没有外定标的情况下可以很好地估计最近斜距误差。最后通过实测数据成像验证了该算法在高分辨率SAR成像中的有效性和适用性。     

基于多核DSP的高分辨距离像运动补偿算法实现

弹载毫米波步进频率雷达能 够得到目标具有较高距离分辨率的一维距离像,但当运动目标相对雷达具有径向速度时,会 使 回波信号产生严重的偏移与畸变,故必须进行运动速度补偿。本文首先分析了步进频率雷达 高分辨距离像成像原理与目标运动速度对成像的影响,然后论述了一种基于正负调频法与最 大脉组求和法的二次速度估计算法,并在TMS320C6678多核DSP平台上提出了该算法的一种并 行实现方案,同时描述了该实现方案的任务分配、调度与核间通信过程。最后在多核DSP平 台上对1 000帧回波信号做算法测试。测试结果表明,基于多核DSP的运动补偿算法具有较高 的精度和良好的实时性。     

一种基于曲率的边折叠简化算法

针对目前三角网格简化算法在低分辨率的状态下往往丢失模型重要几何特征,从而导致视觉上的失真问题,提出了一种改进的边折叠三角网格简化算法。在Garland算法基础上引入了近似曲率的概念,并将其加入到二次误差测度中,使得二次误差测度在能够度量距离偏差的情况下,能够反映模型局部表面几何变化。实验结果表明改进的算法有效保持了模型的细节特征,简化效果更好。     

一种基于回波数据的运动补偿方法

提出一种基于回波数据进行运动参数提取和运动补偿的方法。该方法首先从回波数据估计多个距离单元的多普勒调频率,再利用这些多普勒调频率估计值对运动参数进行加权最小二乘估计,最后利用估计的运动参数完成视线方向和沿航向运动误差的补偿。无人机SAR实测数据的实验结果表明,在运动误差较大的情况下,该方法仍可以取得良好的成像效果。     

斜视模式下基于Legendre展开的改进CS成像算法

在SAR的斜视模式下,针对传统的CS成像算法相位展开精度差的缺点,提出一种改进算法将Legendre展开应用其中。传统的成像算法主要对回波信号的二维频域的传递函数,还有算法流程中的三个相位校正函数应用Taylor多项式进行展开。将Legendre多项式取代Taylor多项式对算法进行处理,并进行了详细的推导,与传统的基于Taylor多项式进行展开的成像算法相比,减少了回波相位近似误差,相位展开精度得到了提高。最后从峰值旁瓣比和积分旁瓣比的角度评估了SAR图像的成像质量。理论推导和仿真结果都表明成像质量得到了进一步的提高。     

An exact wide field digital imaging algorithm

Abstract

A new imaging algorithm is presented for Synthetic Aperture Radar (SAR) that is exact in the sense that it is capable of producing a complex image with excellent geometrical, radiometrical and phase fidelity. No interpolations or significant approximations are required, yet the method accomplishes range curvature correction over the complete range swath. The key to the approach is a quadratic phase perturbation of the range linearly frequency modulated signals while in the range signal, azimuth frequency transform (Doppler) domain. Range curvature correction is completed by a phase multiply in the two-dimensional frequency domain. Other operations required are relatively conventional. The method is generalizable to imaging geometries encountered in squint and spotlight SAR, inverse SAR, seismics, sonar, and tomography.     

一种用于H.264的快速多块类型运动估计算法

针对H.264在多块运动估计中全搜索算法的计算复杂度高的问题,提出了一种基于图像的空间频率特性来预测运动估计的宏块划分的算法。实验结果表明,该算法在保证图像质量和码率的情况下显著地减少了运动估计计算复杂度的24%~53%。     

二维加窗全相位图像卷积滤波模板的设计

基于全相位数据空间的概念以及一种二维加窗全相位图像滤波模板的设计,给出了一修正的二维全相位图像卷积滤波模板设计方法。方法在用全相位概念设计滤波模板中充分考虑了用DFT和循环卷积表达线性卷积时的等价性条件,以确保待设计的全相位滤波器的频率响应特性与给定的原型滤波器的频率特性矩阵有更好的相关性和一致性。实验表明,该全相位滤波模板与已有的二维全相位图像滤波模板比较,有过渡带的宽度窄、滚降陡峭,阻带衰减小等特点,可以进一步有效消除方块效应,提高滤波处理后图像的质量。     

Leveraging polynomial approximation for non-linear image transformations in real time

Applying non-linear image transformations in real time remains a challenge for cost-sensitive embedded systems today. This paper offers a method to perform such transformations efficiently using polynomial approximations. It is based on a previously published method, but is not limited to transformations with little vertical variation, such as lens distortion removal. Instead, a wider range of image transformations is supported.     

Image segmentation using optimal and hierarchical piecewise-constant approximations

The problem of approaching optimal image approximations using quasi-optimal hierarchical approximations, which are close to the optimal ones in terms of the total squared error, is considered. The problem is solved using algorithms for merging and dividing pixel clusters. The algorithms are based on elementary formulas. Information encoded in the image is proposed to be described, based on quasi-optimal approximations, using the invariant isomorphic image representation. The definition of the integer quantity of information is given, and the comparison of the integer estimate with the classical estimates in terms of R. Hartley and K. Shannon is performed. The experimental study of the properties of quasi-optimal hierarchical approximations, which are characteristic and uncharacteristic of nonhierarchical optimal image approximations, is carried out.     

基于小波分解的快速分形图象压缩算法

为了解决传统分形压缩编码时间过长的问题 ,提出了一种基于小波分解的快速分形图象压缩算法 .该算法利用小波分解后各频带间能量分布不均衡的特性 ,将集中图象主要能量的低频域看作是一种图象 ,进行传统分形编码 ;然后利用小波分解后不同分辨率子带图象之间的相似性 ,用低频域的全搜索代替传统算法中整个区域的全搜索 ,将在低频域得到的分形参数进行适当的比例变换得到整幅图象的分形码 ,解码时 ,进行同传统算法相同的迭代过程 ,所得解码图象即为原图的逼近图象 .实验结果表明 ,该算法在压缩比不变 ,且基本不影响恢复图象质量的前提下 ,大大提高了编码速度 ,明显改善了分形图象压缩中编解码时间的不对称 .     

A novel image encryption algorithm based on piecewise linear chaotic maps and least squares approximation

In this paper, we propose a novel image encryption algorithm based on chaotic maps and least squares approximations. The proposed algorithm consists of two main phases, which are applied sequentially in several rounds, namely a shuffling phase and a masking phase. Both phases are based on 1–dimensional piecewise linear chaotic maps and act on the rows/columns of the input plain image. Least squares approximations are used to strengthen the security of the proposed algorithm by providing strong mixing between the rows/columns of the image. Simulation results show that the proposed image encryption algorithm is robust against common statistical and security attacks. We present thorough comparison of the proposed algorithm with some existing image encryption algorithms.

     

Frequency Optimized Computation Methods

In this paper we develop an alternative method to derive finite difference approximations of derivatives on arbitrary distrubutions of data points. The purpose is to find schemes which work for a broader range of frequencies than the usual approximations based on polynomial fitting to the expense of less accuracy for low frequencies. The numerical schemes are obtained as solutions to constrained optimizations problems in a weighted L ²-norm in the frequency domain. We examine the accuracy of these schemes and compare them with the standard approximations. To test the accuracy of the different schemes, we study dispersion errors for a simple wave equation in one space dimension. We examine the number of points per wave length which is needed in order for the relative error in the phase velocity to be below a certain bound. We also apply the technique to solve a simple two-dimensional hyperbolic equation.     

Efficient generation of discontinuity-preserving adaptive triangulations from range images

This paper presents an efficient technique for generating adaptive triangular meshes from range images. The algorithm consists of two stages. First, a user-defined number of points is adaptively sampled from the given range image. Those points are chosen by taking into account the surface shapes represented in the range image in such a way that points tend to group in areas of high curvature and to disperse in low-variation regions. This selection process is done through a noniterative, inherently parallel algorithm in order to gain efficiency. Once the image has been subsampled, the second stage applies a two and one half-dimensional Delaunay triangulation to obtain an initial triangular mesh. To favor the preservation of surface and orientation discontinuities (jump and crease edges) present in the original range image, the aforementioned triangular mesh is iteratively modified by applying an efficient edge flipping technique. Results with real range images show accurate triangular approximations of the given range images with low processing times.