【正文】 20060401/*中文學(xué)位論文*/[20] 安勇 車(chē)牌識(shí)別算法研究[碩士學(xué)位論文] 中國(guó)科學(xué)技術(shù)大學(xué) 20050301/*中文學(xué)位論文*/[21] 王璐 基于MATLAB的車(chē)牌識(shí)別系統(tǒng)研究[碩士學(xué)位論文] 上海交通大學(xué) 20090107 /*中文學(xué)位論文*/ 致　謝首先我要感謝我的導(dǎo)師，在畢業(yè)設(shè)計(jì)的過(guò)程中給予的幫助，還有同組同學(xué)在畢業(yè)設(shè)計(jì)資料選取，編碼過(guò)程中的幫助同時(shí)，在畢業(yè)設(shè)計(jì)過(guò)程中，還得到了同專(zhuān)業(yè)同學(xué)的許多幫助，在此表示感謝最后，向所有曾給予作者關(guān)心和幫助的老師和同學(xué)們?cè)俅伪硎咀钪孕牡母兄x，向參加論文評(píng)審、答辯的專(zhuān)家和老師表示衷心的感謝和崇高的敬意。fr233。 as a result, the scale factors and the transform order offer additional keys. On the other hand, note that the phasemask serves as the key of the system, enlarging the key space can be achieved by encoding the target image into two or more phase masks with a modified POCS algorithm. Chang et al have proposed a multiplephases retrieval algorithm and demonstrated that an optical security system based on it has higher level of security and higher quality for the decrypted image. However, this algorithm retrieves only one phasedistribution with a phase constraint in each iteration. As a result, the masks are not so consistent and may affect the quality of the recovered image. In the present paper, we propose a modified POCS algorithm that adjusts the distributions of both phasemasks synchronously in each iteration. As a result, the convergent speed of the iteration process is expected to significantly increase. And the target image with much higher quality is expected to recover because of the coadjusting of the two masks during the iteration process. When the iteration process is finished, the target image is encoded into the phasemasks successfully. Each of these masks severs as the key of the security system and part of the encrypted image itself as well. Moreover, the algorithm can be extended to generate multiple phasemasks for arbitrary stages correlator. To acquire the maximum security, each key is assigned to different authority so that the decryption cannot be performed but being authorized by all of them. This keyassignment scheme is especially useful for military and government applications. The algorithm description is presented in Section 2. Computer simulation of this algorithm and the corresponding discuss are presented in Section 3. 2. Cascaded Iterative Fourier Transform (CIFT) Algorithm Consider the operation of the encryption system with the help of a 4f correlator as shown in , the phase masks placed in the input and the Fourier planes are denoted asand respectively, where (x, y) and (u, v) represent the space and the frequency coordinate, respectively. Once the system is illuminated with a monochromatic plane wave, a target image f(x,y)(an image to be decrypted or verified) is expected to obtain at the output plane. The phasemasksand contain the information of f(x,y), that is,f(x,y)is encoded into these phasemasks. The encoding process is the optimization of the two phasedistributions. It is somewhat similar with the problems of the image reconstruction and the phase retrieval, which can be solved with the POCS algorithm. However, the present problem es down to the phase retrieval in three (or more, in general) planes along the propagation direction. So the conventional POCS algorithm should be modified for this application. The cascaded iteration Fourier transform (CIFT) algorithm begins with the initialization of the phasedistributions of the masks. Suppose the iteration process reaches the kth iteration (k = 1, 2, 3, …), and the phasedistributions in the input and the Fourier plane are represented as and , respectively. Then an estimation of the target image is obtained at the output of the correlator defined by where FT and IFT denote the Fourier transform and the inverse Fourier transform, fk(x,y)satisfies the convergent criterion, the iteration process stops, and and are the optimized distributions. Otherwise, the fk(x,y) is modified to satisfy the target image constraint as follows Then the modified function is transformed backward to generate both of the phasedistributions as follows where ang{ ] denotes the mean of the image. The convergent behavior of this algorithm is similar to that of the conventional POCS. That is, the MSE reduces rapidly in the foremost few iterations, then it keeps reducing slowly till it reaches the minimum. Correspondingly, the correlation coefficient is expected to increase rapidly at first and keep increasing slowly till the stopping criterion is satisfied. In decryption, the determined phasemasks and (the keys or essentially, the encrypted images) are placed in the input and the Fourier plane, respectively, and then transformed into the output plane through the correlation defined by Eq. (1). The modulus of the output is the decrypted image. The CIFT algorithm retains the property of the conventional iteration algorithm, that is, the final phasedistributions of the masks are determined by the initializations of them. Therefore different initializations will result in different distributions of and . The target image cannot be decrypted if the keys mismatch (that is, the keys were generated from the different iteration process). In practical system, the phases of the masks are quantized to finite levels, which might reduce the solution space and introduce noise to the recovered image. To pensate the loss of the quality, the target image can be encoded into more phasemasks to provide additional freedom for solutions searching, which means to encrypt the image with a multistages (cascaded) correlator. From the point of view of security, this strategy significantly enlarges the key space (because more keys were generated), and makes the intrusion more difficult. Generally, the tstages correlation is defined asfor t is even, or for t is odd, where the matrix I(x, y) represents the input plane wave, and the superscript i (i=1, 2, …, t) denotes the serial number of the masks in the system. The phasedistributions of these masks may be deduced by analogous analysis for Eq. 3. 3. Computer simulation In this section we numerically demonstrate our general jet plane image of the size 128 18