模糊车牌、污迹车牌和多车牌的定位与校正
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摘要
车牌识别技术是智能交通系统中的关键技术,而车牌的定位与校正又是车牌识别技术的主要步骤之一,复杂情况下车牌的定位与校正更是其中的难点。本文在介绍复杂情况下车牌识别的国内外研究概况的基础上,研究了一些新的处理方法,主要工作如下:
     1、提出了基于混沌粒子群和Contourlet域正则化的图像复原算法。Contourlet变换是一种新的多尺度几何变换方法,建立Contourlet域中的正则化模型,引入混沌粒子群算法选取正则化参数,利用其快速准确的搜索能力,找出泛函的极值解。实验结果表明,该方法具有更好的视觉效果,且峰值信噪比更高。
     2、研究了一种基于对偶树复小波变换的的污迹车牌图像修复方法。将原有的小波方法扩展到复数域,使其具有了优良的平移不变性、方向选择性以及可精确的重构性;然后利用Criminisi算法思想,通过确定像素优先级、搜索最佳匹配块填充待修复区域,再进行小波逆变换得到修复后的图像。结果表明该方法的峰值信噪比高于传统小波方法。
     3、给出了一种基于改进Sobel边缘检测算子和数学形态学的多车牌定位方法。对输入图像先用改进的Sobel算子进行边缘检测,接着再生成边缘密度图,对边缘密度图二值化并进行有效的非线性滤波,然后用数学形态学方法做膨胀处理,通过连通域分析找到目标的外接矩形,最后根据车牌的几何特征确定最终的车牌区域。结果显示该方法有更高的检出率。
     4、实现了基于Tent映射混沌粒子群和车牌纹理特征相结合的车牌精确定位算法。采用两种不同的方法进行车牌纹理特征的提取:①用三组不同的一维滤波器提取车牌的纹理特征,引入车牌纹理的一致性度量作为判决条件,构造一种能够准确反映车牌区域的特征向量;②根据车牌区域的重点特征,提取7种能够准确反映车牌区域的特征向量,构成整个车牌的特征矢量;然后利用Tent映射的混沌粒子群算法,并结合车牌特征矢量,搜索车牌区域的最佳定位参量。实验结果表明该方法误检率更低且运算时间较短。
     5、给出了一种基于混沌粒子群和字符旋转投影的车牌图像倾斜校正方法。分别将车牌字符区域向水平和垂直方向进行投影,利用混沌粒子群优化算法搜索投影的最小距离,从而获得水平错切角度以及垂直倾斜角度,最后按照该角度分别进行旋转得到校正后的车牌图像。结果表明该方法收敛精度更高且运行时间较短。
The vehicle plate recognition technology is the key of intelligent transportation system, and plate position and correction is the main step, which is a challenging task. After the introduction of current development of vehicle plate recognition in condition of complex background, researches on the new methods are included in this paper. All the work of the paper is described as follows:
     Firstly, an image restoration method based on chaotic particle swarm optimization and regularized parameter by contourlet transform is realized. Contourlet transform is a new multi-scale geometric distortion. Then the regularization model based on contourlet-domain is established. Chaotic particle swarm optimization is adopted to choose the optimal regularized parameter. At last, the extrmal solution of function is found by making use of the fast and exact ability of chaotic particle swarm optimization.The results show that this method has better visual effects and higher peak signal-to-noise ratio(PSNR).
     Secondly, an image inpainting method of stain vehicle plate based on dual--tree complex wavelet transform is given. It expands primary wavelet transform to plural-domain, which has the good shift invariance, directional selectivity and perfect reconstruction. Then this method confirms the pixel priority and search the optimal matching patch to fill in the inpainting area, which is based on Criminisi idea. At last, the inpainting image is got by wavelet inverse transform. The results show that this method has higher PSNR than primary wavelet transform.
     Thirdly, a multi-vehicle plate position method based on improved Sobel operator and morphology is given. The image edge is detected by the improved Sobel operator, and then the edge density chart is generated. Afterward, it is binaryzationed and filtered based on non-linear. Then, the image is dilated by morphology method. The external rectangle of the object is found by connected component analyse, and the final vehicle plate is confirmed areas based on the geometric character of it. The experimental results show that the method has higher relevance ratio.
     Fourthly, an accurate license plate location method based on Tent chaotic particle swarm optimization and texture features is realized. It adapts two different methods to extract the vehicle plate texture features: (1) get the texture feature vector by three one-dimensional filters, and construct the fitness function by leading the texture coherence into the judgment, which could stand for the plate area accurately; (2) get the texture feature vector by seven different vectors and construct the fitness function based on the important characters of plate area. Then the optimal character parameter with tent chaotic particle swarm optimization by combining the texture features of plate area is found. The results show that this method has lower miss ratio and shorter running time.
     Fifthly, a vehicle plate tilt correction method based on chaotic particle swarm optimization and character rotation projection is proposed. The projection on horizontal and vertical coordinate is used on character region of the rotated vehicle license plate. Then the projection minimum distance is searched by chaotic particle swarm optimization, which is used to get the optimal vertical tilt and horizontal sheer angle. At last, it rotates the image with the anle to get the corrected result. The results show that this method has higher constringent precision and shorter running time.
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