Optical recognition of motor vehicle license plates

A system for the recognition of car license plates is presented. The aim of the system is to read automatically the Italian license number of a car passing through a tollgate. A CCTV camera and a frame grabber card are used to acquire a rear-view image of the vehicle. The recognition process consists of three main phases. First, a segmentation phase locates the license plate within the image. Then, a procedure based upon feature projection estimates some image parameters needed to normalize the license plate characters. Finally, the character recognizer extracts some feature points and uses template matching operators to get a robust solution under multiple acquisition conditions. A test has been done on more than three thousand real images acquired under different weather and illumination conditions, thus obtaining a recognition rate close to 91%     

复杂光照下的车牌图像二值化新方法

为解决车牌识别系统中,车牌区域由于受到光照过度、不足和不均等复杂光照的影响,使得车牌图像的二值化效果不理想的问题,文中提出一种复杂光照车牌图像的二值化新方法。根据国内车牌颜色特点,重新分配R、G、B颜色分量的权重来构造新灰度图,并设计不同颜色车牌灰度图转化的规则。采用全局和局部阈值相结合的双阈值方法实现车牌图像二值化,同时利用空域同态滤波进一步提高图像对光照的鲁棒性。实验结果表明,文中方法运算时间仅为传统方法的61.95%,同时能更有效地克服不均匀光照对车牌图像二值化的干扰,使二值图像的汉字和字符更清晰,鲁棒性更好。     

Real‐Time License Plate Detection in High‐Resolution Videos Using Fastest Available Cascade Classifier and Core Patterns

We present a novel method for real‐time automatic license plate detection in high‐resolution videos. Although there have been extensive studies of license plate detection since the 1970s, the suggested approaches resulting from such studies have difficulties in processing high‐resolution imagery in real‐time. Herein, we propose a novel cascade structure, the fastest classifier available, by rejecting false positives most efficiently. Furthermore, we train the classifier using the core patterns of various types of license plates, improving both the computation load and the accuracy of license plate detection. To show its superiority, our approach is compared with other state‐of‐the‐art approaches. In addition, we collected 20,000 images including license plates from real traffic scenes for comprehensive experiments. The results show that our proposed approach significantly reduces the computational load in comparison to the other state‐of‐the‐art approaches, with comparable performance accuracy.     

基于空间转换网络的端到端车牌检测与识别

在复杂场景中,许多现有的车牌检测和识别方面 的研究方法存在数据集单一且有限、算法复杂等问题。因此提出了一个端到端的统一网络: 残差-空间变换-连接时序分类融合的 车辆号牌检测识别网络(LPDR-RSCNet)。该网络结合残差神经网络、空间变压器网络和连 接主义者时间分类,联合训练检测和识别模块,以减少中间错误积累。通过在残差神经网络 提取特征过程中引入空间变换网络,使特征提取器具有平移不变性、旋转不变性和缩放不变 性;在分类器引入连接时序分类,可以自动识别图片标签和特征之间的关系。同时,还可以 适应可变长度序列的识别。在中国城市停车场数据集(CCPD)上进行了比较实验,CCPD是一 个大规模、多样的中文车牌数据集。实验证明LPDR-RSCNet模型在实际应用中可实现98.8％ 的识别精度和34 fps的速度,并且相较于YOLO9000、Faster-RCNN、SSD300, 具有更好的检测准确度,可满足智能交通系统中对移动车辆实时车牌检测和识别的要求。     

Temporal matching prior network for vehicle license plate detection and recognition in videos

In real‐world intelligent transportation systems, accuracy in vehicle license plate detection and recognition is considered quite critical. Many algorithms have been proposed for still images, but their accuracy on actual videos is not satisfactory. This stems from several problematic conditions in videos, such as vehicle motion blur, variety in viewpoints, outliers, and the lack of publicly available video datasets. In this study, we focus on these challenges and propose a license plate detection and recognition scheme for videos based on a temporal matching prior network. Specifically, to improve the robustness of detection and recognition accuracy in the presence of motion blur and outliers, forward and bidirectional matching priors between consecutive frames are properly combined with layer structures specifically designed for plate detection. We also built our own video dataset for the deep training of the proposed network. During network training, we perform data augmentation based on image rotation to increase robustness regarding the various viewpoints in videos.     

基于注意力机制的车牌快速检测方法研究

针对光照、车辆密集和低分辨率等复杂场景下车牌定位困难、检测速度慢和准确率 低等问题,提出了一种基于注意力机制的车牌快速检测方法。首先,综合车牌的特征,设计 了轻量级网络单元LeanNet,并使用该单元构建一种计算量低且精准的骨干网络。其次,设 计了MLA(muti-scale light attention)模块,用于引导网络关注不同尺度的车牌,生成 基 于车牌的局部显著图,抑制背景噪声。最后,设计了一个四尺度预测网络,其中的FSPF(fo ur scale pyramid fusion)模块能够生成四尺度特征金字塔,有利于实现不同尺度车牌的检测 。 实验结果表明,本文方法在CCPD(Chinese city parking dataset)数据集中的准确识别率 为 99.12%,与最新的YOLOv4(you only look once v4)检测方法相比,准确率提高了1.9%,运行速度提高了6倍,能 够在嵌入式设备中实现复杂场景下的车牌检测。     

基于改进BP神经网络的车牌字符识别

在分析了BP网络学习算法的缺陷的基础上引入动量项和遗传算法对BP网络学习算法进行改进,大大提高了BP网络的收敛速度.对车牌字符图像进行分割并利用sobel算子进行边缘检测来提取字符特征.然后利用改进的BP网络来自动识别车牌字符,提高了识别的速度和正确率.     

雾霾环境下基于PLATE-YOLO的车牌检测方法

针对目前车牌识别领域中，雾霾环境下车牌检测准确率低的问题，本文提出一种基于深度学习的抗雾霾车牌检测方法，该方法能够检测民用车牌和机场民航车辆车牌。该方法首先利用一种基于卷积神经网络的去雾算法对车牌图片进行去雾预处理，然后将处理过的无雾霾图片送入PLATE-YOLO网络中检测车牌的位置。该PLATE-YOLO网络是本文针对车牌检测的特点，对YOLOv3网络做了修改后得到的适用于车牌检测的网络。主要改进点有两处:第一，提出了一种基于层次聚类算法的锚盒(anchor box)个数和初始簇中心的计算方法；第二，针对车牌目标较大的特点，对网络的多尺度特征融合做了优化。优化后的PLATE-YOLO网络更适合于车牌检测，且提高了检测速度。实验证明，PLATE-YOLO网络检测车牌的速度较YOLOv3提高了5 FPS；在雾霾环境下，经去雾预处理的 PLATE-YOLO车牌检测方法比未经去雾处理的车牌检测方法准确率提高了9.2%。      

Finding suits in images of people in unconstrained environments

Clothing style analysis is a critical step for understanding images of people. To automatically identify the style of clothing that people wear is a challenging task due to various poses of person and large variations for even the same clothing category. Suit as one of the clothing style is a key element in many important activities. In this paper, we propose a novel suits detection method for images of people in unconstrained environments. In order to cope with various human poses, human pose estimation is incorporated. By analyzing the style of clothing, we propose the color features, shape features and statistical features for suits detection. Experiments with four popular classifiers have been conducted to demonstrate that the proposed features are effective and robust. Comparative experiments with Bag of Words (BoW) method demonstrate that the proposed features are superior to BoW which is a popular method for object detection. The proposed method has achieved promising performance over our dataset, which is a challenging web image set with various human poses and diverse styles of clothing.     

多特征级联筛选的高鲁棒车牌检测

现有的车牌检测算法在车牌较模糊时往往难于取得很好的检测效果.针对监控图像的特点,首先提取清晰和模糊车牌所共有的归一化梯度特征,进行初步车牌检测;然后结合车牌区域的颜色直方图特征,进行级联筛选、去除非车牌样本,得到一种高鲁棒的车牌检测方法.基于真实监控图像的实验结果表明,此方法具有较高的稳定性和鲁棒性,尤其对模糊车牌具有明显优于已有方法的召回率.     

采用二次定位的车牌图像定位算法研究

提出了一种二次定位的车牌图像定位方法.在第一次定位中首先对车辆图像进行灰度化、边缘检测及直线检测处理,根据车牌区域的特征初步找出包含车牌边框的车牌图像区域;再根据车牌边框对车牌图像进行倾斜校正;在此基础上,对车牌图像采用投影法进行二次定位,最终获得精确的车牌区域.测试证明,提出的二次定位方法能够适应不同背景,对光照、环境及车牌种类不敏感,得到的车牌图像不会包含车牌边框等无用的信息,为后续的车牌字符识别打下良好的基础.     

复杂环境下基于ROI的车牌提取算法

感兴趣区域（ROI）是最能体现图像内容的区域,基于ROI的图像特征提取技术有效提高了图像处理和分析的效率,在图像处理与分析领域有着重要的应用.首先简要介绍现有的车牌提取方法,针对目前复杂环境下的车牌ROI提取算法提取效果不明显、计算繁琐、漏检率高等缺点,提出一种采用颜色特征和模板匹配的车牌ROI提取算法,该算法避免大量运算,具有相对于图像平移、尺度变化的低敏感性,并且取得了较好的实验效果.     

分块思想在汽车牌照粗定位中的应用

分块思想,是相对于模板匹配而言的一种图像处理思想。它是采用相对静止的方式进行图像匹配,首先根据待匹配图像的某些特征选择块的尺寸,对整幅的图进行分块,利用某些特征值来进行匹配。在以往的车牌定位算法中,有很多文献提出利用模板匹配方法进行车牌定位的思想,但是由于模板匹配方法本身存在计算量大的缺点,直接明显地影响到车牌定位速度,无法满足车牌照自动识别系统的实时性要求,所以本文提出把分块思想应用到车牌照粗定位过程中,利用车牌区域字符自身的一些纹理特征,采用固定分块划分出若干大小相同的特征区域,在各区域内通过水平差分突出垂直方向纹理特征,寻找水平差分累加值最大的区域即为车牌的大致位置,这样就大大提高了车牌定位速度,同时也为后续的车牌精确定位奠定了基础。     

基于边缘检测的车牌识别算法的实现

针对传统车牌识别的不足,本文提出了基于边缘检测的车牌识别的算法.该算法首先对摄像头获取的车牌图像预处理,去除图像无用信息,然后运用Robert算子检测车牌边缘,并对车牌区域进行图像较正,用高斯滤波法去除噪声并且提取车牌信息特征,接着对车牌区域水平和竖直方向运用触点定位法分割字符,对车牌分割后与相应字符模版匹配,利用预测模型预测识别结果,最后识别出车牌字符.     

Multinational vehicle license plate detection in complex backgrounds

Many methods for multinational License Plate Detection (LPD) have been proposed in recent times but most of them are not sophisticated enough to handle complex backgrounds. Moreover, their ability to handle various environmental and illumination conditions has been limited and still needs improvement. In this paper, we propose a novel technique to detect license plates of vehicles regardless of their color, size, and content. As the rear vehicle lights are an essential part of any vehicle, we reduce the image processing area to eliminate the complex background by detecting the rear-lights as the license plates are in a certain range of these lights. Heuristic Energy Map (HEM) of the vertical edge information in the Region of Interest (ROI) is calculated and area with the dense edges is selected using a unique histogram approach which is considered to be the license plate. The proposed algorithm is tested on 855 images from various countries including China, Pakistan, Serbia, Italy and various states of America. Experimental results show that the proposed method is able to detect license plates 90.4% of times despite of complex backgrounds in 0.25 s on average that can achieve real time performance.     

Automatic license plate detection in hazardous condition

Automatic detection of license plate (LP) is to localize a license plate region from an image without human involvement. So far a number of methods have been introduced for automatic license plate detection (ALPD), but most of them do not consider various hazardous image conditions that exist in many real driving situations. Hazardous image condition means an image can have rainy or foggy weather effects, low contrast environments, objects similar to LP in the background, and horizontally tilted LP area. All these issues create challenges in developing effective ALPD method. In this paper, we propose a new ALPD method which effectively detects LP area from an image in the hazardous conditions. For rain removal we apply a novel method that uses frequency domain mask to filter rain streaks from an image. A new contrast enhancement method with a statistical binarization approach is introduced in the proposed ALPD for handling low contrast indoor, night, blurry and foggy images. For correcting tilted LP, we apply Radon transform based tilt correction method for the first time. To filter non-LP regions, a new condition is used which is based on image entropy. We test the proposed ALPD method on 850 car images having different hazardous conditions, and achieve satisfactory results in LP detection.     

基于边缘颜色分布的车牌定位新方法

根据车牌的综合特征,提出了一种新的基于边缘颜色分布的车牌定位算法.该算法抓住了车牌背景与字符具有固定颜色搭配的重要特点,利用车牌区域内特有的边缘颜色分布信息并结合车牌的纹理特征,有效地滤除了大量的背景和噪声边缘,然后利用车牌的结构特征和边缘信息,并结合形态滤波的方法,以进一步确定车牌区域.实验结果表明,该算法定位准确率高、鲁棒性好,而且适用于对复杂背景下的多车牌图像进行分割.     

Local consistent hierarchical Hough Match for image re-ranking

Geometric image re-ranking is a widely adopted phrase to refine the large-scale image retrieval systems built based upon popular paradigms such as Bag-of-Words (BoW) model. Its main idea can be treated as a sort of geometric verification targeting at reordering the initial returning list by previous similarity ranking metrics, e.g. Cosine distance over the BoW vectors between query image and reference ones. In the literature, to guarantee the re-ranking accuracy, most existing schemes requires the initial retrieval to be conducted by using a large vocabulary (codebook), corresponding to a high-dimensional BoW vector. However, in many emerging applications such as mobile visual search and massive-scale retrieval, the retrieval has to be conducted by using a compact BoW vector to accomplish the memory or time requirement. In these scenarios, the traditional re-ranking paradigms are questionable and new algorithms are urgently demanded. In this paper, we propose an accurate yet efficient image re-ranking algorithm specific for small vocabulary in aforementioned scenarios. Our idea is inspired by Hough Voting in the transformation space, where votes come from local feature matches. Most notably, this geometry re-ranking can easily been aggregated to the cutting-edge image based retrieval systems yielding superior performance with a small vocabulary and being able to store in mobile end facilitating mobile visual search systems. We further prove that its time complexity is linear in terms of the re-ranking instance, which is a significant advantage over the existing scheme. In terms of mean Average Precision, we show that its performance is comparable or in some cases better than the state-of-the-art re-ranking schemes.     

复杂背景下的车牌自动分割方法

车牌分割是汽车牌照自动识别的基础。本文针对含有车牌的灰度图像，利用车牌字符具有明显垂直纹理的特征．提出了一种适合字符纹理特征的边缘检测方法，获取垂直边缘图。根据车牌固有的特征，运用改进的投影算法确定车牌区域。文中详细给出了车牌分割步骤。实验结果表明，在复杂的背景下，该方法能够快速有效地实现车牌自动分割。