Research Of License Plate Location Algorithm

License plate recognition(LPR) is a special computer vision system, which can recognize the license plate automatically from the video。The study of LPR is related to many kinds of technologies such as digital image processing, computer vision, pattern recognition and artificial intelligence. Its key techniques include license plate location, character segmentation and recognition. License plate location, which is to locate and segment the License Plate from a vehicle image, providing the inputting information for the next processes, is the basis of LPR system.Based on the detail analysis of the typical LPR system over the world and the characteristics of Chinese license plate, three different algorithms of license plate location are presented in this paper:①.< The application of Top-hat Transform in Multi-license Plate Localization >: Combining with the texture feature of license plates and the characteristics of Top-hat transform, the Top-hat coefficients are retained. Then the license plate can be located through smoothing, vertical direction information, four connected region detection, and density calculation and so on. The proposed algorithm was suitable for the single-license and multi-license plate detection in various illumination conditions with high accuracy.②.:In this chapter a new algorithm which can improve the accuracy of vehicle license plate location using dyadic wavelet coefficients in vertical direction is presented. The performance of the proposed algorithm has been tested on monitoring images obtained from the practical application. Experimental results show that our algorithm has better performance than Mallat's on location precision.③.: A new algorithm for license plate localization using the discrete cosines transform (DCT) coefficients is proposed in this section. According to the characteristic of DCT, six AC coefficients of an 8*8 block were selected to represent the vertical and diagonal textures of a block. Then following the next processes, the license plate would have been located at last.These three algorithms having their own advantages were suitable for the different LPR systems respectively. The algorithm based on Top-hat was easily realized and suitable for the real time LPR system. The algorithms based on dyadic wavelet transform performed best, but was complicated and time-consuming. So it was suitable for the background system. The algorithm based compressed domain using DCT coefficients in JPEG files to capture the license plate enhanced localization-efficiency rapidly. Although its effect was not as good as the two algorithms mentioned before, it was better than the traditional methods.