A sliding window technique for efficient license plate localization based on discrete wavelet transform

Real-time license plate recognition (LPR) is an interesting but complicated research topic. Some previous works use discrete wavelet transform (DWT) to extract license plate (LP), however, most of them are not capable of dealing with complex environments such as the low-contrast source images and the dynamic-range problems. In this paper, we propose an algorithm for license plate localization (LPL) based on DWT. The LP can be extracted from different quality of source images under complex environments by using two frequency subbands. We first use the HL subband to search the features of LP and then verify the features by checking whether in the LH subband there exists a horizontal line around the feature or not. The proposed method can extract both front and back LPs of various vehicles. The experimental results show that the proposed method can achieve good LPL results with both short run-time and high accurate detection rate.

Research highlights
► License plate localization (LPL) is the most important step in the license plate recognition (LPR).
► By employing discrete wavelet transform (DWT) instead of edge detection, the proposed algorithm achieves fast real-time LPL.
► The proposed method employs CMOS camera and does not restrict the distance between camera and car.
► The experimental results show that the proposed algorithm can quickly detect the location of LP in different environments with high accuracy.