2-Level-Wavelet-Based License Plate Edge Detection

Against well-known vertical Sobel operator adopted by most license plate detection (LPD) algorithms, this paper presents a robust and real-time method to enhance both edge density and intensity of license plates under various outdoor environments. The proposed method applies HL subband feature of 2D discrete wavelet transform (DWT) twice to significantly highlight the vertical edges of license plates and suppress the surrounding background noise. Then, license plates can easily be extracted and located by the orthogonal projection histogram analysis with local recursive Otsu segmentation and morphological closing operator. Experimental results show LPD system based on the proposed 2-level 2D Haar DWT is better than that based on conventional vertical Sobel operator or 1-level 2D Haar DWT in terms of the detection rate.