Detection of license plate using matched filter

Abstract

AbstractIn this paper the authors describe a license plate (LP) detection method that can eliminate the restrictions on LP position, three‐dimensional rotation and size. In the method, LP reference images, which replaced characters on the plate image by rectangles, are prepared beforehand. Using a frontal LP image, they are calculated by perspective projection at regular direction intervals and are calculated by a process of expanding and shrinking at regular size intervals. Using an input image of a vehicle passing through a scene and one reference image, a correlation map as a distribution of matching degree is obtained by a matched filter. By using the matched filter with every reference image, the correlation maps corresponding to the reference images are obtained. Detecting the highest value in all correlation maps, the matched reference image and the coordinates as LP position are decided. Furthermore, the shape of the LP in the input image is detected by fitting the LP frame, which is calculated using the matched reference image. In the paper, the proposed method is explained and an example of the experimental results is given. © 2009 Wiley Periodicals, Inc. Electron Comm Jpn, 91(9): 20–27, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10004