Detection of license plate using matched filter

In this paper, the authors describe a license plate (LP) detection method which can exclude a restriction 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 processing of expanding and shrinking at regular size intervals. Using an input image of vehicle passing scene and one reference image, a correlation map as a distribution of matching degree is obtained by matched filter. By 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 coordinate as LP position are decided. Furthermore, a shape of LP in the input image is detected by fitting of LP frame which is calculated using the matched reference image. In the paper, the proposal method is explained and an example of the experimental results is shown.