A flexible method for recognizing four-digit numbers on a license-plate in a video scene

In order to realize a simple system for monitoring car movements, a flexible method was developed for recognizing four-digit numbers on a license-plate in a video scene. The method consisted of three parts; global search for finding a license plate in a scene, corner detection for removing shape deformation, and numeral character recognition. The spatial features and the spectral (color) ones were merged and processed at a time in the global search. The Hough transformation was adopted for selecting sides of the license-plate among various edges in the following corner detection. The corner points found as cross points of the neighboring sides were used for removing the shape deformation. The four-digit numbers were recognized by using a virtual pixel algorithm with centroid compensation. The global search achieved 97 % of correct detection rate for 100 sets of scenes. The corner detection achieved 65 % (39 of 65 scenes) of correct detection rate, so far. The four-digit numbers on the license-plates, whose corners were correctly detected, were recognized perfectly.