Automatic car license plate extraction using modified generalized symmetry transform and image warping

This paper proposes a method which evaluates symmetry of plate corners and extracts car license plates captured from the arbitrary directions. The generalized symmetry transform (GST) produces continuous features of symmetry between two points by combining locality constraint and reflectional symmetry. Also, we propose scan line based GST to improve time complexity of the GST significantly due to selective attention to particular distortions in case of a large searching window. The corners of a plate are detected from our modified scan line based GST with selective directions using the fact that a corner is formed by two neighboring straight lines acquired in a symmetry map of GST. However, a rotated or perspectively distorted car license plate image is very difficult for segmentation of interior characters. Image normalization by image warping is adopted to make such segmentation of license plate and later identification much easier. We also adopt the verifier which evaluates a candidate license plate to enhance extraction rate. Our experiments show that the proposed method for extracting perspectively distorted license plates is fairly reliable.