Automated Sub-Pixel License Plate Localization via Mathematical Morphology and Wavelet Analysis

International Journal of Computer Science and Technology | Vol. 13, No. 8, August 2011 | pp. 50–56
DOI: 10.46882/2011/IJCST/000114

Research Article

Title: Automated Sub-Pixel License Plate Localization via Mathematical Morphology and Wavelet Analysis

Names of Authors: Hans-Werner Schmidt¹, Katarzyna J. Kowalski²

Authors’ Affiliations: ¹Institute for Pattern Recognition, Technical University of Vienna, Vienna, Austria; ²Department of Computer Vision, Warsaw University of Technology, Warsaw, Poland

Abstract: Automatic License Plate Recognition (ALPR) remains a key component of modern electronic toll collection, parking access management, and traffic monitoring architectures. Classic localization routines struggle when analyzing video frames captured under adverse illumination conditions, variable camera viewpoints, or complex background clutter. This article details a high-precision Automated Sub-Pixel License Plate Localization (ASPL) algorithm that merges mathematical morphology operators with discrete wavelet transforms (DWT). The proposed method applies vertical morphological edge enhancement to separate license plate text clusters from vehicle bodies before isolating the high-frequency sub-bands of the image via DWT. Sub-pixel interpolation is then performed to resolve blurry borders caused by high-velocity vehicle movement. Evaluations using an empirical dataset of 1,500 outdoor traffic images yielded a 96.8% accurate localization success rate across diverse environmental profiles. The system successfully managed variations in perspective tilt up to ±15°, preserving an average execution speed of 38 ms per frame on a standard 2.4 GHz workstation. This framework offers an industrial pipeline for real-time video enforcement architectures.

Keywords: Image Processing, License Plate Recognition, Mathematical Morphology, Discrete Wavelet Transform, Edge Detection

Manuscript Timeline: Received May 22, 2011; Revised July 05, 2011; Accepted July 20, 2011; Published August 24, 2011.

Citation: Schmidt, H. W., & Kowalski, K. J. (2011). Automated Sub-Pixel License Plate Localization via Mathematical Morphology and Wavelet Analysis. International Journal of Computer Science and Technology, 13(8), 50–56.

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