A rank filter method for localizing non-uniform, 2-column Indian license plate with unequal plate and character dimensions

The first and the most important stage for any ALPR system is the correct localization of the license plate within the image captured by the LPR camera. Most of the varieties of techniques reported for localization of license plate are applicable for a very controlled environment. This work focuses on localizing the license plate regions from true color still snapshots captured in a very realistic situation. The technique is based on a novel multi-stage approach for analysis of vertical edge gradients from contrast stretched gray-scale images. Horizontally and vertically oriented rank filters are used to detect clusters of high density of bright edges in the area of the number plate. The proposed approach embeds multi resolution and multi scale edge detection, adaptive searching, color analysis, and affine rectification in a hierarchical frame work for sign detection, with different emphasis at each phase to handle the text in different sizes, orientations, color distributions and backgrounds.