206 An Offline Technique for Localization of License Plates for Indian Commercial Vehicles

Satadal Saha 1, Subhadip Basu 2, Mita Nasipuri 2, Dipak Kumar Basu # 2

AICTE Emeritus Fellow

1 CSE Department, MCKV Institute of Engineering, Howrah, India 2 CSE Department, Jadavpur University, Kolkata, India

Abstract—Automatic License Plate Recognition (ALPR) is a challenging area of research due to its importance to variety of commercial applications. The overall problem may be subdivided into two key modules, firstly, localization of license plates from vehicle images, and secondly, optical character recognition of extracted license plates. In the current work, we have concentrated on the first part of the problem, i.e., localization of license plate regions from Indian commercial vehicles as a significant step towards development of a complete ALPR system for Indian vehicles. The technique is based on color based segmentation of vehicle images and identification of potential license plate regions. True license plates are finally localized based on four spatial and horizontal contrast features. The technique successfully localizes the actual license plates in 73.4% images. I. INTRODUCTION Automatic License Plate Recognition from vehicle images has long been an active area for the researchers. In general, objective of such systems is to localize the license plate region(s) from the vehicle images, captured through a road- side camera, and interpret them using an Optical Character Recognition (OCR) system.
ALPR systems are widely implemented for automatic ticketing of vehicles at car parking facilities, tracking vehicles during traffic signal violations and related applications with huge saving of human energy and cost. Any ALPR system may be broadly categorized into two types, namely, an online ALPR system and an offline ALPR system. In an online ALPR system, the localization and interpretation of license plates take place instantaneously from the incoming video frames, enabling real-time tracking of moving vehicles through the surveillance camera. An offline ALPR system, in contrast, captures the vehicle images and stores them in a centralized data server for further processing, i.e. , for interpretation of vehicle license plates. The current work, discussed in this paper, comes under the later category of solutions. Various techniques have been developed recently for the purpose for efficient detection of license plate regions from offline vehicular images. Most of these works [1-4] concentrate on localizing standardized license plate regions using edge based features. Some of these works [2, 5, 6] use the image of a vehicle, well placed in front of a camera, to get a clear view of the license plate. But in the practical scenario, there may be multiple vehicles of different types in a single scene along with partial occlusions of the license plates from other objects. In one of the earlier works [1], Rank filter is used for localization of license plate regions giving bad result for skewed license plates. An analysis of Swedish license plate is done in [2] using vertical edge detection followed by binarisation. This does not give better result for non-uniformly illuminated plates. An exhaustive study of plate recognition is done in [3] for different European countries. In Greece the license plate uses shining plate. The bright white background is used as a characteristic for license plate in [4]. Spanish license plate is recognized in [5] using Sobel edge detection operator. It also uses the aspect ratio and distance of the plate from the center of the image as characteristics. But it is constrained for single line license plates. During the localization phase the position of the characters is used in [6]. It assumes that no significant edge lies near the license plate and characters are disjoint.
In the developed countries and in most of the developing countries the attributes of the license plates are strictly maintained. For example, the size of the plate, color of the plate, font face / size / color of each character, spacing between subsequent characters, the number of lines in the license plate, script etc. are maintained very specifically. Some of the images of standard license plates, used in developed countries, are shown in Fig 1 (a). However, in India, the license plates are not yet standardized across different states, making localization and subsequent recognition of license plates extremely difficult. Moreover, in India license plates are often written in multiple scripts. Fig. 1(b) shows some of the typical Indian license plates with variations in shape, size, script etc. This large diversity in the features of the license plate makes its localization a challenging problem for the research community.

(a)

(b) Fig. 1. Lic

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