Reading Car License Plates Using Deep Convolutional Neural Networks and LSTMs

TL;DR

This paper presents a deep learning cascade framework combining CNNs and LSTMs for effective, segmentation-free license plate detection and recognition in natural scenes, achieving high accuracy and robustness.

Contribution

It introduces a novel cascade approach with high-level CNN features for detection and an LSTM-based sequence labeling method for recognition, improving over traditional segmentation-based methods.

Findings

High recall and precision in license plate detection.

State-of-the-art accuracy in license plate recognition.

Segmentation-free approach reduces errors.

Abstract

In this work, we tackle the problem of car license plate detection and recognition in natural scene images. Inspired by the success of deep neural networks (DNNs) in various vision applications, here we leverage DNNs to learn high-level features in a cascade framework, which lead to improved performance on both detection and recognition. Firstly, we train a $37$-class convolutional neural network (CNN) to detect all characters in an image, which results in a high recall, compared with conventional approaches such as training a binary text/non-text classifier. False positives are then eliminated by the second plate/non-plate CNN classifier. Bounding box refinement is then carried out based on the edge information of the license plates, in order to improve the intersection-over-union (IoU) ratio. The proposed cascade framework extracts license plates effectively with both high recall and precision. Last, we propose to recognize the license characters as a {sequence labelling} problem. A recurrent neural network (RNN) with long short-term memory (LSTM) is trained to recognize the sequential features extracted from the whole license plate via CNNs. The main advantage of this approach is that it is segmentation free. By exploring context information and avoiding errors caused by segmentation, the RNN method performs better than a baseline method of combining segmentation and deep CNN classification; and achieves state-of-the-art recognition accuracy.