Boosting One-Stage License Plate Detector via Self-Constrained Contrastive Aggregation

Scene Text Detection (STD) has applied in many fields successfully. One of the important applications of STD is License Plate Detection (LPD). As a unique identity of vehicle, License Plate (LP) facilitates the intelligent transportation in many fields, such as traffic enforcement, intelligent transportation dispatching, etc. However, there are many scene texts similar to LPs causing misjudgment of LP detector. To alleviate these disturbances, more discriminative features are necessary. In latent feature space, discriminative features should aggregate into a tight cluster to widen decision boundary. We assume three perspectives about how to aggregate features and boost feature expression. From these assumptions, a special contrastive triad is designed. Then, we propose a Self-Constrained Contrastive Aggregation (SCCA) method to lead the feature aggregation in latent space and boost the feature expression of backbone. The proposed SCCA is jointly trained with supervised learning for detection to improve the detection performance. The experiments show that our proposed SCCA prompts the baseline significantly and exceeds recent LP detectors, reaching 99.7 on both F1-score and AP on UFPR-ALPR dataset. Meanwhile, we compare the self-constrained contrastive learning with vanilla contrastive learning in experiments and visualize their LP features. The results show that our proposed SCCA reaches better performance and verifies our assumptions are reasonable.