Prohibited item detection based on X-ray images is one of the most effective security inspection methods. However, the foreground-background feature coupling caused by the overlapping phenomenon specific to X-ray images makes general detectors designed for natural images perform poorly. To address this issue, we propose a Category Semantic Prior Contrastive Learning (CSPCL) mechanism, which aligns the class prototypes perceived by the classifier with the content queries to correct and supplement the missing semantic information responsible for classification, thereby enhancing the model sensitivity to foreground features. To achieve this alignment, we design a specific contrastive loss, CSP loss, which comprises the Intra-Class Truncated Attraction (ITA) loss and the Inter-Class Adaptive Repulsion (IAR) loss, and outperforms classic contrastive losses. Specifically, the ITA loss leverages class prototypes to attract intra-class content queries and preserves essential intra-class diversity via a gradient truncation function. The IAR loss employs class prototypes to adaptively repel inter-class content queries, with the repulsion strength scaled by prototype-prototype similarity, thereby improving inter-class discriminability, especially among similar categories. CSPCL is general and can be easily integrated into Deformable DETR-based models. Extensive experiments on the PIXray, OPIXray, PIDray, and CLCXray datasets demonstrate that CSPCL significantly enhances the performance of various state-of-the-art models without increasing inference complexity. The code is publicly available at https://github.com/Limingyuan001/CSPCL.

Computer-aided prohibited item detection is applied in X-ray security inspection to maintain public safety. However, existing prohibited item detectors are limited to a small set of categories in current X-ray datasets, posing potential risks to public security. Since constructing bigger datasets and annotating hundreds of categories is time-consuming and labor-intensive, scaling detectors to more categories with minimal supervision is of great importance. To this end, in this paper, we adopt an open-vocabulary object detection (OVOD) method to detect arbitrary unlabeled novel categories of prohibited item. OVOD methods typically rely on datasets with caption annotations, which are lacking in the domain of prohibited item detection. To support the research on OVOD in X-ray security inspection scenarios, we contribute PIXray Caption dataset, the first X-ray dataset with image-caption pair annotations, which could benchmark and facilitate researches in the community. Further, we propose a novel Open-Vocabulary Prohibited Item Detection (OVPID) network to leverage textual information from captions. OVPID contains two core modules, i.e., Interference Resistant Module (IRM) and Prediction Module (PM). Specifically, IRM includes two submodules, namely Edge Perception (EP) and Foreground Activation (FA), which are designed to address the dilemma of interference caused by overlapping problem and complex background in X-ray images. PM consists of two branches for classification and localization. In classification branch, PM generates more accurate prompts for X-ray dataset via large multimodal model (LMM). In localization branch, PM aligns the student embeddings with both teacher and caption embeddings. Extensive experiments on PIXray Caption dataset demonstrate that OVPID outperforms other OVOD methods by delivering a higher accuracy on novel categories.

Prohibited item detection in X-ray images is one of the most essential and highly effective methods widely employed in various security inspection scenarios. Considering the significant overlapping phenomenon in X-ray prohibited item images, we propose an anti-overlapping detection transformer (AO-DETR) based on one of the state-of-the-art (SOTA) general object detectors, DETR with improved denoising anchor boxes (DINO). Specifically, to address the feature coupling issue caused by overlapping phenomena, we introduce the category-specific one-to-one assignment (CSA) strategy to constrain category-specific object queries in predicting prohibited items of fixed categories, which can enhance their ability to extract features specific to prohibited items of a particular category from the overlapping foreground-background features. To address the edge blurring problem caused by overlapping phenomena, we propose the look forward densely (LFD) scheme, which improves the localization accuracy of reference boxes in mid-to-high-level decoder layers and enhances the ability to locate blurry edges of the final layer. Similar to DINO, our AO-DETR provides two different versions with distinct backbones, tailored to meet diverse application requirements. Extensive experiments on the PIXray, OPIXray, and HIXray datasets demonstrate that the proposed method surpasses the SOTA object detectors, indicating its potential applications in the field of prohibited item detection. The source code will be available at: https://github.com/Limingyuan001/AO-DETR.

Dual-view baggage inspection has been widely applied in real-world scenarios, where orthogonal viewpoints are deployed to capture diverse and complementary information. Compared with single-view, it can effectively improve the identification performance when rotation and overlay hinder the viewability of the objects. However, this topic has not been rigorously explored due to the scarcity of datasets. To overcome this limitation, we contribute the first fully public large-scale Dual-view X-ray dataset. Our dataset, named DvXray, contains 16,000 pairs, 32,000 X-ray images, in which 15 common classes of 5,496 prohibited items are manually labeled. Besides, we propose an approach named Adaptive Hierarchical Cross Refinement (AHCR) to establish a strong baseline for prohibited item discovery in dual-view X-ray images. AHCR hypothesizes that each input pair is sampled from one mixture distribution, hence gathering the non-overlapping and position-aware cues along the shared axis and complementarily delivering to the other in a hierarchical structure to enrich the feature discriminability of the objects of interest from background overlaps. Upon this structure, we propose an adaptive control strategy and a confidence-weighted view fusion term to make it robust to difficult samples. Extensive experiments on DvXray show that AHCR not only brings significant classification gains over various backbones, such as recent Swin Transformer and ConvNeXt, but also exhibits an impressively better ability to localize objects. In addition, AHCR performs favorably against the counterparts and some recent multi-view learning approaches, moving a step closer towards potential application in practice. Dataset and code are available at https://github.com/Mbwslib/DvXray.