Unsupervised Visible-Infrared Person Re-Identification (USVI-ReID) aims to match visible and infrared person images without relying on prior annotations. Recently, unsupervised contrastive learning methods have become the mainstream approach for USVI-ReID, leveraging clustering algorithms to generate pseudo-labels. However, these methods often suffer from inherent noisy pseudo-labels, which significantly hinders their performance. To address this challenge, we propose a Adaptive Pseudo-label Purification and Debiasing (APPD) framework for USVI-ReID, which is designed to calibrate noisy pseudo-labels and dynamically detects clean pseudo-labels, thereby enhancing the model’s performance and reliability. Specifically, we propose an Adaptive Pseudo-label Calibration and Division (APCD) module, which calibrates noisy pseudo-labels by assessing their reliability and divides pseudo-labels into clean and noisy subsets, ensuring a more focused and accurate learning process. Based on the calibrated pseudo-labels, we develop an Optimal Transport Prototype Matching (OTPM) module to establish robust cross-modality correspondences. For clean pseudo-labels, we propose a Debiased Memory Hybrid Learning (DMHL) module, which jointly captures modality-specific and modality-invariant information while addressing sampling bias to enhance feature representation. To effectively utilize noisy pseudo-labels, we introduce a Neighbor Relation Learning (NRL) module that mitigates intra-class variations by exploring neighbor relationships in the feature space. Comprehensive experiments conducted on two widely recognized USVI-ReID benchmarks demonstrate that APPD achieves state-of-the-art performance, significantly outperforming existing methods. The source code will be made available at https://github.com/XiangboYin/RPNR.

Federated learning (FL) provides a decentralized machine learning paradigm where a server collaborates with a group of clients to learn a global model without accessing the clients' data. User heterogeneity is a significant challenge for FL, which together with the class-distribution imbalance further enhances the difficulty of FL. Great progress has been made in large vision-language models, such as Contrastive Language-Image Pre-training (CLIP), which paves a new way for image classification and object recognition. Inspired by the success of CLIP on few-shot and zero-shot learning, we use CLIP to optimize the federated learning between server and client models under its vision-language supervision. It is promising to mitigate the user heterogeneity and class-distribution balance due to the powerful cross-modality representation and rich open-vocabulary prior knowledge. In this paper, we propose the CLIP-guided FL (CLIP2FL) method on heterogeneous and long-tailed data. In CLIP2FL, the knowledge of the off-the-shelf CLIP model is transferred to the client-server models, and a bridge is built between the client and server. Specifically, for client-side learning, knowledge distillation is conducted between client models and CLIP to improve the ability of client-side feature representation. For server-side learning, in order to mitigate the heterogeneity and class-distribution imbalance, we generate federated features to retrain the server model. A prototype contrastive learning with the supervision of the text encoder of CLIP is introduced to generate federated features depending on the client-side gradients, and they are used to retrain a balanced server classifier. Extensive experimental results on several benchmarks demonstrate that CLIP2FL achieves impressive performance and effectively deals with data heterogeneity and long-tail distribution. The code is available at https://github.com/shijiangming1/CLIP2FL.

Unsupervised Visible-Infrared Person Re-identification (USVI-ReID) presents a formidable challenge, which aims to match pedestrian images across visible and infrared modalities without any annotations. Recently, clustered pseudo-label methods have become predominant in USVI-ReID, although the inherent noise in pseudo-labels presents a significant obstacle. Most existing works primarily focus on shielding the model from the harmful effects of noise, neglecting to calibrate noisy pseudo-labels usually associated with hard samples, which will compromise the robustness of the model. To address this issue, we design a Robust Pseudo-label Learning with Neighbor Relation (RPNR) framework for USVI-ReID. To be specific, we first introduce a straightforward yet potent Noisy Pseudo-label Calibration module to correct noisy pseudo-labels. Due to the high intra-class variations, noisy pseudo-labels are difficult to calibrate completely. Therefore, we introduce a Neighbor Relation Learning module to reduce high intra-class variations by modeling potential interactions between all samples. Subsequently, we devise an Optimal Transport Prototype Matching module to establish reliable cross-modality correspondences. On that basis, we design a Memory Hybrid Learning module to jointly learn modality-specific and modality-invariant information. Comprehensive experiments conducted on two widely recognized benchmarks, SYSU-MM01 and RegDB, demonstrate that RPNR outperforms the current state-of-the-art GUR with an average Rank-1 improvement of 10.3%. The code is available at https://github.com/XiangboYin/RPNR.

Unsupervised visible-infrared person re-identification (USVI-ReID) aims to match specified persons in infrared images to visible images without annotations, and vice versa. USVI-ReID is a challenging yet underexplored task. Most existing methods address the USVI-ReID through cluster-based contrastive learning, which simply employs the cluster center to represent an individual. However, the cluster center primarily focuses on commonality, overlooking divergence and variety. To address the problem, we propose a Progressive Contrastive Learning with Hard and Dynamic Prototypes for USVI-ReID. In brief, we generate the hard prototype by selecting the sample with the maximum distance from the cluster center. We reveal that the inclusion of the hard prototype in contrastive loss helps to emphasize divergence. Additionally, instead of rigidly aligning query images to a specific prototype, we generate the dynamic prototype by randomly picking samples within a cluster. The dynamic prototype is used to encourage variety. Finally, we introduce a progressive learning strategy to gradually shift the model's attention towards divergence and variety, avoiding cluster deterioration. Extensive experiments conducted on the publicly available SYSU-MM01 and RegDB datasets validate the effectiveness of the proposed method.

Visible-infrared person re-identification (VI-ReID) aims to match a specific person from a gallery of images captured from non-overlapping visible and infrared cameras. Most works focus on fully supervised VI-ReID, which requires substantial cross-modality annotation that is more expensive than the annotation in single-modality. To reduce the extensive cost of annotation, we explore two practical semi-supervised settings: uni-semi-supervised (annotating only visible images) and bi-semi-supervised (annotating partially in both modalities). These two semi-supervised settings face two challenges due to the large cross-modality discrepancies and the lack of correspondence supervision between visible and infrared images. Thus, it is diffi-cult to generate reliable pseudo-labels and learn modality-invariant features from noise pseudo-labels. In this paper, we propose a dual pseudo-label interactive self-training (DPIS) for these two semi-supervised VI-ReID. Our DPIS integrates two pseudo-labels generated by distinct models into a hybrid pseudo-label for unlabeled data. However, the hybrid pseudo-label still inevitably contains noise. To eliminate the negative effect of noise pseudo-labels, we introduce three modules: noise label penalty (NLP), noise correspondence calibration (NCC), and unreliable anchor learning (UAL). Specifically, NLP penalizes noise labels, NCC calibrates noisy correspondences, and UAL mines the hard-to-discriminate features. Extensive experimental results on SYSU-MM01 and RegDB demonstrate that our DPIS achieves impressive performance under these two semi-supervised settings.