Weakly supervised group activity recognition (WSGAR) aims to identify the joint activity of a group of people without relying on hand-annotated human bounding boxes. Existing WSGAR methods typically acquire coarse human-level features by pooling from detected bounding boxes or applying human queries with cross attentions. These approaches focus on learning human relations from the acquired features. However, discriminative person-specific clues might be confused with irrelevant backgrounds, hindering the effectiveness of downstream human relation learning. To address this limitation, we propose a Human Feature Refinement framework that enhances human-level information with graph convolutional networks and self-attention. We define in-box regions as tokens and learn their spatial correspondence through GCN and self-attention. By explicitly extracting in-box details and suppressing irrelevant regions, our method acquires more discriminative human-level features for relation learning and group activity prediction. We further propose a Graph-based Token Merging algorithm to reduce the computation cost of Human Feature Refinement, while minimizing information loss and overfitting risk. Experiments show that our method outperforms previous WSGAR methods on Volleyball, NBA and JRDB-PAR benchmarks, with reduced computation cost.

Generalizable object keypoint localization is a fundamental computer vision task in understanding the object structure. It is challenging for existing keypoint localization methods because their limited training data cannot provide generalizable shape and semantic cues, leading to inferior performance and generalization capability. Instead of relying on large scale training data, this work tackles this challenge by exploiting the rich priors from large generative models. We propose a data-efficient generalizable localization method named GenLoc. GenLoc extracts the generative priors from a pre-trained image generation model by calculating the correlation map between image latent feature and condition embedding. Those priors are hence optimized with our proposed heatmap expectation loss to perform object keypoint localization. Benefited by the rich knowledge of generative priors in understanding of object semantics and structures, GenLoc achieves superior performance on various object keypoint localization benchmarks. It shows more substantial performance enhancements in cross-domain, few-shot and zero-shot evaluation settings, e.g., getting 20%+ AP enhancement over CLAMP [43] in various zero-shot settings.

Regular unsupervised domain adaptive person re-identification (ReID) focuses on adapting a model from a source domain to a fixed target domain. However, an adapted ReID model can hardly retain previously-acquired knowledge and generalize to unseen data. In this paper, we propose a Dual-level Joint Adaptation and Anti-forgetting (DJAA) framework, which incrementally adapts a model to new domains without forgetting source domain and each adapted target domain. We explore the possibility of using prototype and instance-level consistency to mitigate the forgetting during the adaptation. Specifically, we store a small number of representative image samples and corresponding cluster prototypes in a memory buffer, which is updated at each adaptation step. With the buffered images and prototypes, we regularize the image-to-image similarity and image-to-prototype similarity to rehearse old knowledge. After the multi-step adaptation, the model is tested on all seen domains and several unseen domains to validate the generalization ability of our method. Extensive experiments demonstrate that our proposed method significantly improves the anti-forgetting, generalization and backward-compatible ability of an unsupervised person ReID model.

Fine-grained text-to-image retrieval aims to retrieve a fine-grained target image with a given text query. Existing methods typically assume that each training image is accurately depicted by its textual descriptions. However, textual descriptions can be ambiguous and fail to depict discriminative visual details in images, leading to inaccurate representation learning. To alleviate the effects of text ambiguity, we propose a Multi-Modal Reference learning framework to learn robust representations. We first propose a multi-modal reference construction module to aggregate all visual and textual details of the same object into a comprehensive multi-modal reference. The multi-modal reference hence facilitates the subsequent representation learning and retrieval similarity computation. Specifically, a reference-guided representation learning module is proposed to use multi-modal references to learn more accurate visual and textual representations. Additionally, we introduce a reference-based refinement method that employs the object references to compute a reference-based similarity that refines the initial retrieval results. Extensive experiments are conducted on five fine-grained text-to-image retrieval datasets for different text-to-image retrieval tasks. The proposed method has achieved superior performance over state-of-the-art methods. For instance, on the text-to-person image retrieval dataset RSTPReid, our method achieves the Rank1 accuracy of 56.2%, surpassing the recent CFine by 5.6%.

Person search is a challenging task that aims to jointly detect and identify a target person from a large-scale scene image dataset. Fully supervised person search requires both bounding boxes and person identity annotations, making it hard to deploy in real-world applications. Although recent weakly supervised person search methods can alleviate annotation workloads, they often result in severe performance degradation when compared to supervised methods. To pursue better performance with a lower annotation budget, we propose to integrate active learning into weakly supervised person search, where a small number of pairwise identity annotations are actively acquired from oracles. Specifically, we propose a context-assisted active learning framework that selects informative instance pairs for labeling and refines pseudo labels for representation learning. The proposed framework consists of a split module and a merge module, which leverage two types of contextual cues for label refinement. Besides, a pairwise relationship predictor is introduced to estimate relations between instances so that annotation cost can be further reduced. Extensive experiments demonstrate that the proposed method could achieve comparable or even better performance than recent fully supervised methods at a much lower annotation cost. Notably, our method achieves 61.4% mAP on PRW dataset, which outperforms recent fully supervised methods at a much lower annotation cost.

Recent advances in pre-trained vision-language models have successfully boosted the performance of unsupervised image representation in many vision tasks. Most of existing works focus on learning global visual features with Transformers and neglect detailed local cues, leading to suboptimal performance in fine-grained vision tasks. In this article, we propose a text-guided patch token exploitation framework to enhance the discriminative power of unsupervised representation by exploiting more detailed local features. Our text-guided decoder extracts local features with the guidance of texts or learned prompts describing discriminative object parts. We hence introduce a local-global relation distillation loss to promote the joint optimization of local and global features. The proposed method allows to flexibly extract either global or global-local features as the image representation. It significantly outperforms previous methods in fine-grained image retrieval and base-to-new fine-grained classification tasks. For instance, our Recall@1 metric surpasses the recent unsupervised retrieval method STML by 6.0% on the SOP dataset. The code is publicly available at https://github.com/maosnhehe/TPTE .

This work focuses on unsupervised representation learning in person re-identification (ReID). Recent self-supervised contrastive learning methods learn invariance by maximizing the representation similarity between two augmented views of a same image. However, traditional data augmentation may bring to the fore undesirable distortions on identity features, which is not always favorable in id-sensitive ReID tasks. In this article, we propose to replace traditional data augmentation with a generative adversarial network (GAN) that is targeted to generate augmented views for contrastive learning. A 3D mesh guided person image generator is proposed to disentangle a person image into id-related and id-unrelated features. Deviating from previous GAN-based ReID methods that only work in id-unrelated space (pose and camera style), we conduct GAN-based augmentation on both id-unrelated and id-related features. We further propose specific contrastive losses to help our network learn invariance from id-unrelated and id-related augmentations. By jointly training the generative and the contrastive modules, our method achieves new state-of-the-art unsupervised person ReID performance on mainstream large-scale benchmarks.

Recent self-supervised contrastive learning provides an effective approach for unsupervised person re-identification (ReID) by learning invariance from different views (transformed versions) of an input. In this paper, we incorporate a Generative Adversarial Network (GAN) and a contrastive learning module into one joint training framework. While the GAN provides online data augmentation for contrastive learning, the contrastive module learns view-invariant features for generation. In this context, we propose a mesh-based view generator. Specifically, mesh projections serve as references towards generating novel views of a person. In addition, we propose a view-invariant loss to facilitate contrastive learning between original and generated views. Deviating from previous GAN-based unsupervised ReID methods involving domain adaptation, we do not rely on a labeled source dataset, which makes our method more flexible. Extensive experimental results show that our method significantly outperforms state-of-the-art methods under both, fully unsupervised and unsupervised domain adaptive settings on several large scale ReID dat-sets. Source code and models are available under https://github.com/chenhao2345/GCL.

Unsupervised person re-identification (ReID) aims at learning discriminative identity features without annotations. Recently, self-supervised contrastive learning has gained increasing attention for its effectiveness in unsupervised representation learning. The main idea of instance contrastive learning is to match a same instance in different augmented views. However, the relationship between different instances has not been fully explored in previous contrastive methods, especially for instance-level contrastive loss. To address this issue, we propose Interinstance Contrastive Encoding (ICE) that leverages interinstance pairwise similarity scores to boost previous classlevel contrastive ReID methods. We first use pairwise similarity ranking as one-hot hard pseudo labels for hard instance contrast, which aims at reducing intra-class variance. Then, we use similarity scores as soft pseudo labels to enhance the consistency between augmented and original views, which makes our model more robust to augmentation perturbations. Experiments on several large-scale person ReID datasets validate the effectiveness of our proposed unsupervised method ICE, which is competitive with even supervised methods. Code is made available at https://github.com/chenhao2345/ICE.

The objective of unsupervised person re-identification (Re-ID) is to learn discriminative features without laborintensive identity annotations. State-of-the-art unsupervised Re-ID methods assign pseudo labels to unlabeled images in the target domain and learn from these noisy pseudo labels. Recently introduced Mean Teacher Model is a promising way to mitigate the label noise. However, during the training, self-ensembled teacher-student networks quickly converge to a consensus which leads to a local minimum. We explore the possibility of using an asymmetric structure inside neural network to address this problem. First, asymmetric branches are proposed to extract features in different manners, which enhances the feature diversity in appearance signatures. Then, our proposed cross-branch supervision allows one branch to get supervision from the other branch, which transfers distinct knowledge and enhances the weight diversity between teacher and student networks. Extensive experiments show that our proposed method can significantly surpass the performance of previous work on both unsupervised domain adaptation and fully unsupervised Re-ID tasks.

In Person Re-Identification (Re-ID) task, combining local and global features is a common strategy to overcome missing key parts and misalignment on models based only on global features. Using this combination, neural networks yield impressive performance in Re-ID task. Previous part-based models mainly focus on spatial partition strategies. Recently, operations on channel information, such as Group Normalization and Channel Attention, have brought significant progress to various visual tasks. However, channel partition has not drawn much attention in Person Re-ID. In this paper, we conduct a study to exploit the potential of channel partition in Re-ID task. Based on this study, we propose an end-to-end Spatial and Channel partition Representation network (SCR) in order to better exploit both spatial and channel information. Experiments conducted on three mainstream image-based evaluation protocols including Market-1501, DukeMTMC-ReID and CUHK03 and one video-based evaluation protocol MARS validate the performance of our model, which outperforms previous state-of- the-art in both single and cross domain Re-ID tasks.