Unsupervised cross-modal hashing has gained significant attention for efficient retrieval between heterogeneous modalities through encoding data into the unified binary representations, offering low storage cost and fast response. However, the constraints of existing methods persist in bridging the cross-modal semantic gap and capturing fine-grained global semantic structures without explicit labels. In this paper, we propose an innovative unsupervised Stationary distribution and soft Clustering Transformer Hashing approach for cross-modal retrieval, denoted as SCTH. Initially, a Transformer-based modality fusion encoder is employed to extract abundant cross-modal semantic representations, further integrated with contrastive hashing to minimize the semantic gap. To enhance the inter-modal alignment, a pseudo-classifier clustering module with entropy-regularized contrastive loss is presented, ensuring balanced and diverse cluster assignments in unsupervised settings. Additionally, a Markovian stationary distribution strategy stabilizes the feature representations through mitigating the interference of noise and outliers. Comprehensive experiments on MIRFlickr, NUS-WIDE, and IAPR-TC12 datasets validate that SCTH outperforms state-of-the-art hashing methods in cross-modal retrieval tasks, demonstrating superior generalization performance.

Unsupervised deep cross-modal hash retrieval aims to map multi-modal features into binary hash codes without labels, which is of interest due to its storage efficiency, query speed and convenient applications. However, existing approaches suffer from two main limitations: (1) Slightly insufficient consideration of text instance similarity, along with independent or redundant fusion to learn multi-modal similarity information. (2) They ignore the noisy adjacent correlations between multi-modal instances, leading to a lack of discriminative power in the generated hash codes. To address these challenges, we propose a new approach called Statistical Model-driven Similarity Hashing (SMSH). Specifically, we introduce Jaccard similarity when constructing the text similarity matrix. It reduces the similarity error between text instances while better considering the asymmetry of the elements in the text features. After that, we integrate the original similarity information between various modalities to construct a unified similarity matrix. The gaps between modalities are bridged while reducing the redundant information in them. In addition, we introduce a Statistical Model-driven Similarity Enhancement (SMSE) approach, which reduces the noise of similarity relations between multi-modal instances by using a Gaussian Mixture Model to keep instances with lower semantic similarity as far away from each other as possible. Experiments on three benchmark datasets demonstrate the excellent performance of the SMSH method.

Hashing has been widely applied in large-scale multimodal retrieval by mapping heterogeneous modalities data into binary codes. However, most cross-modal hashing methods cannot make the most of semantic information to construct the association relations of sample pairs, resulting in unsatisfactory retrieval accuracy. Concept lattice is a powerful tool for data mining and information retrieval, and for all we know, this is the first time to combine formal concept analysis and hash learning to improve cross-modal hashing retrieval performance. In this paper, we propose a novel framework for Asymmetric Cross-modal Hashing based on Formal Concept Analysis, denoted as ACHFCA. Initially, a flash-projection three-layer semantic enhancement descriptor is designed to extract latent representations from heterogeneous modalities. Subsequently, an asymmetric hash learning framework is established to enhance the semantics of different layers based on the fine-grained similarity values reconstructed from concept lattice to reinforce the discriminative competence of the model. Finally, an effective discrete optimization algorithm is proposed, which can directly learn compact hash codes. Comprehensive experiments on MIRFlickr, NUS-WIDE and IAPR-TC12 datasets demonstrate the superior performance of ACHFCA to state-of-the-art hashing approaches.

Unsupervised cross-modal hashing methods have become a core technology for retrieving vast amounts of heterogeneous multimedia information due to their advantages in retrieval speed and storage efficiency. Although these methods have made significant progress in the field of multimedia retrieval, they still face challenges related to inaccurate similarity measurements and incomplete embedding of relational information. To address these issues, we propose Unsupervised Hierarchical Dynamic Similarity Hashing(UHDSH) for multimedia retrieval. First, the Semantic Similarity Measurement Layer extracts common semantic information within multimedia data to construct a dynamic fluctuation similarity hypergraph, which guides the training of the hash function. Second, the Relational Constraint Hashing Layer, based on the dynamic fluctuation similarity hypergraph embedding technique and multimodal feature reconstruction, ensures the precise embedding of both semantic and relational information. Finally, we conducted comprehensive experiments on two widely used datasets, MIR Flickr and NUS-WIDE. Our proposed UHDSH method achieves a maximum improvement of 5.06% over the best baseline methods. The code is publicly available at https://github.com/YunfeiChenMY/UHDSH.

The global distribution of large-scale, multi-source, heterogeneous data has posed an urgent challenge for the efficient organization and retrieval of massive heterogeneous datasets. Hashing learning, known for its advantages in storage efficiency and retrieval speed, has emerged as a key technology to address the challenges of large-scale heterogeneous data. However, challenges related to the decentralized distribution of data and inter-institutional privacy security during transmission remain unresolved. To tackle these issues, we propose a Decentralized Multi-agent Distributed Hashing (DMDH) framework for multimedia retrieval. First, we innovatively introduce a multi-agent collaborative strategy, where each client is treated as an independent agent capable of selecting corresponding agents for model interaction based on its own needs, enabling efficient organization and management of distributed data. Second, we propose an adaptive client fusion mechanism, which leverages model exchange, model aggregation, and model updates to enable distributed model training for any cross-modal hashing method. Comprehensive experiments demonstrate the superiority and efficiency of the proposed DMDH framework, validating its effectiveness in addressing these critical challenges.

Unsupervised hashing is applied in large-scale multimodal retrieval by mapping original data from heterogeneous modalities into compact binary codes. Transformer-based retrieval augmented generation possesses significant advantages in retrieval accuracy and context-awareness, yet faces scalability challenges due to the computational overhead of dense embedding. Thus, the integration of hash learning and Transformer provides a feasible improvement scheme, which can achieve efficient retrieval preserving semantic association. This paper proposes a novel Unsupervised Similarity-Fusion Transformer Hashing for multimodal retrieval, denoted as USFTH. Initially, the modal fusion similarity matrix based on Gaussian kernel, sigmoid function, and Laplacian transformation is introduced to construct a discriminative similarity matrix, ensuring that semantic correlation among samples can be captured precisely. Then, cross-modal multiplex joint construction via Transformer-based attention mechanisms is designed, realizing effective integration of heterogeneous modalities in the similarity matrix through multi-path fusion. Furthermore, the consensus fusion strategy is proposed to ensure that hash codes generated under unsupervised conditions possess a uniform distribution and achieve accurate retrieval. In addition, comprehensive experiments on MIRFlickr, NUS-WIDE, and IAPR-TC12 datasets demonstrate the superior performance of USFTH to state-of-the-art hashing approaches.

Cross-modal hashing enables efficient cross-modal retrieval by compressing multi-modal data into compact binary codes, but traditional methods primarily rely on centralized training, which is limited when handling large-scale distributed datasets. Federated learning presents a scalable alternative, yet existing federated frameworks for cross-modal hashing face challenges like data heterogeneity and imbalance, such as non-IID data distribution across clients. To address these challenges, we propose Personalized Federated learning with Lookahead for Adaptive cross-modal Hashing (PFedLAH) method, which combines Feature Adaptive Personalized Learning (FAPL) and Weight-aware Lookahead Adaptive Selection (WLAS) mechanism together. Initially, the FAPL module is designed for the client, enabling personalized learning to mitigate the effect of divergence between server and client resulting from non-IID data distribution, while the local optimization constraint mechanism is also integrated to avoid local optimization shift and ensure better alignment with global convergence. On the server side, WLAS module combines weight-aware adaptive client selection and gradient momentum lookahead to form a dynamic and intelligent client selection scheme, while enhancing the overall convergence and consistency through lookahead gradient prediction. Comprehensive experiments on widely used datasets, including MIRFlickr-25K, MS COCO, and NUS-WIDE, comparing state-of-the-art federated hashing methods, demonstrate the superior retrieval performance, robustness, and scalability of the PFedLAH method.

Unsupervised hashing has been extensively applied in large-scale multi-modal retrieval by mapping original data from heterogeneous modalities into unified binary codes . However, there still remain challenges especially how to balance the individual modality-specific representations and common representation preserving intrinsic linkages among heterogeneous modalities. In this paper, we propose a novel fast U nsupervised M ulti-modal H ashing based on P iecewise L earning, denoted as UMHPL , to deal with the mentioned issue. Initially, we formulate the problem as matrix factorization to derive the individual modality-specific latent representations and common latent representation with consensus matrices in a brief time. To maintain the integrality of multi-modal data, we integrate them by adaptive weight factors and nuclear norm minimization. Subsequently, we establish a connection between the individual modality-specific latent representations and common latent representation based on the piecewise hash learning framework to reinforce the discriminative competency of model, which leads the hash codes more compact. Finally, an effective discrete optimization algorithm in mathematical logic and functional analysis is proposed. Comprehensive experiments on Wiki, MIRFlirck, NUS-WIDE, and MSCOCO datasets demonstrate the superior performance of UMHPL to state-of-the-art hashing methods.

Hashing technologies have been widely applied for large-scale multimodal retrieval tasks owing to their excellent performance in search and storage tasks. Although some effective hashing methods have been proposed, it is still difficult to handle the intrinsic linkages that exist among different heterogeneous modalities. Moreover, optimizing the discrete constraint problem through a relaxation-based strategy results in a large quantization error and leads to a suboptimal solution. In this article, we present a novel asymmetric supervised fusion-oriented hashing method, named (ASFOH), which investigates three novel schemes to remedy the above issues. Specifically, we first explicitly formulate the problem as matrix decomposition into a common latent representation and a transformation matrix, combined with an adaptive weight scheme and nuclear norm minimization to ensure the information completeness of multimodal data. Then, we associate the common latent representation with the semantic label matrix, thereby increasing the discriminative capability of the model by constructing an asymmetric hash learning framework, thus, making the generated hash codes more compact. Finally, an efficient discrete optimization iterative algorithm based on nuclear norm minimization is proposed to decompose the nonconvex multivariate optimization problem into several subproblems with analytical solutions. Comprehensive experiments on the MIRFlirck, NUS-WIDE, and IARP-TC12 datasets testify that ASFOH outperforms the compared state-of-the-art approaches.

Action segmentation plays an important role in video understanding, which is implemented by frame-wise action classification. Recent works on action segmentation capture long-term dependencies by increasing temporal convolution layers in Temporal Convolution Networks (TCNs). However, high layers in TCNs are more coarse access to video features, resulting in the loss of fine-grained information for frame-wise action classification. To address the above issues, we propose a novel Attention-based Temporal Convolution (ATC) block to capture fine-grained information of temporal dependencies for frame-wise action classification by self-attention mechanism. Via stacking ATC blocks, we design a Stacking-based Attention Temporal Convolutional Network (SATC) to adaptively capture long-term and short-term dependencies, according to the semantic similarity of features on different temporal receptive fields simultaneously. The experimental results demonstrate that our SATC outperforms other baselines on all three challenging datasets: GTEA, 50Salads and Breakfast.

The contact window scheduling technology supports various space mission requirements of the satellite information network. The continuous emergence of new space missions challenges the scarce satellite contact window scheduling. In this article, We propose a dynamically updated microcloud structure based on digital twin and multiagent system technology. Then, we build an on-satellite autonomous scheduling model based on the microcloud structure and design an efficient solution algorithm to solve the maximum revenue problem of inefficient task scheduling caused by the dynamic change of task priority and satellite position over time. Simulation experiments show that the scheme proposed in this article can significantly improve task scheduling revenue and resource utilization, and have specific enlightening significance for the innovation of satellite task scheduling methods in the future.

Hashing techniques have recently been successfully applied to solve similarity search problems in the information retrieval field because of their significantly reduced storage and high-speed search capabilities. However, the hash codes learned from most recent cross-modal hashing methods lack the ability to comprehensively preserve adequate information, resulting in a less than desirable performance. To solve this limitation, we propose a novel method termed Nonlinear Robust Discrete Hashing (NRDH), for cross-modal retrieval. The main idea behind NRDH is motivated by the success of neural networks, i.e., nonlinear descriptors, in the field of representation learning, and the use of nonlinear descriptors instead of simple linear transformations is more in line with the complex relationships that exist between common latent representation and heterogeneous multimedia data in the real world. In NRDH, we first learn a common latent representation through nonlinear descriptors to encode complementary and consistent information from the features of the heterogeneous multimedia data. Moreover, an asymmetric learning scheme is proposed to correlate the learned hash codes with the common latent representation. Empirically, we demonstrate that NRDH is able to successfully generate a comprehensive common latent representation that significantly improves the quality of the learned hash codes. Then, NRDH adopts a linear learning strategy to fast learn the hash function with the learned hash codes. Extensive experiments performed on two benchmark datasets highlight the superiority of NRDH over several state-of-the-art methods.