Recently, pseudo-label-based deep learning methods have shown excellent performance in semi-supervised hyperspectral image (HSI) classification. These methods usually select high-confidence unlabeled samples to help optimize backbone classification networks. However, a large number of remaining low-confidence unlabeled samples, which contain rich land-covers information, are underutilized. In this paper, we propose a pseudo-label-based unreliable sample learning (PUSL) method to fully exploit low-confidence unlabeled samples for semi-supervised HSI classification. Firstly, to avoid overfitting the spatial distribution of labeled samples, we build a position-free transformer (PFT) as the backbone classification network. Secondly, PFT is initially trained with labeled samples in a supervised learning manner to obtain an initial classifier, which is then used to split unlabeled samples into reliable and unreliable unlabeled samples based on the predicted confidence. Thirdly, reliable unlabeled samples participate in training along with labeled samples. Finally, unreliable unlabeled samples are treated as negative samples for corresponding categories to improve the discrimination of PFT in a contrastive learning paradigm. Extensive experiments on three HSI datasets demonstrate that PUSL outperforms compared methods.

Semi-supervised hyperspectral image (HSI) classification methods focus on exploring the spectral and spatial information of unlabeled samples. However, existing methods generally follow the closed-set setting, assuming that unlabeled samples do not contain novel classes, which is hard to hold in practical applications. This paper aims to study semi-supervised HSI classification in the open-set setting, i.e., unlabeled samples fall into novel classes, and proposes a class-aware consistency learning (CACL) method. First, to explore discriminative spectral-spatial features, a position-aware transformer is developed, which effectively models spatial position priors between the center pixel and its neighboring pixels via a symmetric position-aware encoding. Then, to reduce the interference from novel class samples on the model’s discrimination, a prototype-driven consistency learning is proposed, which accurately selects unlabeled samples belonging to known classes via a known class sampler, and efficiently utilizes their spectral-spatial information by modeling consistent predictions across different views. Finally, to further improve the distinguishability between known classes, a prototype contrastive optimization is proposed to decrease the distance between samples from the same class and increase the distances between those from different classes in the feature domain. Furthermore, an adaptive segmentation threshold is designed to accurately predict known classes and reject novel classes. Extensive experiments verify that our CACL outperforms the state-of-the-art methods, achieving the overall accuracy of 81.65%, 88.61%, and 92.88% on the Indian Pines, Salinas, and Pavia University datasets, with 10 labeled samples in each known class. The code is available at https://github.com/rock-in/CACL-main.

Pseudo-label learning-based methods usually regard class confidence above a certain threshold for unlabeled samples as pseudo-labels, which may result in pseudo-labels still containing wrong labels. In this letter, we propose a prototype-based pseudo-label refinement (PPLR) for semi-supervised hyperspectral image classification. The proposed PPLR filters wrong labels from pseudo-labels using class prototypes, which can improve the discrimination of the network. First, PPLR uses multi-head attentions to extract the spectral-spatial features, and designs an adaptive threshold that can be dynamically adjusted to generate high-confidence pseudo-labels. Then, PPLR constructs class prototypes for different categories using labeled sample features and unlabeled sample features with refined pseudo-labels to improve the quality of pseudo-labels by filtering wrong labels. Finally, PPLR further assigns reliable weights to these pseudo-labels in calculating their supervised loss, and introduces a center loss to improve the discrimination of features. When 10 labeled samples per category are utilized for training, PPLR achieves the overall accuracies of 82.11%, 86.70% and 92.50% on the Indian Pines, Houston2013 and Salinas datasets, respectively.

Obtaining accurate road conditions is crucial for traffic management, dynamic route planning, and intelligent guidance services. The complex spatial correlation and nonlinear temporal dependence pose great challenges to obtaining accurate road conditions. Existing graph-based methods use a static adjacency matrix or a dynamic adjacency matrix to aggregate spatial information between nodes, which cannot fully represent the topological information. In this paper, we propose a Hybrid Graph Model (HGM) for accurate traffic prediction. The HGM constructs a static graph and a dynamic graph to represent the topological information of the traffic network, which is beneficial for mining potential and obvious spatial correlations. The proposed method combines a graph neural network, convolutional neural network, and attention mechanism to jointly extract complex spatial–temporal features. The HGM consists of two different sub-modules, called spatial–temporal attention module and dynamic graph convolutional network, to fuse complex spatial–temporal information. Furthermore, the proposed method designs a novel gated function to adaptively fuse the results from spatial–temporal attention and dynamic graph convolutional network to improve prediction performance. Extensive experiments on two real datasets show that the HGM outperforms comparable state-of-the-art methods.

Domain adaptation in medical image segmentation enables pre-trained models to generalize to new target domains. Given limited annotated data and privacy constraints, Source-Free Active Domain Adaptation (SFADA) methods provide promising solutions by selecting a few target samples for labeling without accessing source samples. However, in a fully source-free setting, existing works have not fully explored how to select these target samples in a class-balanced manner and how to conduct robust model adaptation using both labeled and unlabeled samples. In this study, we discover that boundary samples with source-like semantics but sharp predictive discrepancies are beneficial for SFADA. We define these samples as the most influential points and propose a slice-wise framework using influential points learning to explore them. Specifically, we detect source-like samples to retain source-specific knowledge. For each target sample, an adaptive K-nearest neighbor algorithm based on local density is introduced to construct neighborhoods of source-like samples for knowledge transfer. We then propose a class-balanced Kullback-Leibler divergence for these neighborhoods, calculating it to obtain an influential score ranking. A diverse subset of the highest-ranked target samples (considered influential points) is manually annotated. Furthermore, we design a progressive teacher model to facilitate SFADA for medical image segmentation. Guided by influential points, this model independently generates and utilizes pseudo-labels to mitigate error accumulation. To further suppress noise, curriculum learning is incorporated into the model to progressively leverage reliable supervision signals from pseudo-labels. Experiments on multiple benchmarks demonstrate that our method outperforms state-of-the-art methods even with only 2.5% of the labeling budget.

Pseudo-label learning-based methods have achieved excellent performance by leveraging spatial-spectral features of massive unlabeled samples in hyperspectral image (HSI) classification. Existing methods primarily rely on confidence scores predicted by classification models to produce pseudo-labels. However, both limited labeled samples and complex spatial-spectral information result in these models being prone to overfitting, reducing the reliability of pseudo-labels. In this paper, a relation-aware multi-prototype learning (RAMPL) method is proposed. First, to extract discriminative spatial-spectral features, a hybrid transformer-CNN module is developed, which extracts both global and local features using the attention mechanism and a mixed-scale convolutional block. Then, to generate high-confidence pseudo-labels, a multi-prototype learning module is proposed to construct multiple class prototypes for each category by exploiting both unlabeled samples with pseudo-labels and a small number of labeled samples, which effectively models the intra-class diversity. Finally, to further enhance feature discrimination, a relation-aware contrastive learning module is proposed to learn the complex spatial distribution difference and spectral variability of the HSI by constructing a relation-aware pseudo graph. Extensive experiments demonstrate that the proposed RAMPL achieves superior classification accuracy, achieving 85.76%, 84.36%, 93.14%, and 96.13% on the Indian Pines, Houston2013, Salinas, and Kennedy Space Center datasets with 5 labeled samples in each category, and surpasses 11 state-of-the-art HSI classification methods.