Anomaly detection has become an important remote sensing application due to the abundant spectral and spatial information contained in hyperspectral images. Recently, hyperspectral anomaly detection methods based on the collaborative representation (CR) model have attracted significant attention. Nevertheless, these methods have to face two main challenges: 1) all features (spectral signatures) are constrained to share the same representation coefficient, which ignores the differences among features and 2) existing dictionaries for pixel-by-pixel detection models are usually not reliable. To address these issues, this article proposes a new relaxed CR detector for hyperspectral anomaly detection by using a novel nonglobal dictionary. The proposed detector conducts CR on each feature dimension of the pixel under test and simultaneously constrains the coding vectors of different features to be similar. To the best of our knowledge, this is the first time that a detection model is built from each feature dimension. To adjust the contributions of each feature, an adaptive feature weight constrained version of the method is also proposed. The nonglobal dictionary is constructed by combining the ${k}$ -nearest neighbor method and an existing global dictionary, which is more reliable and practical than the widely used dual-window dictionary. In addition, this article also designs a band selection strategy for the proposed method. Experiments on five real datasets indicate that the proposed method suppresses background well and outperforms other classical and state-of-the-art methods.

Hyperspectral unmixing is a vibrant research field that focuses on the task of decomposing mixed pixels into a collection of pure spectral signatures, known as endmembers, along with their corresponding fractional abundances. Conventional unmixing algorithms often need to combine two techniques, namely endmember extraction and abundance estimation, to accomplish the unmixing task. Recently, deep learning (DL) has succeeded in the field of hyperspectral unmixing due to its strong feature learning and data-fitting capabilities. By extracting the output and weight of a particular layer as abundance maps and endmember signatures, available DL methods can directly unmix hyperspectral images. However, to improve the performance of spectral unmixing, such available DL methods frequently employ the results of endmember extraction algorithms—in most cases, the well-known vertex component analysis (VCA)—as the initial weights, which leads to significant limitations in their performance: 1) the unmixing results are heavily dependent on the initialization given by VCA and 2) the randomness of VCA is passed to the unmixing network. In this article, we design a new method called abundance-guided spectral and spatial network (A2SN) that not only skips the weights to extract endmember features directly from the network, but also estimates the abundance maps and reconstructs images directly. In particular, the proposed A2SN employs different kernels to capture spectral and spatial information. We also propose an abundance-guided attention spectral and spatial attention network (A2SAN) for hyperspectral unmixing by integrating attention mechanisms into A2SN. As a result, A2SAN is a completely innovative unmixing method that employs attention and reconstruction directly for hyperspectral unmixing, rather than just as modules for information extraction. Most importantly, both A2SN and A2SAN use a weighted summation of the feature maps to reconstruct the image and increase the noise immunity of the network. Experimental results, conducted on both synthetic and real datasets, demonstrate the effectiveness and superiority of A2SN and A2SAN over state-of-the-art unmixing methods. Our full code is released at https://github.com/xuanwentao/A2SN-and-A2SAN for public evaluation.

Fast and accurate remote sensing image retrieval from large data archives has been an important research topic in the remote sensing research literature. Recently, hashing-based remote sensing image retrieval has attracted extreme attention because of its efficient search capabilities. Especially, deep remote sensing image hashing algorithms have been developed based on convolutional neural networks (CNNs) and have shown effective retrieval performance. However, implementing a deep hashing network tends to be highly expensive in terms of storage space and computing resources to be suitable for on-orbit remote sensing image retrieval, which usually operates on resource-limited devices such as satellites and unmanned aerial vehicles (UAVs). To address this limitation, we propose to hash a deep network that in turn hashes remote sensing images. Specifically, we develop a quantized deep learning to hash (QDLH) framework for large-scale remote sensing image retrieval. The weights and activation functions in the QDLH framework are binarized to low-bit representations, which require comparatively much less storage space and computing resources. The QDLH results in a lightweight deep neural network for effective remote sensing image hashing. We conduct extensive experiments on two public remote sensing image data sets by incorporating several state-of-the-art network architectures into our QDLH methodology for remote sensing image hashing. The experimental results demonstrate that the proposed QDLH is effective in saving hardware resources in terms of both storage and computation. Moreover, superior remote sensing image retrieval performance is also achieved by our QDLH, compared with state-of-the-art deep remote sensing image hashing methods.

The storage and processing of remotely sensed hyperspectral images (HSIs) is facing important challenges due to the computational requirements involved in the analysis of these images, characterized by continuous and narrow spectral channels. Although HSIs offer many opportunities for accurately modeling and mapping the surface of the Earth in a wide range of applications, they comprise massive data cubes. These huge amounts of data impose important requirements from the storage and processing points of view. The support vector machine (SVM) has been one of the most powerful machine learning classifiers, able to process HSI data without applying previous feature extraction steps, exhibiting a robust behaviour with high dimensional data and obtaining high classification accuracies. Nevertheless, the training and prediction stages of this supervised classifier are very time-consuming, especially for large and complex problems that require an intensive use of memory and computational resources. This paper develops a new, highly efficient implementation of SVMs that exploits the high computational power of graphics processing units (GPUs) to reduce the execution time by massively parallelizing the operations of the algorithm while performing efficient memory management during data-reading and writing instructions. Our experiments, conducted over different HSI benchmarks, demonstrate the efficiency of our GPU implementation.

Convolutional neural networks (CNNs) have proven to be a powerful tool for the classification of hyperspectral images (HSIs). The CNN kernels are able to naturally include spatial information to smooth out the spectral variability and the noise present in HSI data. However, these kernels are composed of a large number of learning parameters that must be correctly adjusted to achieve good performance. This forces the model to consume a large amount of training data, being prone to overfitting when limited labeled samples are available. In addition, the execution of kernels is computationally very expensive, increasing quadratically with respect to the size of the convolution filter. This significantly reduces the performance of the model. To overcome the aforementioned limitations, this work presents a new few-parameter CNN (based on shift operations) for HSI classification that dramatically reduces both the number of parameters and the computational complexity of the model in terms of floating-point operations (FLOPs). The operational module combines a shift kernel (which adjusts the input data in particular directions without involving any parameters nor FLOPs) with pointwise convolutions that perform the feature extraction stage. The newly developed shift-based CNN has been employed to conduct HSI classification over five widely used and challenging data sets, achieving very promising results in terms of computational performance and classification accuracy.

Hyperspectral unmixing extracts pure spectral constituents (endmembers) and their corresponding abundance fractions from remotely sensed scenes. Most traditional hyperspectral unmixing methods require the results of other endmember extraction algorithms to complete the abundance estimation step. Due to the impressive learning and data fitting capabilities of convolutional neural networks (CNNs), deep learning (DL)-based hyperspectral unmixing technologies have rapidly developed in the literature. According to the procedure used to combine different layers (i.e., fully connected layers, convolution layers, and activation layers), these techniques are mainly divided into three main categories, i.e., those based on autoencoder networks, convolutional neural networks, and convolutional autoencoder networks. They usually extract the weight and output of a specific activation layer as endmember signatures and abundance maps, respectively. Moreover, most existing DL-based unmixing approaches usually use 2-D CNNs to learn the features contained in hyperspectral images, and very few approaches employ 3-D CNNs to extract spectral and spatial information. However, 2-D CNN-based techniques cannot capture good discriminative feature maps from the spectral viewpoint, and 3-D CNN-based techniques usually have high computational overload. In this work, to further exploit the feature extraction capability of CNNs, we combine 3- and 2-D convolutions to propose a cross-convolution unmixing network (CrossCUN) for hyperspectral unmixing. Simultaneously, to better illustrate the improvements of our proposed CrossCUN, we also build the corresponding 2-D convolution unmixing network (2-DCUN) and 3-D convolution unmixing network (3-DCUN). We evaluate the performance of our newly developed networks on two types of synthetic datasets and three real hyperspectral images. Experimental results show that the proposed networks not only obtain better results than other DL-based unmixing methods but also do not require any prior knowledge (e.g., the results of other endmember extraction algorithms) to estimate the abundance maps.