Classification is an important technique for remotely sensed hyperspectral image (HSI) exploitation. Often, the presence of wrong (noisy) labels presents a drawback for accurate supervised classification. In this article, we introduce a new framework for noisy label detection that combines a superpixel-to-pixel weighting distance (SPWD) and density peak clustering. The proposed method is able to accurately detect and remove noisy labels in the training set before HSI classification. It considers two weak assumptions when exploiting the spectral–spatial information contained in the HSI: 1) all the pixels in a superpixel belong to the same class and 2) close pixels in spectral space have the same label. The proposed method consists of the following steps. First, a superpixel segmentation step is used to obtain self-adaptive spatial information for each training sample. Then, a metric is utilized to measure the spectral distance information between each superpixel and pixel. Meanwhile, in order to overcome the first weak assumption, we use $K$ nearest neighbors to obtain the closest neighborhoods of pixels around each superpixel, and a Gaussian weight is employed to mitigate the second weak assumption by adapting the original distance information. Next, the noisy labels in the original training set are removed by a density threshold-based decision function. Finally, the support vector machine (SVM) classifier is employed to evaluate the effectiveness of the proposed SPWD detection method in terms of classification accuracy. Experiments performed on several real HSI data sets demonstrate that the method can effectively improve the performance of classifiers trained with noisy training sets in terms of classification accuracy.

Hyperspectral Classification With Noisy Label Detection via Superpixel-to-Pixel Weighting Distance

In hyperspectral image (HSI) analysis, label information is a scarce resource and it is unavoidably affected by human and nonhuman factors, resulting in a large amount of label noise. Although most of the recent supervised HSI classification methods have achieved good classification results, their performance drastically decreases when the training samples contain label noise. To address this issue, we propose a label noise cleansing method based on spectral-spatial graphs (SSGs). In particular, an affinity graph is constructed based on spectral and spatial similarity, in which pixels in a superpixel segmentation-based homogeneous region are connected, and their similarities are measured by spectral feature vectors. Then, we use the constructed affinity graph to regularize the process of label noise cleansing. In this manner, we transform label noise cleansing to an optimization problem with a graph constraint. To fully utilize spatial information, we further develop multiscale segmentation-based multilayer SSGs (MSSGs). It can efficiently merge the complementary information of multilayer graphs and thus provides richer spatial information compared with any single-layer graph obtained from isolation segmentation. Experimental results show that MSSG reduces the level of label noise. Compared with the state of the art, the proposed MSSG method exhibits significantly enhanced classification accuracy toward the training data with noisy labels. The significant advantages of the proposed method over four major classifiers are also demonstrated. The source code is available at https://github.com/junjun-jiang/MSSG.

Multilayer Spectral-Spatial Graphs for Label Noisy Robust Hyperspectral Image Classification.

Recent developments in hyperspectral sensors have made it possible to acquire hyperspectral images (HSI) with higher spectral and spatial resolution. Hence, it is now possible to extract detailed information about relatively smaller structures. Despite these advantages, HSI suffers from many challenges also, like higher spatial variability of spectral signatures, the Hughes effect due to higher dimensionality, and a limited number of labeled training samples compared to the dimensions of the spectral space. Superpixels can be a potentially effective tool in tackling these challenges. Superpixel segmentation is a process of segmenting the spatial image into several semantic subregions with similar characteristic features. Such grouping by similarity can significantly ease the subsequent processing steps. Because of this, superpixels have been successfully applied to various fields of HSI processing such as classification, spectral unmixing, dimensionality reduction, band selection, active learning (AL), denoising, and anomaly detection. This article focuses on classification, presenting a detailed survey of superpixel segmentation approaches for the classification of HSI. The superpixel creation algorithm framework and postprocessing frameworks for superpixels in HSI are also analyzed. Also, a brief description of various application areas of superpixels is provided. An experimental analysis of existing superpixel segmentation approaches is also provided in this article, supported by quantitative results on standard benchmark datasets. The challenges and future research directions for the implementation of superpixel algorithms are also discussed.

/pdf/a-survey-on-superpixel-segmentation-as-a-preprocessing-step-1bix2jh2zd.pdf

A Survey on Superpixel Segmentation as a Preprocessing Step in Hyperspectral Image Analysis

Vision-guided autonomous underwater vehicles based on remote sensing play an important role in ocean missions. However, some problems exist in underwater visual perception, such as color distortion, low contrast, and fuzzy details, which restrict the applications of underwater visual tasks. Most of the state-of-the-art image enhancement methods are still limited in scene adaptability, recovery accuracy, and real-time processing. To solve these problems, we propose an underwater sensing scene image enhancement method called a multiscale feature fusion network (MFFN). To extract the multiscale feature, the measure merging the feature extraction module, the feature fusion module, and the attention reconstruction module is designed. This measure can also enhance the adaptability and visual effect of the scene. Moreover, we propose multiple objective functions for supervised training to match the nonlinear mapping. Based on the qualitative and quantitative evaluations, the proposed method produces competitive performance compared with some state-of-the-art methods, and the perception and statistical quality of underwater images are enhanced effectively. 

MFFN: An Underwater Sensing Scene Image Enhancement Method Based on Multiscale Feature Fusion Network

Vision-guided autonomous underwater vehicles based on remote sensing play an important role in ocean missions. However, some problems exist in underwater visual perception, such as color distortion, low contrast, and fuzzy details, which restrict the applications of underwater visual tasks. Most of the state-of-the-art image enhancement methods are still limited in scene adaptability, recovery accuracy, and real-time processing. To solve these problems, we propose an underwater sensing scene image enhancement method called a multiscale feature fusion network (MFFN). To extract the multiscale feature, the measure merging the feature extraction module, the feature fusion module, and the attention reconstruction module is designed. This measure can also enhance the adaptability and visual effect of the scene. Moreover, we propose multiple objective functions for supervised training to match the nonlinear mapping. Based on the qualitative and quantitative evaluations, the proposed method produces competitive performance compared with some state-of-the-art methods, and the perception and statistical quality of underwater images are enhanced effectively.

In a real hyperspectral image classification task, label noise inevitably exists in training samples. To deal with label noise, current methods assume that noise obeys the Gaussian distribution, which is not the real case in practice, because in most cases, we are more likely to misclassify training samples at the boundaries between different classes. In this paper, we propose a spectral–spatial sparse graph-based adaptive label propagation (SALP) algorithm to address a more practical case, where the label information is contaminated by random noise and boundary noise. Specifically, the SALP mainly includes two steps: First, a spectral–spatial sparse graph is constructed to depict the contextual correlations between pixels within the same superpixel homogeneous region, which are generated by superpixel image segmentation, and then a transfer matrix is produced to describe the transition probability between pixels. Second, after randomly splitting training pixels into “clean” and “polluted,” we iteratively propagate the label information from “clean” to “polluted” based on the transfer matrix, and the relabeling strategy for each pixel is adaptively adjusted along with its spatial position in the corresponding homogeneous region. Experimental results on two standard hyperspectral image datasets show that the proposed SALP over four major classifiers can significantly decrease the influence of noisy labels, and our method achieves better performance compared with the baselines.

https://www.mdpi.com/2072-4292/11/9/1116/pdf

Label Noise Cleansing with Sparse Graph for Hyperspectral Image Classification

The number of superpixels (i.e., segmentation scale) is crucial for spectral–spatial hyperspectral image (HSI) classification. Existing methods always set the segmentation scale through a manually experimental strategy, which is time-consuming and unsuitable for various complicated practical applications. The information fusion of complementary multiple scales is proven to be more effective than the single scale for HSI classification, but the scale level is still set manually. In this article, we propose a novel adaptive multiscale segmentations (AMSs) method that can automatically provide a set of suitable scales that are adapted to different hyperspectral data. Specifically, based on the assumption that the segmentation scale of HSI is related to the image complexity itself, the texture ratio and the number of land cover classes are used to examine a candidate scale pool. A good scale means that it contains a small spectral difference between pixels within the same superpixel (intrasuperpixel discrimination index) and a large discrepancy between neighboring superpixels (intersuperpixel discrimination index). Thus, an intra–interscale discrimination index is defined and applied to depict the characteristics of the scale. Then, the scale with the best intra–inter discrimination index, which usually has satisfactory performance, is treated as the initially selected scale. The remaining suitable scales are iteratively compared with the selected ones and then added to the target scale pool, until the newly added scale can no longer provide significantly complementary information. Extensive experimental results on three HSI data sets have demonstrated the effectiveness of the proposed AMS when compared with state-of-the-art methods.

Adaptive MultiScale Segmentations for Hyperspectral Image Classification

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Label Noise Cleansing with Sparse Graph for Hyperspectral Image Classification

Adaptive MultiScale Segmentations for Hyperspectral Image Classification