Multi-view Contrastive Coding of Remote Sensing Images at Pixel-level
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Our planet is viewed by satellites through multiple sensors (e.g., multi-spectral, Lidar and SAR) and at different times. Multi-view observations bring us complementary information than the single one. Alternatively, there are common features shared between different views, such as geometry and semantics. Recently, contrastive learning methods have been proposed for the alignment of multi-view remote sensing images and improving the feature representation of single sensor images by modeling view-invariant factors. However, these methods are based on the pretraining of the predefined tasks or just focus on image-level classification. Moreover, these methods lack research on uncertainty estimation. In this work, a pixel-wise contrastive approach based on an unlabeled multi-view setting is proposed to overcome this limitation. This is achieved by the use of contrastive loss in the feature alignment and uniformity between multi-view images. In this approach, a pseudo-Siamese ResUnet is trained to learn a representation that aims to align features from the shifted positive pairs and uniform the induced distribution of the features on the hypersphere. The learned features of multi-view remote sensing images are evaluated on a liner protocol evaluation and an unsupervised change detection task. We analyze key properties of the approach that make it work, finding that the requirement of shift equivariance ensured the success of the proposed approach and the uncertainty estimation of representations leads to performance improvements. Moreover, the performance of multi-view contrastive learning is affected by the choice of different sensors. Results demonstrate both improvements in efficiency and accuracy over the state-of-the-art multi-view contrastive methods.
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