Machine Learning in Pansharpening: A benchmark, from shallow to deep networks

Machine learning (ML) is influencing the literature in several research fields, often through state-of-the-art approaches. In the past several years, ML has been explored for pansharpening, i.e., an image fusion technique based on the combination of a multispectral (MS) image, which is characterized by its medium/low spatial resolution, and higher-spatial-resolution panchromatic (PAN) data. Thus, ML for pansharpening represents an emerging research line that deserves further investigation. In this article, we go through some powerful and widely used ML-based approaches for pansharpening that have been recently proposed in the related literature. Eight approaches are extensively compared. Implementations of these eight methods, exploiting a common software platform and ML library, are developed for comparison purposes. The ML framework for pansharpening will be freely distributed to the scientific community. Experimental results using data acquired by five commonly used sensors for pansharpening and well-established protocols for performance assessment (both at reduced resolution and at full resolution) are shown. The ML-based approaches are compared with a benchmark consisting of classical and variational optimization (VO)-based methods. The pros and cons of each pansharpening technique, based on the training-by-examples philosophy, are reported together with a broad computational analysis. The toolbox is provided in https://github.com/liangjiandeng/DLPan-Toolbox.