I. Introduction
Single-image super-resolution (SISR) is a classic computer vision task, which aims to reconstruct the corresponding high-resolution (HR) image from a given low-resolution (LR) image. As a low-level problem of image processing, super-resolution (SR) has been paid attention by more and more researchers. And SR methods have been widely used in other advanced visual tasks, such as video codecs [1], security and surveillance imaging [2], satellite and aerial imaging [3], and facial analysis [4]. Therefore, how to reconstruct high-quality SR images is of great significance.