  The task of single image super-resolution (SISR) aims at reconstructing a
high-resolution (HR) image from a low-resolution (LR) image. Although
significant progress has been made by deep learning models, they are trained on
synthetic paired data in a supervised way and do not perform well on real data.
There are several attempts that directly apply unsupervised image translation
models to address such a problem. However, unsupervised low-level vision
problem poses more challenge on the accuracy of translation. In this work,we
propose a novel framework which is composed of two stages: 1) unsupervised
image translation between real LR images and synthetic LR images; 2) supervised
super-resolution from approximated real LR images to HR images. It takes the
synthetic LR images as a bridge and creates an indirect supervised path from
real LR images to HR images. Any existed deep learning based image
super-resolution model can be integrated into the second stage of the proposed
framework for further improvement. In addition it shows great flexibility in
balancing between distortion and perceptual quality under unsupervised setting.
The proposed method is evaluated on both NTIRE 2017 and 2018 challenge datasets
and achieves favorable performance against supervised methods.
