Ref-ZSSR: Zero-Shot Single Image Superresolution with Reference Image.

Single image superresolution (SISR) has achieved substantial progress based on deep learning. Many SISR methods acquire pairs of low-resolution (LR) images from their corresponding high-resolution (HR) counterparts. Being unsupervised, this kind of method also demands large-scale training data. However, these paired images and a large amount of training data are difficult to obtain. Recently, several internal, learning-based methods have been introduced to address this issue. Although requiring a large quantity of training data pairs is solved, the ability to improve the image resolution is limited if only the information of the LR image itself is applied. Therefore, we further expand this kind of approach by using similar HR reference images as prior knowledge to assist the single input image. In this paper, we proposed zero-shot single image superresolution with a reference image (Ref-ZSSR). First, we use an unconditional generative model to learn the internal distribution of the HR reference image. Second, a dual-path architecture that contains a downsampler and an upsampler is introduced to learn the mapping between the input image and its downscaled image. Finally, we combine the reference image learning module and dual-path architecture module to train a new generative model that can generate a superresolution (SR) image with the details of the HR reference image. Such a design encourages a simple and accurate way to transfer relevant textures from the reference high-definition (HD) image to LR image. Compared with using only the image itself, the HD feature of the reference image improves the SR performance. In the experiment, we show that the proposed method outperforms previous image-specific network and internal learning-based methods.