Towards Extreme Image Rescaling with Generative Prior and Invertible Prior

The goal of image rescaling is to embed the information from high-resolution images into low-resolution images and then reconstruct the high-resolution images in reverse. Existing methods either focus on small scaling factors or do not generalize well to natural images with diverse content in extreme settings, i.e., using extreme scaling factors (e.g., 16× and 32×). When performing extreme rescaling, previous methods often fail to produce plausible high-quality results due to insufficient cues in low-resolution images. In this work, we propose an extreme natural image rescaling framework that exploits the rich generative prior integrated into the GAN model trained on large-scale natural images to reduce the ambiguity of extreme upscaling. Considering the invertible bijective transformation between quantized features and low-resolution image, we develop an invertible feature recovery module that generates semantically sound low-resolution image while maximizing the preservation of useful features for the subsequent upscaling. Furthermore, we propose a multi-scale refinement module that explicitly introduces the supervised ground truth information to mitigate unpleasant artifacts and distortions. Extensive experiments show that the proposed rescaling framework formulated by the above components achieves significantly better visual performance than state-of-the-art methods.