Progressive image painting outside the box with edge domain

Motivated by a widely studied computer vision task: image inpainting, we became interested in a less concerned problem image outpainting. By which, contents beyond the image boundaries may be extrapolated. In recent years, deep learning methods have achieved remarkable improvements in image inpainting, these techniques can be considered to be applied to image outpainting as solutions. However, many of these inpainting methods generate image blocks generally resulting in blur or smooth. Recently, hallucinating edges for the missing holes before completion has been proved to be a state-of-the-art image inpainting method. Refer to the aforementioned method, we propose a three-phase outpainting model that consists of an edge generation phase, an image expansion phase and a refinement phase. In order to depict the edge lines more accurately, we adopt a comparatively effective focal loss for edge prediction. An optimization stage with a refinement network is also added since large portions outside the image need to be inferred, and discriminator in this stage works on a decreased patch size with a coarse-to-fine fashion. In addition, with recursive outpainting, an image could be expanded arbitrarily. Experiments show that an image can be effectively expanded by our method, and our outpainting method of predicting edges and then coloring is generally superior to other methods both quantitatively and qualitatively.