Coarse-to-Fine Document Image Registration for Dewarping

Document dewarping has made great progress in recent years, however it usually requires huge document pairs with pixel-level annotation to learn a mapping function. Although photographed document images are easy to obtain, the pixel-level annotation between warped and flat images is time-consuming and almost impossible for large-scale datasets. To overcome this issue, we propose to register photographed documents with corresponding flat counterparts, obtaining the auto-annotation of pixel-level mapping labels. Due to the severe deformation in the real photographed documents, we introduce a coarse-to-fine registration pipeline to learn global-scale transformation and local details alignment respectively. In addition, the lack of registration labels motivates us to tailor a teacher-student dual branch under semi-supervised training, where the model is initialized on synthetic documents with labels. Furthermore, we contribute a large-scale dataset containing 12,500 triplets of synthetic-real-flat documents. Extensive experiments demonstrate the effectiveness of our proposed registration method. Specifically, trained by our registered pixel-level documents, the dewarping model can obtain comparable performance with SOTAs trained by almost 100x scale of samples, showing the high quality of our registration results. Our dataset and code are available at https://github.com/hanquansanren/DIRD.