Bleed-through cancellation in non-rigidly misaligned recto-verso archival manuscripts based on local registration

摘要

Ancient manuscripts written on both pages of the sheet are frequently affected by ink bleeding from the reverse side. This phenomenon produces a significant degradation of both the foreground text and the general appearance of the manuscript. Effective digital image restoration techniques may require the use of the content of both document sides, thus needing their perfect alignment. Although often available, recto and verso are usually not aligned, either for rigid misalignments occurring during acquisition, or for non-rigid deformations of the sheet. In this paper, we propose a novel method to restore color recto-verso manuscript images in a piecewise manner, without the need of a preliminary, global registration of the two sides. We assume that at the local level any deformation can be approximated by a displacement and subdivide the two images into small patches of same size. For each pair of patches at the same location, their relative shift is estimated by cross-correlation, thus allowing their straightforward alignment. A bleed-through removal algorithm is then applied to the registered patches. By spanning the entire images, this procedure returns free-of-interferences versions of the images in their original acquisition layout. The experiments show that the restoration results so obtained are better than those obtained with the classical approach that first registers the whole recto-verso pair and then performs restoration. Further advantages are a much lower computational cost, the possibility to manage non-global and non-rigid deformations, and the unaltered geometry and color appearance of the two restored images.