Document Recto-verso Registration Using a Dynamic Time Warping Algorithm

Recto verso registration is an important step allowing detection of missing digitized pages, or location of the bleed-through defect over a page. An efficient way to restore or evaluate the bleed-through of a digitized document consists in analyzing at the same time both the recto side and the verso side. This method requires the two images to be aligned, registered. Without particular knowledge about document, recto verso registration is complex. Indeed, the only information that we can use to register the two is the bleed-through. Recto verso registration is complex because the recto's bleed-through is a highly degraded version of verso's ink pixels. Therefore, in this particular context, usual image comparison methods [1] are not very relevant. Nevertheless, document recto verso registration algorithms has been proposed [2], [3] [4], but these methods have important time computation costs, are noise sensitive and even fail in some cases where bleed-through is too light. The previous techniques are based on a pixel to pixel approach where the bleed-through is considered to be just a set of grey pixels. In this article, we consider the structure of the ink pixels on the verso page. The recto verso registration method presented here is based on the fact that bleed-through has the same structure that the ink on the verso side. The method registers the recto's bleed-through layout and the verso's ink layout, in two main steps, first a de-skewing algorithm is applied to both pages then, horizontal and vertical profiles are extracted and aligned with a dynamic time warping. The time complexity of our method is linear according to the image size. Moreover, experiments detailed at the end show the accuracy of our method.