Textline alignment on the image domain

Editing and publishing a historical manuscript involves a research phase to recover the original manuscript and reconstruct the transmission of its text based on the relations between its surviving copies. Manuscript alignment, which aims to locate the shared and the different text among a set of copies of the same manuscript, is essential for this phase. In this paper, we present an alignment algorithm for historical handwritten documents that works directly on the image domain due to the absence of an accurate handwritten text recognition (HTR) system for handwritten historical documents and the necessity to visualize the original manuscripts in parallel to examine features beyond the transcribed text. Our approach extracts subwords, estimates the similarity among these subwords, and establishes an alignment among them. We extract subwords from textlines images and convert them into sequences of subword images. It estimates the similarity between two subwords using a Siamese network model and applies Longest Common Subsequence (LCS) to establish the alignment between two image sequences. We have implemented our algorithm, trained the Siamese model, and evaluate its performance using textline images from historical documents. Our algorithm outperformed the state-of-the-art by large margins. Unlike the state-of-the-art, the framework builds the alignment from scratch without requiring any prior knowledge concern subwords boundaries. In addition, we build a new dataset for textline alignment for historical documents, which include ten pairs of pages taken from two copies of two Arabic manuscripts and annotated at the subword level.