Joceli Mayer, Kapali Viswanathan, Yogesh Sankarasubramaniam, and Marcelo Thielo
Hardcopy document watermarking, Print-scan channel, Black-and-white photocopy resilience
We provide a method for robust authentication of contents in a printed document. The authentication message is digitally embedded into the document before printing and can be retrieved after scanning the printed media. The method can be employed for all types of documents as the proposed additive embedding function is independent of the document components such as graphics, text or images. The detection can be significantly improved by segmentation of typical interfering components (text, graphics, equations, drawings) found in office documents. The remaining segmentation noise plays a major role in the detection as shown in the provided print and scan channel analysis. This noise is mitigated by improved segmentation considering the size of the connected interfering components. The message encoding employs M-ary modulation and the resulting patterns are additively spread into the document aiming low perceptual impact for a given message size. We illustrate the robustness performance by embedding message sizes over 100 bytes per printed page by employing Time Division Modulation (TDM). The approach is shown to be robust to the print scan channel and also to multiple black-and-white photocopies (multiple passes over the print scan channel) of the document when carefully considering the resulting print dot gain. Results also indicate that the method transparency is specially improved using recycled paper media resulting in a small reduction of the detection performance.
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