Text luminance modulation for hardcopy watermarking

This paper improves a recently proposed hardcopy watermarking method by introducing new approaches to decode the embedded information. The proposed method, coined as text luminance modulation (TLM), embeds hidden data in office-like documents, while presenting robustness to the print–scan (PS) channel. The hidden data is embedded by slightly modulating the luminance of characters and symbols. This change can be set unperceivable to the human eye and detected with the aid of a scanner. In previous works, the hidden data was retrieved by using the average luminance level or the halftone pattern as a detection metric. In this paper, however, the detection process combines different metrics into a single metric, significantly improving the detection performance. This paper proposes a new PS model where characteristics induced by the halftoning in the printing process are considered, allowing the use of the variance of a character as a new detection metric. Performance analyses validate the proposed PS model. Experiments illustrate the precision of the analyses and the applicability of the method.