Document watermarking via character luminance modulation

In this paper we propose a new method to embed hidden data in office-like documents, presenting robustness to the print-scan channel. The method slightly modulates the luminance of documents characters and symbols, being this change unnoticeable to the human eye, yet detectable by a scanner. It is based on pulse amplitude modulation communications, using "character shaped" pulses instead of traditional pulses, using text spacing to avoid intersymbol interference. We present an error analysis for the method, along with a new print-scan channel model for text documents. The analyses and applicability of the method are validated by experiments.