Chapter 22: HDR Image Watermarking

This Section introduces the basic notions of digital watermarking, to give to the reader a self-contained, succinct yet quite complete coverage of digital watermarking basics while presenting the watermarking system in a way congenial to the rest of this Chapter. Digital watermarking [Barni(2004)] [Cox(2008)] belongs to the data hiding field. Data (or information) hiding is a field as old as History itself [Cox(2008), Chapter 1]. One can hide an object, as a piece of information, for many possible reasons to obtain the more disparate intended results. The most common reason for hiding information is to protect it against inappropriate or prohibited or sometimes even perfectly licit uses from subjects who have not the authority to do so. A different but somewhat correlated reason is secrecy: one may want to hide informations to keep their very existence unknown (secrecy is correlated to protection since the former can be a mean for protection on its own and because secrecy and protection are often simultaneously present, and sometimes confused, in real world applications). Hidden information may also be utilized as in surveillance systems to trigger some actions in response to subjects, unaware of their presence, performing (potentially illicit) operations, this way constituting a repressive rather than a preventing way of protection. Last but not least, informations could be hidden because, in spite of their perhaps indispensable presence, should they be in plain sight in some cases they would degrade the perceptual value of the object they correspond to. An excellent introduction to data hiding history in its totality can be found in [Petitcolas(1999)]. For a mathematical analysis of the problem of secret communication, see the classic paper by Shannon [Shannon(1949)]. Digital data hiding is the direct derivation of these concepts into the digital world. Since digital content is only another representation of the same information the human senses naturally perceive, it is quite natural that the same needs as the ones previously cited arise in the digital world as well. Hence, digital information hiding applications account to hide some kind of digital information into digital documents. Digital information hiding can be thought as a composition of three main techniques: cryptography, steganography and digital watermarking. The best known branch of information hiding is probably cryptography [Menezes(1996)], which is the art of hiding the content of a transmission between two subjects to a potentially malicious eavesdropper by making it unintelligible to all except the intended recipients. Another very old, though less known than cryptography, information hiding technique is steganography [Provos(2003)], sometimes also called covert communication. In steganography, the very existence of the communication is hidden; the information is conveyed by proper, imperceptible modifications of an innocent-looking object. The most recent information hiding technique, digital watermarking, is the subject of this Chapter, applied in the high-dynamic range imaging context. Loosely speaking, digital watermarking tries to introduce information in a certain domain (this process is called watermark embedding [Barni(2004), Chapter 4]) inside a digital object while preserving its perceptual content (like steganography) and while being in a “hostile” environment (like cryptography), i.e. populated by intelligent attackers (embodying the so-called watermark channel [Barni(2004), Chapter 7]) interested in disrupting the watermarking system operativeness while preserving the perceptual quality of the host object. This information then will be retrieved by another entity, or perhaps the same one