The watermarking or digital watermarking is a steganographic technique for hiding information on License Celebrity Photos. Its main objective is to highlight the illegal use of certain digital service by an unauthorized user.

Specifically, this technique involves inserting a message (hidden or not) within a digital object (images, audio, video, text, software), a group of bits containing information about the author or copyright owner of the digital object.

Another related art is the fingerprinting or digital fingerprint, which is inserted into the digital object (License Celebrity Photos) in addition to a brand that contain copyright information contains user information that has acquired the rights to use that object. This way you can identify the illegal distribution of digital items.

Features

A watermarking technique must meet the following requirements. Be imperceptible: the invisible or visible to the observer and must not degrade the object that is intended to protect. It must be unambiguous: the brand must unambiguously identify the copyright owner, so that he can claim ownership.

Despite these premises, there are watermarks that are perceptible and degrade the object, such as water marks superimposed images indicating the property of the same. The process of generating a marked object is as follows:

Also, the reverse process, to see if an object is marked or not, and thus obtain information stored on that mark, will be: To extract the information contained in the mark (M) or to see if an object is marked or not, certain data are needed. Based on these and what is desired, there are different types of digital watermark :

Techniques

Two major types of techniques digital watermark are known: Space techniques: involve modification of any component in the spatial domain, are easy to implement and are fragile against attacks (eg substitution of least significant bits).

Spectral Techniques: involve modification of any component in the frequency domain transformed, are complicated to implement and robust to changes. (examples: DCT coefficients modification, spread spectrum, etc.).

Examples of insertion of marks in objects are as follows. (For images and audio, the inclusion of such marks need not influence your visual or audio representation). Modification of the least significant bit (LSB : less significant bit ) of some pixels of the image, chosen by a pseudo-random sequence. Being the least significant bit of a pixel, it is subjected to a perceptible color change of the celebrity photo.

These techniques are susceptible to various attacks, which will differentiate into types according to their purpose. Attacks to the robustness: be achieved by reducing or completely eliminating the presence of the brand.