20-07-2013, 04:12 PM
Digital Watermarking
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INTRODUCTION
Internet evolution, along with the advancement of digital multimedia tools have create a major impact in making the storage and distribution of multimedia content a straight forward task. Issues on security have emerged and there is a vital need in protecting the digital content against counterfeiting, piracy and malicious manipulations. In order to protect the information from such manipulations the most prevalent technique is Information hiding. It can be mainly divided into three processes - cryptography, stenography and watermarks.
Watermarking
What is Watermark?
A watermark is a visible embedded overlay on a digital photo consisting of text, a logo, or a copyright notice. The purpose of a watermark is to identify the work and discourage its unauthorized use. Though a visible watermark can't prevent unauthorized use, it makes it more difficult for those who may want to claim someone else's photo or art work as their own.
Watermarks vary greatly in their visibility; while some are obvious on casual inspection, others require some study to pick out. Various aids have been developed, such as watermark fluid that wets the paper without damaging it. Watermarks are often used as security features of banknotes, passports, postage stamps, and other documents to prevent counterfeiting.
History of Watermark:
The history of watermark dates back to the 13th century. Watermarks were used to indicate the paper brand and the mill that produced it in Italy. By the 18th century watermarks began to be used as anti- counterfeiting measures on money and other documents and in 1995 interest in digital watermarking began to mushroom. Intense research has been carried out in this field for the past few years which has led to the discovery of various algorithms. Throughout this report, some of these techniques are discussed and one such technique is implemented.
Principle
A watermarking system is usually divided into three distinct steps, embedding, attack and detection. In embedding, an algorithm accepts the host and the data to be embedded and produces a watermarked signal. The watermarked signal is then transmitted or stored, usually transmitted to another person. If this person makes a modification, this is called an attack. There are many possible attacks. Detection is an algorithm which is applied to the attacked signal to attempt to extract the watermark from it. If the signal was not modified during transmission, then the watermark is still present and it can be extracted. If the signal is copied, then the information is also carried in the copy. The embedding takes place by manipulating the content of the digital data, which means the information is not embedded in the frame around the data, it is carried with the signal itself. The figure given below shows the basic block diagram of watermarking process.
What is a Wavelet?
A wavelet is a wave-like oscillation with an amplitude that begins at zero, increases, and then decreases back to zero. It can typically be visualized as a "brief oscillation" like one might see recorded by a seismograph or heart monitor. Generally, wavelets are purposefully crafted to have specific properties that make them useful for signal processing. Wavelets can be combined, using a "reverse, shift, multiply and sum" technique called convolution, with portions of an unknown signal to extract information from the unknown signal.
For example, a wavelet could be created to have a frequency of Middle C and a short duration of roughly a 32nd note. If this wavelet were to be convolved at periodic intervals with a signal created from the recording of a song, then the results of these convolutions would be useful for determining when the Middle C note was being played in the song. Mathematically, the wavelet will resonate if the unknown signal contains information of similar frequency – just as a tuning fork physically resonates with sound waves of its specific tuning frequency. This concept of resonance is at the core of many practical applications of wavelet theory.