Watermarking digital images for copyright protection

Abstract

A watermark is an invisible mark placed on an image that is designed to identify both the source of an image as well as its intended recipient. The authors present an overview of watermarking techniques and demonstrate a solution to one of the key problems in image watermarking, namely how to hide robust invisible labels inside grey scale or colour digital images

Rotation, scale and translation invariant spread spectrum digital image watermarking

“…Spread spectrum systems are also capable of approaching the Shannon limit for reliable communication. The fundamental information theoretic limits to reliable communication and its implications to digital watermarking have been discussed by some authors [16,25]. Note that the smaller is the number of bits of core information or \payload" contained in a watermark, the greater the chance of it being communicated without error.…”

“…Information has been embedded using the Discrete Cosine Transform (DCT) [16,34,5,6] Discrete Fourier Transform magnitude and phase [15], Wavelets [16], Linear Predictive Coding [13] and Fractals [9,22]. The key to making watermarks robust has been the recognition that in order for a watermark to be robust it must be embedded in the perceptually signi cant components of the image [16,5,6]. The term \perceptually signi cant" is somewhat subjective but it suggests that a good watermark is one which takes account of the behaviour of human visual system.…”

“…Objective criteria for measuring the degree to which an image component is signi cant in watermarking have gradually evolved from being based purely on energy content [16,5,6] to statistical [20] and psychovisual [27,10] criteria. Digital watermarking is also fundamentally a problem in digital communications [16,25,5,6]. In parallel with the increasing sophistication in modelling and exploiting the properties of the human visual system, there has been a corresponding development in communication techniques.…”

“…In parallel with the increasing sophistication in modelling and exploiting the properties of the human visual system, there has been a corresponding development in communication techniques. Early methods of encoding watermarks were primitive and consisted of no more than incrementing an image component to encode a binary '1' and decrementing to encode a '0' [3,16]. Tirkel and Osborne [29] were the rst to note the applicability of spread spectrum techniques to digital image watermarking.…”

“…Since then there has been an increasing use of spread spectrum communications in digital watermarking. It has several advantageous features such as cryptographic security [29,30,6], and is capable of achieving error free transmission of the watermark near or at the limits set by Shannon's noisy channel coding theorem [16,25]. Spread spectrum is an example of a symmetric key [24] cryptosystem.…”