University of Texas at Arlington Spring-2011 Digital Watermarking Final Report Ehsan SyedEhsan Syed Digital Watermarking 2 DIGITAL WATERMARKING - Final Report (EE 5359: Multimedia Processing) Under the guidance of, Dr. K. R. Rao Submitted By, Ehsan Syed 1000671971 [email protected] The University of Texas at Arlington – Spring 2011Ehsan Syed Digital Watermarking 3 DIGITAL WATERMARKING Introduction: Watermarking is defined as the practice of altering a work to embed a message about that work [1]. Embedding a digital signal (audio, video or image) with information which cannot be removed easily is called digital watermarking. Digital watermarking is based on the science of steganography [2] or data hiding. Steganography comes from the Greek meaning ‘covered writing’. Steganography is defined as the practice of undetectably altering a work to embed a secret message. It is an area of research of communicating in a hidden manner. Steganography and watermarking rely on imperfections of human senses. The eyes and ears are not perfect detectors and cannot detect minor change therefore can be tricked into thinking two images or sounds are identical but actually differ, for example in luminance or frequency shift. The human eye has a limited dynamic range so low quality images can be hidden within other high quality images [3]. Basic Principle: There are three main stages in the watermarking process [4]: • generation and embedding • attacks • retrieval/detection Generation of watermarks is an important stage of the process. Watermarks contain information that must be unique otherwise the owner cannot be uniquely identified. In embedding, an algorithm accepts the host and the data to be embedded and produces a watermarked signal. Various algorithms have been developed so far [5-14].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 possibleEhsan Syed Digital Watermarking 4 attacks. Detection is an algorithm which is applied to the attacked signal to attempt to extract the watermark from it. If the signal is 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 contents of the digital data, which means the information is not embedded in the frame around the data, it is carried with the signal itself. Figure 1 shows the basic block diagram of watermarking process. Figure 1: Basic block diagram of watermarking Embedding Watermark Image Host Image Watermarked Image Extraction Private Key Private Key Extracted watermark signalEhsan Syed Digital Watermarking 5 Types of Watermarking: There are mainly three types of watermarking [15]: • Visible Watermarking • Invisible Watermarking • Dual watermarking Below, are discussed the various types of digital watermarkings: Visible Watermarking: In visible watermarking of images, a secondary image (the watermark) is embedded in a primary image such that watermark is intentionally perceptible to a human observer [23]. Invisible Watermarking: In invisible watermarking the embedded data is not perceptible, but may be extracted by a computer program [23]. Invisible Robust Watermarking Invisible Robust watermark cannot be manipulated without disturbing the host signal. This is by far the most important requirement of a watermark. There are various attacks, unintentional (cropping, compression, scaling) and unintentional attacks which are aimed at destroying the watermark. So, the embedded watermark should be such that it is invariant to various such attacks. They are designed to resist any manipulations that may be encountered. All applications where security is the main issue use robust watermarksEhsan Syed Digital Watermarking 6 Invisible Fragile Watermarks These watermarks are designed with very low robustness and are used to check the integrity of objects. Invisible Public and Private Watermark They are differentiated in accordance with the secrecy requirements for the key used to embed and retrieve watermarks. If the original image is not known during the detection process then it is called a public or a blind watermark and if the original image is known it is called a non blind watermark or a private watermark. Dual watermarking: The dual watermark is a combination of a visible watermark and an invisible watermark. A visible watermark is first inserted in the host image and then an invisible watermark is added to the already visible-watermarked image. The final watermarked image is the dual watermarked image [23]. Capacity The length of the embedded message determines two different main classes of digital watermarking schemes: Zero-bit: The message is conceptually zero-bit long and the system is designed in order to detect the presence or the absence of the watermark in the marked object. This kind of watermarking scheme is usually referred to as zero-bit or presence watermarking schemes. Sometimes, this type of watermarking scheme is called 1-bit watermark, because a 1 denotes the presence (and a 0 the absence) of a watermark. N-bit: The message is a n-bit-long stream and is modulated in the watermark. These kinds of schemes usually are referred to as multiple-bit watermarking or non-zero-bit watermarking schemes.Ehsan Syed Digital Watermarking 7 Perceptibility: A watermark is called imperceptible if the original cover signal and the marked signal are perceptually indistinguishable. A watermark is called perceptible if its presence in the marked signal is noticeable. Techniques of Watermarking: There are mainly two major techniques of watermarking [16]: • Spatial domain: slightly modifies the pixels of one or two randomly selected subsets of an image • Frequency domain: this technique is also called transform domain. Values of certain frequencies are altered from their original.
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