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The research field is still relatively young and has contributors from several disciplines with varying traditions, but the terminology used is still quite diverse. This section provides a formal introduction to watermarking and their types and classification.
Watermarking – The term stands for the hiding of some message or other kind of information in to an image. This information is called as Watermarking and the image that hosts it is the cover image.
Image watermarking imperceptibly embeds data into the host image. The general process of image watermarking is shown in Figure 1.1. The original image is modified using the signature data to form the watermarked image. In this process, some distortion may be introduced. To ensure transparency of the embedded data, the amount of distortion introduced should be small. The watermarked image is then distributed and which may circulate from legal customers to illegal customers. Thereby, subjecting to various kinds of image distortions e.g., loss compression.
The extraction process may or may not require the knowledge of the original image for getting back the hidden signature. The extracted watermark is then compared with the original signature; the difference must be as low as possible.
1.2 Types of watermarks
• Robust watermarks are designed to resist against heterogeneous manipulations, all applications presupposing security of the watermarking systems require this type of watermark.
• Fragile watermarks are embedded with very low robustness. Therefore, this type of watermark can be destroyed even by the slightest manipulations. In this sense they are comparable to the hidden messages in steganographic methods. They can be used to check the integrity of objects.
• Public and private watermarks are differentiated in accordance with the secrecy requirements for the key used to embed and retrieve markings.
According to the basic principle of watermarking, the same key is used in the encoding and decoding process. If the key is known, this type of watermark is referred to as public, and if the key is hidden, as private watermarks. Public watermarks can be used in applications that do not have security-relevant requirements.
• Visible or localized watermarks can be logos or overlay images in the field of image or video watermarking. Due to the implicit localization of the information, these watermarks are not robust.
1.3 Classification of Watermarking
According to signal processing methods
1) Spatial domain – This technique is susceptible to image processing operations like image compression, cropping, filtering.
2) Transform domain – The mark embedding in frequency domain like DCT and DWT.
According to application point of view
1) Robust Watermarking – Is mainly aimed at Copyright protection.
2) Fragile Watermarking – Is mainly aimed at content authentication. It can be altered or destroyed when the digital content is modified.
Besides the various types and classifications of watermarks, four different watermarking systems are classified according to the input and output during the detection process. Using more information at the detector site increases the reliability of the whole watermarking system but limits the practicability of the watermarking approach on the embedded side.
The side information in the detection process can be the original co and the watermark w itself in Figure 1.2. Therefore, four permutations of side information requirements are possible. For a general watermark decoder, the following four are the most essential inputs
ii. Marked data
iii. Key and
iv. Original Image
1.4 Applications of Watermarking
Watermarking techniques are applied to images because of various reasons. Each of these possible applications involves typical processing operations that a watermarking technique must survive. Content protection scenarios may include operations like colour to gray-scale conversion, global or local affine transforms, and printing and scanning. Authentication watermarks must not be affected by legal operations, while illegal attacks must destroy them. Metadata labelling scenarios may include media transform. A typical example is the transmission of information in printed images. This information is revealed if the printed image is shown to a webcam whose data is processed with the watermark reader software as presented by Digi-marc.
Yet robustness is not a general requirement for data hiding techniques: Undetectibility is essential. A typical scenario for data hiding is the distribution of hidden information via newsgroups, bulletin boards, or simply by images on homepages. Steganalysis is a new research area dealing with the detection of hidden data as presented, for example, by Fridrich and Goljan. A possible application of these techniques is the so-called StegoWall as proposed by Voloshynovskiy et al. This StegoWall can be compared with a firewall that analyzes the data that should be transmitted and prevents the transmission of any data containing hidden information.
1.5 Reversible Watermarking
Reversible watermarking is a subset of fragile watermarking. It has an additional advantage of recovering the image which is same as the original image pixel by pixel, after the image is authenticated.
Merits of Reversible Watermarking are listed as follows.
It has an additional advantage of recovering the image which is same as the original image pixel by pixel, after the image is authenticated
During the watermark embedding process, the degradation in the original image can happen. So this technique provides the distortion free data embedding
The reversible watermarking can recover the original image from the watermarked image directly. Therefore, the reversible watermarking is blind, which means the retrieval process does not need the original image.
Due to the reversible watermarking schemes having to embed the recovery information and watermark information into the original image, the required embedding capacity of the reversible watermarking schemes is much more than the conventional watermarking schemes.
Application point of view, it can be used as an information carrier
The advantages of embedding watermarks in frequency domains are naturally resisting some attacks, immune to several destructions or others.
1.5.1 Reversible Watermarking Process
Watermarking for valuable and sensitive images such as military and medical image presents a major challenge to most of watermarking algorithms. First, such applications may require the embedding of several kilobytes of data, but most of robust watermarking algorithms can embed only several hundred bits of data. Second, the watermarking process usually introduces a slight but irreversible degradation in the original image. This degradation may cause the loss of significant artifacts in military and medical images. These artifacts may be crucial for an accurate diagnosis from the medical images or for an accurate analysis of the military images.
As a basic requirement, the quality degradation on the digital content after data embedding should be low. A feature of reversible data embedding is the reversibility, that is, when the digital content has been authenticated, one can remove the embedded data to restore the original content.
The motivation of reversible data embedding is distortion-free data embedding. In sensitive images such as military and medical image, every bit of information is important. Reversible data embedding will provide the original data when the digital content is authenticated.
From the flow diagram shown in Figure 1.3, Let I represent the digital image, W be the watermark then the watermarked image I’ is formed after reversibly embedding the watermark in original image. This watermarked image I’ is then distributed. The image is finally received at the destination but this received image may or may not be tampered by third party. The received image is then given to the authenticator, if it is authenticated then the recovered image I" will be exactly same as the original image pixel by pixel otherwise we can say that the image is trampled.
1.6 Review of existing methods
The earliest reference to reversible data embedding we could find is the Barton patent, filed in 1994. In his invention, the bits to be overlayed will be compressed and added to the bit string, which will be embedded into the data block. Honsinger, et al., reconstruct the payload from an embedded image, and then subtract the payload from the embedded image to losslessly recover the original image. Macq proposes an extension to the patchwork algorithm to achieve reversible data embedding. Fridrich, et al., develop a high capacity reversible data-embedding technique based on embedding message on bits in the status of group of pixels. They also describe two reversible data-embedding techniques for loss image format JPEG. Kalker, et al., provide some theoretical capacity limits of lossless data compression based reversible data embedding and give a practical code construction. Celik, et al., present a high capacity, low distortion reversible data-embedding algorithm by compressing quantization residues.
In the past few years different reversible watermarking techniques had been proposed. But all of these used to remove bits from block of the image and by losslessly compressing these bits providing the space for the watermark to be embedded in the same block. Tian proposed an algorithm using difference expansion and using Haar wavelet transform. This method chooses the expandable coefficients and embeds an extra bit into these coefficients. The watermarked image formed in this method is imperceptible and exact recovery is also possible. But the embedding capacity in this method is less. Alattar proposed an algorithm which uses difference expansion of a generalized integer wavelet transform there by embedding a set of watermark bits in a vector of pixels.
A new algorithm is suggested for reversible watermarking which makes use of difference expansion concept in Tian’s algorithm and the concept of watermark embedding in a set of pixels presented in Alattar’s algorithm. The watermarking embedding and recovery blocks can be introduced in JPEG2000 standard without making many changes to the normal coding flow. In the suggested algorithm, the watermark bits are embedded in the transformed coefficients which don’t cause any overflow or underflow. Table 1.1 the list of papers that has been reviewed.
1.7 Current Status and Key Issues
In recent years a special kind of digital watermarking is discussed widely, called reversible watermarking. It not only provides the protection of the copyright by embedding the assigned watermark into the original image but also can recover the original image from the suspected image. The retrieved watermark can be used to determine the ownership by comparing the retrieved watermark with the assigned one. Similar to conventional watermarking schemes, reversible watermarking schemes have to be robust against the intentional or the unintentional attacks, and should be imperceptible to avoid the attraction of attacks and value lost. Therefore, the reversible watermarking also has to satisfy all requirements of the conventional watermarking such as robustness, imperceptibility, and readily embedding and retrieving.