Abstract
Watermarking, which belong to the information hiding field, has seen a lot of research interest recently. There is a lot of work begin conducted in different branches in this field. Steganography is used for secret conmunication, whereas watermarking is used for content protection, copyright management, content authentication and tamper detection. In this paper we present a detailed survey of existing and newly proposed steganographic and watenmarking techniques. We classify the techniques based on different domains in which data is embedded. Here we limit the survey to images only. Index Terms- Watermarldng, Watenmark Detection, Spatial Domain, Image Transforms, DWT, DCT, DFT.
L. INTRODUCTION
Watermarking is a branch of information hiding which is used to hide proprietary information in digital media like photographs, digital music, or digital video. The ease with which digital content can be exchanged over the Internet has created copyright infringement issues. Copyrighted material can be easily exchanged over peer-to-peer networks, and this has caused major concerns to those content providers who produce these digital contents. In order to protect the interest of the content providers, these digital contents can be watermarked. In this paper we provide a survey of the latest techniques that are employed to watermark images. Image watermarking techniques can be applied to digital videos as well. However, in this paper we limit our self to the image domain. The paper is organized in the following sections. In Section II we describe digital watermarking process. In Section III we identify the main requirements of image watermarking. In Section IV we discuss the main application of watermarking. In Section V we provide a detailed survey of image watermarking techniques. We conclude this paper in section VI where we give some guidelines on developing robust watermarking algorithms.
II. DIGITAL WATERMARKING TECHNIQUE
The process of embedding a watermark in a multimedia objectl is termed as watermarking. Watermark can be considered as a kind of a signature that reveals the owner of the multimedia object. Content providers want to embed watermarks in their multimedia objects (digital content) for several reasons like copyright protection, content authentication, tamper detection etc. A watermarking algorithm embeds a visible or invisible watermark in a given multime- 1 Multimedia object refers to images, video and audio clips or any digital content that can be used for the purpose of information hiding. dia object. The embedding process is guided by use of a secret key which decided the locations within the multimedia object (image) where the watermark would be embedded. Once the watermark is embedded it can experience several attacks because the multimedia object can be digitally processed. The attacks can be unintentional (in case of images, low pass filtering or gamma correction or compression) or intentional (like cropping). Hence the watermark has to be very robust against all these possible attacks. When the owner wants to check the watermarks in the possibly attacked and distorted multimedia object, s/he relies on the secret key that was used to embed the watermark. Using the secret key, the embedded watermark sequence can be extracted. This extracted watermark may or may not resemble the original watermark because the object might have been attacked. Hence to validate the existence of watermark, either the original object is used to compare and find out the watermark signal (non-blind watermarking) or a correlation measure is used to detect the strength of the watermark signal from the extracted watermark (blind waternarking). In the correlation based detection the original watermark sequence is compared with the extracted watermark sequence and a statistical correlation test is used to determine the existence of the watermark.


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