25-01-2013, 11:52 AM
DIGITAL WATER MARKING TECHNOLOGY
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Introduction
In the recent time, the rapid and extensive growth in Internet technology is creating a pressing need to develop several newer techniques to protect copyright, ownership and content integrity of digital media. This necessity arises because the digital representation of media possesses inherent advantages of portability, efficiency and accuracy of information content in one hand, but on the other hand, this representation also puts a serious threat of easy, accurate and illegal perfect copies of unlimited number. Unfortunately the currently available formats for image, audio and video in digital form do not allow any type of copyright protection. A potential solution to this kind of problem is an ‘electronic stamp’ or ‘digital watermarking’ which is intended to complement cryptographic process.
The technology
Digital watermarking, an extension of steganography, is a promising solution for content copyright protection in the global network. It imposes extra robustness on embedded information. To put into words, digital watermarking is the art and science of embedding copyright information in the original files. The information embedded is called ‘watermarks’.Digital watermarks don’t leave a noticeable mark on the content and don’t affect its appearance. These are imperceptible and can be detected only by proper authorities. Digital watermarks are difficult to remove without noticeably degrading the content and are a covert means in situations where cryptography fails to provide robustness. The content is watermarked by converting copyright information into random digital noise using a special algorithm that is perceptible only to the content creator. Digital watermarks can be read only by using the appropriate reading software. These are resistant to filtering and stay with the content as long as Originally purposely degraded.
Watermarking principles
All watermarking methods share the same building blocks[3]: an embedding system and the watermark extraction or recovery system. Any generic embedding system should have as inputs: cove (data/image)/hiding medium (I), watermark symbol, (w)(image/text/number) and a key (k) to enforce security. The output of the embedding process is always the watermarked data/image.The generic watermark recovery process needs the watermarked data, the secret key or public key and depending on the method, the original data and /or the original watermark as inputs while the output is the recovered watermark W with some kind of confidence measure for the given watermark symbol or an indication about the presence of watermark in the cover document under inspection. Depending on the combination of inputs and outputs three types namely private, semi private public watermarking system can be defined [2].
• Private watermarking (also called non blind watermarking)
requires at least the cover image and/or watermark symbol and key (if used in embedding) for the recovery of
the hidden information.
Watermark Extraction
The extraction of watermark requires the secret image(s) and the key (k) used for spatial dispersion of the watermark image. The watermarked image under inspection with or without external attacks is partitioned into non-overlapping block of size 8x8 pixels. Now from the secret image, position of the homogenous blocks are selected and gray value of the secret image indicates the corresponding bit positionin mean gray values where watermark pixel was inserted. Hence from the secret image the mean gray value of the blocks of the watermarked image/distorted watermarked image is calculated and watermark pixel is extracted.