Abstract :
Digital Watermarking
describes methods and technologies that hide information, for example a number or
text, in digital media, such as images, video.
The embedding
takes place by manipulating the content of the digital data, which means the information
is not embedded in the frame around the data. The hiding process has to be such
that the modifications of the media are imperceptible. For images this means that
the modifications of the pixel values have to be invisible.
A digital watermark
is a message which is embedded into digital content
(video, images
or text) that can be detected or extracted later.
Moreover, in
image the actual bits representing the watermark must be scattered throughout the
file in such a way that they cannot be identified and manipulated. Watermarking
is the insertion of imperceptible and inseparable information into the host data
for data security & integrity.
They are characterizing
patterns, of varying visibility, added to the presentation media as a guarantee
of authenticity, quality, ownership, and source.
However, in
digital watermarking, the message is supposed not to visible (or at least not interfering
with the user experience of the content), but (only) electronic devices can retrieve
the embedded message to identify the code.
Another form
of digital watermarking is known as steganography, in which a message
is hidden in the content without typical citizens or the public authorities noticing
its presence.
Only a limited
number of recipients can retrieve and decode the hidden message. Unlike a traditional
watermark on paper, which is generally visible to the eye, digital watermarks can
be made invisible or inaudible. They can, however, be read by a computer with the
proper decoding software.
Different attributes
associated with watermarking
The characteristics of a watermarking
algorithm is normally tied to
the application is designed for. The most
important properties of any digital watermarking techniques are robustness, security,
imperceptibility, complexity, and verification. The following merely explain the words used in
the context of watermarking.
Imperceptibility:-In
watermarking, we traditionally seek high fidelity, i.e. the watermarked work must
look or sounds like the original. Whether or not this is a good goal is a different
discussion. Imperceptibility means the watermark is not seen by the human visual
system.
Robustness:- By "robust" we mean the capability of the watermark to resist manipulations of the media, such as lossy compression (where compressing data and then decompressing it retrieves data that may well be different from the original, but is close enough to be useful in some way), scaling, and cropping, just to enumerate some. Robustness is defined as if the watermark can be detected after media (normal) operations such as filtering, lossy compression, color correction, or geometric modifications. In some cases the watermark may need to be fragile. "Fragile" means that the watermark should not resist tampering, or would resist only up to a certain, predetermined extent.
It is more a
property and not a requirement of watermarking. The watermark should be able to
survive any reasonable processing inflicted on the carrier (carrier here refers
to the content being watermarked).
Security:-The
watermarked image should not reveal any clues of the presence of the watermark,
with respect to un-authorized detection, or (statistical) indefectibility or unsuspicious
(not the same as imperceptibility). Security means the embedded watermark cannot
be removed beyond reliable detection by targeted attacks.
Complexity is
described as the effort and time required for watermark embedding and retrieval.
Verification
is a procedure where by there is a private key or public key function.
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