Digital watermarking is essentially evolved from
digital steganography. The word 'steganography' actually
stems from a Greek word meaning 'covered writing'.
One common analogy could relate to a leaf insect
(digital watermark) exploiting the natural surrounding
of leaves (digital content) to camouflage itself.
Digital watermarks can also be best described by comparing
them with conventional watermarks, like paper watermark
and ink watermark are added to the currency notes to enforce
proof of authenticity that can be detected by ultraviolet
light devices for inspection.
Digital watermark can be easily embedded into and retrieved
from a digital image using a proprietary algorithm.
The digital watermark could contain information of the
content creator, time
stamp, company stamp etc.
Digital watermarks fall into two different classes, namely,
visible watermark and invisible watermark. Visible watermarks
are visual patterns which are overlaid on digital image
or digital content. The intention is for the presence of
the visible watermark to be obvious and impossible to be
removed without destroying the digital content. In the
case of an invisible watermark, it is imperceptible to
the human eyes, the digital content is not visibly degraded
by the presence of the watermark and at the same time contains
some digital information of content creator, time stamp,
etc.
Digital watermark could also be robust or fragile depending
of the requirements of the application. Robust watermarks
are those designed to withstand accidental and malicious
attacks, attacks such as content alteration, compression,
filtering
and cropping. Fragile watermarks have just the
opposite characteristics in that they will drastically
change on an event of any alternation of the digital content.
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