Abstract
The growth of new imaging technologies has created a need for techniques that can be used for
copyright protection of digital images. Copyright protection involves the authentication of image
content and/or ownership and can be used to identify illegal copies of a (possibly forged) image.
One approach for copyright protection is to introduce an invisible signal known as a digital
watermark in the image.
In this paper, we describe digital image watermarking techniques, known as perceptually based
watermarks, that are designed to exploit aspects of the human visual system. In the most general
sense, any watermarking technique that attempts to incorporate an invisible mark into an image
is perceptually based. However, in order to provide transparency (invisibility of the watermark)
and robustness to attack, more sophisticated use of perceptual information in the watermarking
process is required. Several techniques have been introduced that incorporate a simple visual
model in the marking procedure. Such techniques usually take advantage of frequency selectivity
and weighing to provide some perceptual criteria in the watermarking process. Even more
elaborate visual models are used to develop schemes that not only take advantage of frequency
characteristics but also adapt to the local image characteristics, providing extremely robust as
well as transparent schemes. We present examples from each category - from the simple schemes
that guarantee transparency to the more elaborate schemes that use visual models to provide
robustness as well as transparency.
