Interference watermarks

               Ryszard Grzaslewicz, Jaroslaw Kutylowski, 
             Miroslaw  Kutylowski,  Wojciech Pietkiewicz

We propose a digital watermarking method for gray tone images  in  which
each  watermark consists of a collection of single points and each point
is encoded in the spatial domain of  the  whole  image.  The  method  is
somewhat  related to physical digital holograms and interference images.
Reconstruction of such watermarks is based on a similar principle as the
reconstruction of physical holograms.

In approaches encoding the watermark in the spatial domain  one  of  the
major problems is to avoid a texture due to an encoding scheme. We avoid
a recognizable pattern by creating pseudo random keyed watermarks, which
resemble random noise.

The method proposed yields robust watermarks that are resistant  against
many  attacks  which  preserve  the  distance between points (filtering,
rotation, JPEG compressing). The watermarking scheme provides means  for
detection  and  reversal  of  scaling  transformations,  thus making the
watermark resistant to this attack.

The original picture is not needed for reconstruction. The watermark  is
very hard to detect in the watermarked image, thus prohibiting to easily
check whether an image was protected.

Our  method  guarantees  an  exact  reconstruction  provided  that   the
watermark  image  consists  of a limited number of white pixels on black
background.