Digital signatures protect image copyright
Many image-processing applications require digital information to be encoded in images for security or copyright reasons. In some applications, for example, it may be necessary to encode source information in satellite images. Using digital signatures or watermarking techniques to protect image data can be accomplished by encoding imperceptible changes into images. Such a method of image watermarking based on amplitude modulation has been developed at the Ecole Polytechnique Fédérale de Lausanne (Lausanne, Switzerland) by Martin Kutter ([email protected]) and his colleagues.
To encode color images, single signature bits are embedded by modifying pixel values in their blue channel. These modifications are either additive or subtractive, depending on the value of the bit and pixels` luminance. According to Kutter, this method is invariant to filtering and geometrical alteration, allowing signatures to be retrieved from altered images. At present, Kutter and his colleagues have developed two water-marking algorithms for still images and a third for MPEG video. To realize the algorithm, Kutter has developed JK_PGS tool, Version 1.0, a software for SGI, SUN, and LINUX-based workstations. The program can be downloaded from http://lts www.epfl.ch/ ~kutter/watermarking/JK_PGS.html. But, as Kutter admits, the robustness of the current algorithm is not yet perfect, and a more sophisticated version resistant to image rotation, scaling, and cropping is under development.
