Programming Research Group Research Report RR-01-14

Andrew D. Ker

August 2001, 30pp.

Abstract

We present an idea for digital watermarking of still colour images. The scheme described is blind, so that the watermark detector does not require access to the unmarked image, or an undistorted copy of the watermarked image. We make use of the correlation of signals in the different colour components of a colour image, even after special desynchronisation attacks which usually defeat blind watermarks, using some components of the image to synchronise the others. We term this the dual channel approach to watermarking of colour images. A mathematical problem describing the difficulties with this approach is formulated, and a solution so simple as to be almost trivial is exhibited. We show how the solution can be used to motivate a practical watermarking scheme. An extremely crude implementation of this scheme is made. Despite its basic nature, this scheme performs well under some preliminary testing, exhibiting robustness to filtering attacks, JPEG compression, small amounts of rotation, scaling, and other linear transformations, and the StirMark tool even with greater than default parameters.