Discrete angle watermark encoding and recovery
Imants Svalbe, Andrew Z. Tirkel, Ron van Schyndel · 2002
Angles derived from sets of image pixel values present an effective medium for embedding "invisible" watermarks. The derived angles can be dithered by the addition of small offset angles. The watermark is a pseudo-noise sequence of dither angles. The watermark embedding is followed by re-quantisation for image storage or transmission. Watermark recovery is achieved by performing a complex correlation between the watermarked image and the reference pseudo-noise sequence. This occurs without recourse to the original image. The paper analyses the form of the observed decorrelation as the watermark to image ratio is decreased. The dependence of sequence recovery on: (1) the magnitude of the added dithered angle; (2) the angle derivation scheme used; and (3) on the level of quantisation in the original data is discussed. The spatially distributed embedding method used also offers potential robust data encryption advantages.