METHOD OF DETECTING WATERMARKS

摘要

It is difficult to detect a watermark (40) in a video image sequence (20) if the image has been subjected (possibly by a hacker) to affine transforms such as scaling, rotation, flipping, etc. The transform carried out is generally unknown. Therefore, one or more inverse transforms (60) are performed to the image prior to detection (90) until a reliable decision can be made. The inverse transforms are performed with small stepsize variations of adequate parameters. In a preferred embodiment, an initial search for correlation is done between the inverse transformed image an a blurred version of the reference watermark, the blurred reference watermark being obtained by combining a number of e.g. rotated versions of the reference watermark. If some correlation has been found, the amount of blur and/or the stepsize is decreased. This requires fewer steps to detect the watermark.