Detection of a watermark in blocks of video or image data which has been subjected to affine geometric distortion is accomplished by approximating the distortion by a spatially varying translation of the blocks. The watermark is extracted by spatial translations of the blocks based on an assumed affine...http://www.google.com.au/patents/US6108434?utm_source=gb-gplus-sharePatent US6108434 - Counteracting geometric distortions for DCT based watermarking
