Quantization effects on digital watermarks

Digital watermarking is the process of introducing small modifications into a copy of a digital document that can be detected later. The embedded info rmation can be used to determine the document's owner or simply to distingu ish several copies. However, coincidental or malicious "attacks" can degrad e the robustness of watermark detection. Here, uniform scalar quantization of watermarked documents is investigated theoretically, extending results f rom theory of dithered quantization, and experimentally. The watermark is e mbedded by an independent additive pseudo-noise sequence. The statistical d istribution of the quantization errors depending on the statistics of the h ost signal and the watermark is used to determine the robustness of waterma rk detection via correlation. Experiments with JPEG compression of an image with a DCT-domain additive watermark demonstrate the usefulness of the pre sented theory. (C) 2001 Elsevier Science B.V. All rights reserved.