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 information can be used to determine the document's owner or simply to distinguish several copies. However, coincidental or malicious "attacks" can degrade the robustness of watermark detection. Here, uniform scalar quantization of watermarked documents is investigated theoretically, extending results from theory of dithered quantization, and experimentally. The watermark is embedded by an independent additive pseudo-noise sequence. The statistical distribution of the quantization errors depending on the statistics of the host signal and the watermark is used to determine the robustness of watermark detection via correlation. Experiments with JPEG compression of an image with a DCT-domain additive watermark demonstrate the usefulness of the presented theory. (C) 2001 Elsevier Science B.V. All rights reserved.