Document identification for copyright protection using centroid detection

A way to discourage illicit reproduction of copyrighted or sensitive documents is to watermark each copy before distribution. A unique mark is embedded in the text whose recipient is registered. The mark can be extracted from a possibly noisy illicit copy, identifying the registered recipient. Most image marking techniques are vulnerable to binarization attack and, hence, not suitable for text marking. We propose a different approach where a text document is marked by shifting certain text lines slightly up or down or words slightly left or right from their original positions. The shifting pattern constitutes the mark and is different on different copies. In this paper we develop and evaluate a method to detect such minute shifts. We describe a marking and identification prototype that implements the proposed method. We present preliminary experimental results which suggest that centroid detection performs remarkably well on line shifts even in the presence of severe distortions introduced by printing, photocopying, scanning, and facsimile transmission.