Watermarking text document images using edge direction histograms

This paper proposes a novel watermarking algorithm for grayscale text document images. The algorithm inserts the watermark signals through edge direction histograms. The concept of sub-image consistency is developed. The concept means the sub-images have similar-shaped edge direction histograms and it is shown to be valid over a wide range of document images. Algorithms to insert and detect watermark signals are proposed. The experiments performed with various document images produced plausible results in terms of robustness.