Stable Regions and Object Fill-Based Approach for Document Images Watermarking

In the literature, the document image watermarking schemes in spatial domain mainly focus on text content, so they need to be further improved to be applied on general document content. In this paper, we propose a blindly invisible watermarking approach for grayscale document images in spatial domain, which is based on stable regions and object fill. In order to detect stable regions, the document is transformed into an intermediate form by taking advantage of image processing operations prior to applying nonsubsampled contourlet transform (NSCT). Next, the separated objects in stable regions are obtained by object segmentation. The stroke and fill of obtained objects are detected, and only the locations of object fill are marked as referential ones for mapping to gray level values where data hiding and detection are conducted. Then, the watermarking algorithm is developed by using every group of gray level values corresponding to locations of each object fill for carrying one watermark bit. The experiments are performed with various document contents, and our approach shows high performance in terms of imperceptibility, capacity and robustness against distortions like JPEG compression, geometric transformation and print-and-scan process.