Reversible Watermarking Algorithm for Halftone Images Based on Overlapping Blocks Scanning and Central Pixels Flipping

The halftone image is a special kind of image. In order to authenticate halftone images, this research proposes a reversible watermarking algorithm for halftone images by considering both the central pixels’ flippability and the overlapping blocks scanning. The proposed method takes the flippability of the central pixels in 3 × 3 binary blocks as the fundamental method. Then the special pattern pairs are taken as the embedding objects, whose central pixel flippability is nonzero and won’t change after flipping. In this way, the visual quality can remain satisfying after embedding. Moreover, the proposed method uses overlapping blocks scanning to enlarge the embedding capacity. Thus, this research presents a reversible watermarking algorithm for halftone images with high capacity. Test shows that the algorithm can make the watermarked images maintain good visual quality after embedding a large watermark. Compared to original images, the watermarked images are of low visual distortion and human eyes can’t detect the difference.