Steganography in halftone images: conjugate error diffusion

In this paper, we propose a method called data hiding by conjugate error diffusion (DHCED) to hide an invisible binary visual pattern in two or more error diffused halftone images, which can be from the same or different multi-tone images. In DHCED, distortions of limited magnitude are added to the multi-tone image such that the pixel values of the various halftone images tend to be conjugate to each other. With the conjugate relationship, the dark regions of the hidden pattern would appear on the halftone images when they are overlaid. Simulation results show that the DHCED watermarked halftone images have good visual quality, and the hidden pattern is revealed with good contrast when the watermarked halftone images are overlaid. Even better contrast can be achieved when more than two watermarked halftone images are used.