Watermarking of Digital Images With the Substitution of Low-Value Bits to Increase Capacity

2. Literature Review

In the visible watermarking, usually transparent watermarked image is produced by adding weight to the edges in an image below watermarking signal is properly preserved. Mintzer et al. (1997) presented a reversible watermarking model image that watermarked image on the host image is visible and the original image can be retrieved. Also, Jung Jin et al. in (2006) presented a visible watermarking method for image authentication that is fully transferable and replaceable. This model is particularly useful for military or medical images. The reversible, visible watermarking method in Yongjian et al. (2006) is a technique based on user key that is applied on visible watermarking signal on human visual system (HVS) to produce visible watermarked image. A user-key method allows authorized users to retrieve high-quality images, while unauthorized users are foreclosed. Shocky et al. in (2006) provided a visible watermarking method using two mapping algorithms: pixel value mapping algorithm (PVMA) and pixel switching algorithm (PPSA). In their method, the pixels are processed by PVMA, and by the PPSA the visible watermarked pixel is formed. By using reverse PPSA and reverse PVMA on watermarked image, the original image is retrieved. In this method for more security random number is added to the watermarking pixels.