Hiding watermark in watermark [image watermarking]

A hide-watermark-in-watermark approach, based on vector quantisation (VQ), for increasing embedding capacity is presented. The image used for carrying the watermark is first decomposed into nonoverlapping blocks, and the codebook used in the traditional VQ system is split into a number of subcodebooks according to a user key. Then, by referring to the watermark bits to be hidden, a subcodebook is selected and used in the VQ system to code the corresponding block. By applying the above hiding strategy, a binary watermark can be hidden in a gray watermark, and the output result - the watermarked watermark - is then hidden in a cover image.