Reversible watermark using difference expansion of quads

A reversible watermarking algorithm with very high data hiding capacity has been developed for colored images. The algorithm allows the watermarking process to be reversed to restore the original image exactly. The algorithm hides triplets of bits in the difference expansion of quads of adjacent pixels. The necessary difference expansion transform and its inverse is derived for quads. Also, the necessary conditions to avoid under- and overflow are derived. The algorithm can also be applied recursively, to maximize the amount of data that can be hidden into an image. Simulation results show that the algorithm can hide a bit-rate as high as 3.3 bits/colored pixel while maintaining an image quality level of 33.5 dB.