In hiding watermarks in bit-planes of a grayscale image, this paper focuses on reduction of level change caused by bit inversion for improving image quality. First, the level transformation to invert one of bits representing a pixel level while minimizing the whole change is defined. According to its level change properties, a scheme for implementing the transformations so that actual level changes can appear below given limits is presented. In addition, domains of the transformation are defined so that watermarks can be recovered from the transformed images alone. On the other hand, allowable level changes are estimated according to human visual properties. Simulations using test images demonstrated effects of the level transformation on preserving image quality. Also, an example of limited level changes was given and then, an implementation scheme for achieving the limitation was specified.
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