T. Kimoto (Japan)
Digital watermark, bit-plane, level transformation, limita tion on level change, self-decodability
A major problem in the hiding of digital watermarks in bit-planes of source signals is the level change caused by the inverting of bits. A level transformation has previously been proposed not only for inverting a specified bit of an input level but also for minimizing the level change. This transformation produces fixed bit patterns in the output lev els, and such patterns are improper for secret watermarks. To reduce this drawback the transformation is modified so that the output levels can include random bits. The relations between input and output levels of the modified transforma tion and the level differences are analyzed in terms of the number of the random bits. Two aspects of the performance of the modified trans formation are then presented. One aspect describes a method for altering the number of the random bits so that the output levels can include as many random bits as possi ble while achieving secure watermarks with level changes less than given limitations. The other one shows theoret ical boundaries of input domains of the transformation to make embedded watermarks decodable alone from the out put signals. These methods are also demonstrated using an example.
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