A holographic technique is applied for digital watermarking by a computer. A
digital-watermark image to be hidden is phase modulated in a random fashion, and
its Fourier-transformed hologram is superposed on a content image. The watermark
is reconstructed by means of a holographic-reconstruction technique from the
bit-map image that hides it. In the study the processes of constructing and
reconstructing a digital hologram are described on the basis of the theory of
Fourier optics. The conditions for superposing the hologram onto the content
images are investigated in detail. The validity of the present method is
verified by changing the weighting of the hologram relative to that of the
content image. The effect of image size is also discussed with respect to
reconstruction of the watermark, and it is shown that watermark information in a
form of a diffuse-type Fourier-transform hologram cannot be removed by cutting
it out of the host image.
© 2002 Optical Society of America
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