Abstract
We previously proposed a novel technology with which the
images of real objects with no copyright protection could contain invisible
digital watermarking, using spatially modulated illumination. In this “optical
watermarking” technology, we used orthogonal transforms, such as a discrete
cosine transform (DCT) or a Walsh-Hadamard transform (WHT), to produce watermarked
images, where 1-b binary information was embedded into each pixel block. In
this paper, we propose an optimal condition for a technique of robust optical
watermarking that varies the size of pixel blocks by using a trade-off in
the efficiency of embedded watermarking. We conducted experiments where ${\hbox{4}}\times{\hbox{4}}$, ${\hbox{8}}\times{\hbox{8}}$, and ${\hbox{16}}\times{\hbox{16}}$ pixels were used in one block. A detection accuracy of 100% was
obtained by using a block with ${\hbox{16}}\times{\hbox{16}}$ pixels when embedded watermarking was extremely weak, although
the accuracy did not necessarily reach 100% by using blocks with ${\hbox{4}}\times{\hbox{4}}$ or ${\hbox{8}}\times{\hbox{8}}$ pixels under the same embedding conditions. We also examined the
effectiveness of using a Haar discrete wavelet transform (Haar DWT) as an
orthogonal transform under the same experimental condition, and the results
showed that the accuracy of detection was slightly inferior to DCT and WHT
under very weak embedding conditions. The results from experiments revealed
the effectiveness of our new proposal.
© 2012 IEEE
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