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| Experiments with paper photocarriers |
Synthesis procedure of the hologram, its integration into the photographic container and how to restore the watermark, were carried out using a specially designed software package among Builder Borland C + +. In the software package implements a method of Fourier holograms with SSB. In the experiments, the size of the image-container is 256x256 pixels and the size of the watermark was equal to 32x32 pixels. The resulting image with a hidden watermark, output to photo printer (typical mini photo lab Gretag NetPrinter-812) and then recorded on paper. Equal size images 4.3x4.3 centimeters, and the spatial data density was 300 dpi. Contact information recovery watermark involves scanning prints, a hidden watermark. Scanning was performed with a scanner DUOSCAN T1200-Agfa. Spatial sampling density of 600 dpi. Then perform the recovery of watermarks using the software package. Figure 1 shows a monochrome image-container containing a watermark symbol ©. In Fig. 2 shows the results of the recovery of the watermark for the two cases. In the first case (2a) used hologram SSB without diffuser, and the second (2b) - Hologram SSB with diffuser.
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| Fig. 1. Picture with a hidden watermark |
Fig. 2. Recovering watermarks |
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| Experiments with plastic cards |
In addition to experimenting with paper media embedding hidden watermarks and photography on plastic. Plastic cards were kindly provided by the company "Banking Technology", which also provided a record still images on the plastic container. We used the color printer SP 35 Printer Data Card Corporation. In Fig. 3 is a plastic card with a color portrait of the container with a hidden watermark ©. In Fig. 4 is an enlarged detail of the portrait, scanned with a plastic card. The full size of the container is 1026h1029 pixels with spatial density 600dpi. In Fig. 5 shows the result of recovery hidden watermark.
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| Fig. 3. Plastic card with a hidden watermark |
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| Fig. 4. Fragment of the portrait container with water marks |
Fig. 5. Recovered watermark |
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| Conclusions |
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It is established that the Fourier transform watermark ", built in the frequency domain, is
a digital Fourier hologram. The developed method of Fourier hologram with SSB (holographic approach) is much
simplifies deployment and recovery hidden watermark. Shown that the recovery of watermarks from the hologram
sufficient to perform the Fourier transform, or only real or only the imaginary part of the complex hologram.
Experiments with real photo media showed a steady recovery of watermarks using standard equipment.
The best results were obtained for the Fourier holograms without diffuser. These holograms as well withstand impact
through MTF recording equipment and scan, and random geometric distortions caused
offset, rotation and scaling of facsimiles.
The main disadvantage of holograms without diffuser is the presence of abnormal disturbances in picture-carrier.
Smallest perturbations are obtained in the case of holograms with a diffuser. These holograms are resistant to the effects of the MTF
through path, but more sensitive to random geometric distortion listed above. This is due to the appearance of
spotty structures (speckles), which is caused by phase fluctuations. The basic approach to reduce the influence of speckles is
posterior image processing.
Effect of graded characteristics (Fig. 3.)
relate more photocarriers on plastic cards, which is associated with significant nonlinearity gradation curve plastic.
Essentially nonlinear character revealed in a dark area
gradation characteristics for each color (up to inversion of the darkest tones), and the
maximum nonlinearity found for the blue component (Fig. 6.) color image.
Fig. 6. Gradation curve of a plastic card
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