A framework for clustered-dot color halftone watermarking is considered, wherein watermark patterns are embedded in individ- ual colorant halftones prior to printing and embedded watermarks are detected from scans of the printed images after obtaining esti- mates of the individual halftone separations. The principal challenge in this methodology arises in the watermark detection phase. Typical three-channel RGB scanner systems do not directly provide good es- timates of the four CMYK colorant halftones that are commonly used in color printing systems. To address this challenge, we propose an estimation method that, when used with suitably selected halftone periodicities, jointly exploits the differences in the spatial periodicities and the color (spectra) of the halftone separations to obtain good estimates of the individual halftones from conventional RGB scans. We demonstrate the efficacy of this methodology experimentally us- ing continuous phase modulation for the embedding of independent visual watermark patterns in the individual halftone separations. Wa- termarks detected from the estimates of halftone separations ob- tained using the proposed estimation method have a much higher contrast than those detected directly. We also evaluate the accuracy of the estimated halftones through simulations and demonstrate that the proposed estimation method offers high accuracy. © 2010 SPIE
['Basak Oztan', 'Gaurav Sharma']
Per-separation clustered-dot color halftone watermarks: separation estimation based on spatial frequency content