Clustered-dot periodic halftones : modeling, modulation, and applications

Thesis (Ph. D.)--University of Rochester. Dept. of Electrical and Computer Engineering, 2010.This thesis addresses analyses and applications of periodic clustered-dot halftones that are widely utilized in electrophotographic (laser) and lithographic (offset) printing. Within this broad setting, we address three specific problems: 1) the modeling and analysis of the color shifts induced by inter-separation misregistration, 2) phase and frequency modulation in clustered-dot halftones for watermark embedding and content adaptive halftoning, and 3) per-separation color halftone watermarking by exploiting differences in spatial periodicity and colorant spectra of the individual colorant halftones. The work and contributions in each of these areas are summarized in the following. Misregistration-induced color shifts for periodic clustered-dot color halftones are characterized by combining the Neugebauer model with a periodic lattice representation for the individual halftones to obtain a spatio-spectral model for the color print. Conditions for color misregistration invariance are identified by using the model in terms of colorant spectra, periodicity of the individual separation halftones, dot shapes, and misregistration displacements. Furthermore, quantitative estimates of the color shifts induced by interseparation misregistration are obtained via a hybrid analytical-numerical simulation model that allows assessment of the impact of different halftone parameters such as halftone periodicities, spot functions, inter-separation misregistration displacements, and optical dot gain on the color shift. Our simulation results show good qualitative agreement with the experimental data. We introduce continuous phase-modulated halftones as a general class of clustereddot halftones, wherein the phase of the halftone spots is modulated using a secondary signal. The process is accomplished by using an analytic halftone threshold function that allows halftones to be generated with controlled phase variation in different regions of the printed page. The method can also be used to modulate the screen frequency, albeit with additional constraints. Visible artifacts are minimized/eliminated by ensuring the continuity of the modulation in phase. Limitations and capabilities of the method are analyzed through a quantitative model. We exploited this technique for: a) embedding watermarks in the halftone image by encoding information in phase or in frequency, and b) modulating the screen frequency according to the frequency content of the continuous tone image in order to improve spatial and tonal rendering. Experimental performance is demonstrated for both applications. Finally, we propose a framework for clustered-dot color halftone watermarking, wherein watermark patterns are embedded in individual colorant halftones prior to printing and embedded watermarks are detected, from scans of the printed images, after obtaining estimates 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 estimates of the four CMYK colorant halftones. 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 by embedding independent visual watermark patterns via continuous phase modulation in the individual halftone separations. Watermarks detected from the estimates of halftone separations obtained using the proposed estimation method have a much higher contrast then those detected directly. The accuracy of the estimated halftones is also evaluated through simulations and is seen to be higher than other alternatives