Continuous Phase-Modulated Halftones

Abstract

Abstract-A generalization of periodic clustered-dot halftones is proposed, 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. The technique can be exploited for two applications that are presented in this paper: 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.

Correcting illumination variations in photomicrograph mosaics of daguerreotypes

“…The origins of the tiles are then modeled as x 0 lk = lPx +T kl , and y 0 lk = kPy +W kl , where T kl , and W kl are random variables representing the positioning errors in relation to these nominal displacement pitches, which we assume are smoothly varying function of l and k. The power spectrum of l f (x, y), the random pattern of illumination variation can then be analyzed by making assumptions on the statistics of the positioning errors, or by considering the case where they correspond to a slow periodic function (see, for example [2]). The analysis reveals that the power is concentrated at DC and about harmonics of the 2-D frequency [1/Px, 1/Py].…”

Per-separation clustered-dot color halftone watermarks: separation estimation based on spatial frequency content

“…Continuous phase-modulated halftoning 13 alters the halftone phase through a modulation of the phase of an underlying analytic halftone threshold function. This method has the benefit that the watermark pattern may be decided upon dynamically just prior to halftoning, as opposed to some of the alternate embedding strategies where the pattern must be incorporated in the design of the halftone thresholds and is therefore static.…”

“…These estimates, though not perfect (due to moiré), do allow for a detection of the embedded watermarks on a per-channel basis. We exploit this methodology to extend halftone watermarking using continuous phase modulation that was introduced for monochrome halftones 13 to color halftones. Note that the present work builds upon and extends our prior work in this area 14 that addressed the more limited setting of three color CMY printing.…”

Multiplexed clustered-dot halftone watermarks using bi-directional phase modulation and detection

“…A majority of these methodologies [4][5][6][7][8] mimic the conventional paper watermarks and utilize visual watermark patterns such as bi-level text images or logos. The watermarks are embedded in the form of changes in the halftone structures that are normally imperceptible but can be revealed by appropriate detection methods.…”

“…3]. As digital printing methods become commonplace, the flexibility that these methods offer over conventional analog printing has also been exploited in order to develop a number of alternative digital watermarking methods for hardcopy prints.Halftoning is typically utilized for the reproduction of images in most digital printing systems [3] and many of the digital hardcopy watermarking techniques exploit the characteristics of the halftoning process for embedding watermarks in the printed images [4][5][6][7][8][9][10]. A majority of these methodologies [4][5][6][7][8] mimic the conventional paper watermarks and utilize visual watermark patterns such as bi-level text images or logos.…”