Halftone phase

This page applies to Harlequin v13.1r0 and later; both Harlequin Core and Harlequin MultiRIP.

HPS is designed to produce moiré-free screens within a manageable amount of memory, while using the discrete raster grid. To do this HPS must make a compromise between memory use and accuracy of angles (which is paramount for moiré-free reproduction) and frequencies. The maximum error in angles and frequencies is controlled with HPS system parameters described in Harlequin Precision Screening .

One effect of this is that over very large pages, the compromises will start to become evident in a shift in registration between different colors. Clear centered rosettes will drift.

There are two ways of reducing this effect without consuming the additional memory that more accurate settings would require: center the screen on the middle of the page so that any drift is halved, and when imposing, re-align the screening separately for each imposed page. The HqnImpose2 procset, described in Advanced Imposition using the HqnImpose2 procset aligns the screens at the center of each page, and setpagedevice automatically sets the phase to the center of the output raster.

These effects are controlled by the sethalftonephase operator. The phase can be determined using currenthalftonephase. The operators, which are modeled on the Display PostScript-language equivalents, are described in the following section.