Harlequin Precision Screening system parameters
This page applies to Harlequin v13.1r0 and later; both Harlequin Core and Harlequin MultiRIP.
AccurateScreens (system param)
Boolean
Default: false
Controls whether Harlequin Precision Screening will be used. HPS is used if AccurateScreens is true and not if false.
The AccurateScreens parameter in a halftone dictionary specified in [RB3], and the AccurateScreens user parameter listed in Adobe’s various PostScript Level 2 extensions documents, is ignored: HPS can only be turned on explicitly in the GUI, or with a configuration file using the parameters described here. Similarly, setaccuratescreens is ignored (see Screens and color).
HPS2 (system param)
Boolean
Default: true
Assuming AccurateScreens is true (HPS is turned on), when HPS2 is true the RIP uses the HPS2 algorithm, and when false it uses the original HPS algorithm. See HPS2.
ScreenAngleAccuracy (system param)
real
Default: 0.004
This number sets the accuracy for the screen angle selected by HPS relative to the angle requested, in degrees. For example, if ScreenAngleAccuracy is 0.1 and the requested angle is 100 degrees, the angle used by HPS is between 99.9 and 100.1 degrees.
The larger this number, the more likely it is that interference effects are observed in printing. On the other hand, smaller numbers usually require more memory to satisfy the constraint (though often the angle is more accurate than the constraint, so changing the number by a small amount may make no difference to particular screens: the inaccuracy is reported to the console and the log file, so this can be used to judge).
ScreenFrequencyAccuracy (system param)
real
Default: 0.02
This number sets the precision (yes, really) in lpi of the frequencies selected by HPS for the various screen angles. For example, if ScreenFrequencyAccuracy is 0.02, then HPS produces a set of screens for the requested angles that all have frequencies falling within a 0.02 lpi range of one another.
This value must be kept small if visible errors are to be avoided when the separations are combined in printing. It should normally be much stricter than ScreenFrequencyDeviation. However, the RIP relaxes constraints on frequency rather than angle when it has to make compromises to achieve suitable screens given the memory available.
ScreenFrequencyDeviation (system param)
real
Default: 0.07 (7%)
The acceptable deviation of frequencies used by HPS from the frequency actually requested. This value may safely be left relatively large, as quite a large deviation is not noticeable in the finished output. Leaving this value large allows the RIP more latitude to choose the most accurate angles, and is therefore advantageous.
ScreenWithinTolerance (system param)
Boolean
Default: false
Failure to achieve the requested accuracy for HPS screens will cause a PostScript-language error when ScreenWithinTolerance is true. When false, the RIP still attempts to satisfy the constraints, but the resultant screen settings do not necessarily achieve them, especially when memory is low, or the constraints are particularly strict. If the accuracies are not achieved, a warning is given.
ScreenExtraGrays (system param)
Boolean
Default: false
If ScreenExtraGrays is true, the RIP produces the number of distinct gray levels specified by ScreenLevels, even when this is more than the number of pixels in the ordinary halftone screen. Its main use is in allowing more gray levels when the frequency is higher than would normally be used for that resolution. If false, HPS will probably still increase the number of gray levels, but perhaps not enough to give good results, especially for smooth vignettes.
ScreenDotCentered (system param)
Boolean
Default: false
Controls the appearance of the rosettes that are formed when process color separations with suitable angle HPS screens are combined. These rosettes can have a dot in the center or be clear in the center, according to the phase of one separation with respect to another. (See the following diagram.) If the parameter is true, the rosettes are dot-centered, otherwise clear-centered.
Clear-centered rosettes
Dot-centered rosettes
The sense of this parameter is the opposite of the Generate clear centered rosettes checkbox in the HPS Options sub-dialog in the HMR Page Setup dialog.
ScreenAngleSnap (system param)
Boolean
Default: true
When this parameter is true the angle specified in setscreen, setcolorscreen, or sethalftone, or overrideangle, is forced to the nearest multiple of 7.5 degrees; when false the angles actually specified to the screening operators are used. (In both cases, the angles may be adjusted afterwards by up to the value of ScreenAngleAccuracy - see earlier in this section.)
Most Level 1 PostScript-language implementations could not achieve acceptable results with the traditional angles of 0, 15, 45, and 75 degrees, but certain sets of other angles that were close to these could give better results. With HPS, the RIP produces good results with the traditional angle sets, and setting this parameter to true overrides any attempt by jobs to use the other angles.
The 7.5-degree interval is used so that typical angle sets used for both offset and flexographic printing can be dealt with using the same mechanism.
ScreenZeroAdjust (system param)
real
Default: 0.07 (+7%)
A fraction (of the frequency) by which the frequency of the yellow HPS screens is varied (relative to the deviated frequency calculated by HPS for the yellow screen).
Formerly, this parameter was applied to zero-degree screens only, hence its name. However, it now applies to the yellow screen angle as identified by ScreenAngles (see below).
yellow here means the screen whose requested angle matches the angle given in the third element of the ScreenAngles parameter array (typically, though not necessarily, zero degrees - see below). The value may be positive (meaning that the frequency will be increased) or negative (decreased).
Varying the frequency of the least noticeable color gives improved results, because there is less chance of interference between the different separations, and we strongly recommend its use.
ScreenAngles (system param)
array of 4 reals
Default: [15.0 75.0 0.0 45.0]
The four values in this array, which must be greater than or equal to zero, but less than 90, tell HPS that screens with these angles, modulo 90 degrees, are going to be set up together as screen sets for the colors Cyan, Magenta, Yellow, and Black - in that order. This information enables HPS to find matching solutions for all components and to identify the Yellow component. A value of [0.0 0.0 0.0 0.0] indicates that no information is available.