(v13) Introduction to halftone screens and calibration

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

The Harlequin RIP incorporates proprietary algorithms for halftone screening, known as Harlequin Precision Screening (HPS^™ ). These algorithms provide moiré-free screening for color reproduction. This chapter describes methods for controlling HPS from PostScript-language extensions; for the most part, the controls are system parameters. HPS does not rely on specialized sets of frequencies and angles, so the controls are mainly for tuning memory use and performance. Most of these controls are available from dialogs in the GUI version of the RIP, so the PostScript-language varieties will not often be needed explicitly in that version.

The RIP also provides methods for overriding various aspects of screens set by a PostScript-language job, whether or not HPS is involved - for example, the frequency and spot shape; again, the GUI version layers dialogs on top of the PostScript-language controls. As well as obtaining a more controlled result, these techniques can make processing a job significantly faster.

The PostScript language expects calibration to be done using transfer functions - see [RB2], and especially the settransfer operator. However, it is common practice for jobs to abuse this to create other effects. For example: to create an image in a color tint, the image is typically represented as 8-bit data using the full range, and the color is produced by varying the range separately in each color component using a scaling transfer function. Therefore, the RIP provides additional facilities for varying the output intensities independently of transfer functions. Also, the RIP directly implements interpolation across a set of values, whereas transfer functions as described in [RB2] have to do this using a complicated series of tests and arithmetic using PostScript-language operators, which is rather slow.

Halftone screens have also been abused to produced special pattern effects. We call these pattern screens. Because of the problems this causes - for example with calibration, or when outputting to a contone device - the RIP detects pattern screens and replaces them with alternatives.