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Pattern and degenerate screens

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

A suitable spot function can be used to produce a pattern rather than a gray-level. However, this can cause problems. Devices which do not require screens reproduce the pattern as a shade of gray. Also, some applications not only use screens to produce patterns, but get it wrong as well, in such a way that they only produce the desired result at multiples of 75 dots per inch (dpi). (In particular, the output works at 300 dpi and 600 dpi, so it appears correct when tested on an Apple LaserWriter or equivalent office printer, but not on high-resolution imagesetters).

Many modern applications have now (in 1997 or earlier) moved to representing patterns as Level 2 Pattern color spaces, or as characters in a font, or otherwise replicating a graphic explicitly. Notable exceptions are FrameMaker, Macromedia Freehand , and ClarisDraw (formerly MacDraw ).

When writing PostScript-language programs, the use of screens as patterns is strongly discouraged. PostScript-language Level 2 patterns are available for this purpose.

To avoid the problems caused by pattern screens, the RIP detects them and replaces them with either a special screen which does not suffer from resolution problems, or a Level 2 PostScript-language pattern, depending on whether the output is halftone or continuous tone.

In some rare circumstances, it may be necessary to turn off the RIP’s automatic pattern screen detection. This is done using the PoorPattern system parameter. Where a halftone screen is approaching degeneracy (has only one intermediate gray level) because it is set to far too high a frequency for the resolution, a screen may be erroneously detected as a pattern; however, this will hardly matter, since the effect would have been wrong anyway.

When a screen is fully degenerate, graphics can only be displayed as either all black (or solid color) or all white. If the RIP produces apparently blank pages, or pages with large black blocks, especially when working at low resolutions, consider whether OverrideFrequency is turned off, and the job is setting a screen with too high a screen ruling.

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