ScreenLevels (screening system param)

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

Integer or array of two integers 

Default: 1024

The number of tone levels the RIP uses is limited by the /ScreenLevels system parameter. It limits the RIP to using no more than the given number of tones (defaulting to 1024) which reduces the memory requirements and improve performance of screens with large cells. This limitation is not applied to modular screens, nor by default to small threshold screens (types 3, 6 and 10); see ScreenExtraGrays in a halftone dictionary. The upper limit on the number of distinct gray levels is 65280.

Screens with very large numbers of gray levels can use a great deal of memory, often for no gain beyond a few thousand levels. Such screens can come about in three ways:

(1)  A low line ruling with a high resolution can generate many gray levels,

(2)  Using HPS at more usual frequencies may have the same effect, and

(3)  An advanced threshold screen may have cell dimensions of hundreds of pixels (Harlequin's HXM, HDS, and AIS screens have them).

ScreenLevels puts an upper limit on the number of gray levels in a screen, and regulates the number of levels to which HPS increases it when the ScreenExtraGrays system parameter is true. If a single number is given, the value is used for both purposes; if an array of two numbers is given, the first sets the upper limit, and the second is the HPS parameter. The second element must not be smaller than the first.

HPS increases the number of levels until it is higher than this parameter, and then only as many of those potential levels are used as the limit allows. In any event, the actual levels made available are always distributed as evenly as possible across the range of potential levels.

To switch off this limitation for type 16 screens, you can add the following PostScript language snippet to each Type 16 halftone dictionary:

/LimitScreenLevels false