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From: rokicki@neon.Stanford.EDU (Tomas G. Rokicki)
Newsgroups: comp.lang.postscript
Subject: Setscreen, again
Message-ID: <1991Mar17.220700.19388@neon.Stanford.EDU>
Date: 17 Mar 91 22:07:00 GMT
Organization: Computer Science Department, Stanford University, Ca , USA
Lines: 19

Under Level 2 PostScript, you can get more accurate screens than the
ones I talked about in my first note to this group.  But that doesn't
mean you should definitely use them.  Accurate screens are good in many
circumstances, such as color separations and the like, but they have
their drawbacks as well (beyond just their computational expense.)

On a low resolution device, the Level 1 screen for any gray scale will
give you identical `dots' across the page---this tends to look good.
With accurate screens, the dots will very; every other dot might be one
pixel larger, for instance.  This can create detectable patterns in
what should be a single gray scale.  These go away as soon as the
resolution increases enough so that a single pixel enlargement in a
screen `dot' is undetectable.  Of course, at high resolutions, the
default screens become more accurate, too.

Color separation is a very tricky business, and handling the screens
correctly is part of the magic.

-tom