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From: jmd@ee.umr.edu (Jim Dumser)
Newsgroups: comp.lang.postscript
Subject: Re: setscreen pixels
Message-ID: <2349@umriscc.isc.umr.edu>
Date: 8 Mar 91 14:19:23 GMT
References: <1454@vidiot.UUCP>
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Organization: University of Missouri - Rolla
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In article <1454@vidiot.UUCP> brown@vidiot.UUCP (Vidiot) writes:
>As an example, if the frequency is 60, and a 300 dpi printer is used,
>that leaves 5 pixels to define a shade of gray.  Square that value to
>get a spot that is 5x5, or 25 shades.  Since you could have all white,
>add one to that, yielding a total number of shades to 26.  The formula
>that I have seen is:
>                      2
>	(dpi/frequency) +1

Quoting from the red (& white) book [p312]:
  "If a cell contains _n_ pixels, then it can render _n_+1 different
  gray levels: all pixels black, one pixel white, two pixels white,
  ..._n_-1 pixels white, all _n_ pixels white.  A desired gray value
  _g_ in the range 0 to 1 is produced by making _i_ pixels white, where
  _i_=floor(_g_*_n_)."

>But, nothing has ever said what to do about fractions.  If the
>frequency is 40, that gives 7.5 to be squared.  You can't have 7.5 dots
>to a side.  Does the printer use floor or round to get the number of
>pixels in the cell?

[p314]:
  "the *sethalftone* or *setscreen* operator may make slight
  adjustments to the requested frequency and angle to ensure that the
  patterns of enclosed pixels remain constant as the screen cells are
  replicated over the entire page."

>                     What affect does the angle have in all of this?

The angle specifies the orientation of the cell borders relative to the
device's coordinates.

>In other words, what is the internal working formula for determining
>what the printer will use for the number of pixels in the cell?  Are
>the formulas different for the Apple LaserWriter IINT and the
>Lintotronic 1270 dpi?

In Level 2, halftone dictionaries have an optional entry,
AccurateScreens, that cause "extremely precise, but computationally
expensive" algorithms to be used.  Also, they can have entries that get
set with the actual frequency and angle used.

Jim

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