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From: brown@vidiot.UUCP (Vidiot)
Newsgroups: comp.lang.postscript
Subject: Re: setscreen pixels
Message-ID: <1474@vidiot.UUCP>
Date: 9 Mar 91 05:39:40 GMT
References: <1454@vidiot.UUCP> <2349@umriscc.isc.umr.edu>
Reply-To: brown@vidiot.UUCP (Vidiot)
Organization: Vidiot's Hangout
Lines: 56

In article <2349@umriscc.isc.umr.edu> jmd@ee.umr.edu (Jim Dumser) writes:
<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_)."

Ya, I read that, but it doesn't answer the question asked next.  It doesn't
say how the the number of pixels in the cell is computed, which is what I
was asking for.

<>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."

Ya, I read that too.  Still doesn't answer the question.

<>                     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.

I know that, still doesn't answer the question.

<>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.

So, what are the internal formulas??????
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