Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!purdue!bu-cs!husc6!endor!stew
From: stew@endor.harvard.edu (Stew Rubenstein)
Newsgroups: comp.lang.postscript
Subject: Re: Getting 255 Grey Levels
Keywords: Apple LaserWriter, image operator, setscreen
Message-ID: <1479@husc6.harvard.edu>
Date: 25 Mar 89 03:55:48 GMT
References: <1597@murdu.oz> <854@draken.nada.kth.se>
Sender: news@husc6.harvard.edu
Reply-To: stew@endor.UUCP (Stew Rubenstein)
Organization: Aiken Computation Lab Harvard, Cambridge, MA
Lines: 36

In article <854@draken.nada.kth.se> jmr@nada.kth.se (Jan Michael Rynning) writes:
>In article <1597@murdu.oz> rab@murdu.oz (Richard Alan Brown) writes:
>>Has anyone out there figured out how to modify the default setscreen
>>parameters to get 255 grey levels?
>
>The default screen frequency is 60 dots/inch on an Apple LaserWriter,
>which has a resolution of 300 pixels/inch.  300/60=5, so each screen
>dot has a maximum size of 5 by 5 pixels.  Making all those 25 pixels
>black, gives you a black printout.  Making 0 of them black, gives you
>a white printout.  That means you can achieve 26 grey levels at this
>screen frequency.  If you want more grey levels, you will either have
>to lower the frequency or use a printer with a higher resolution.  As
>you need at least 16 by 16 pixels to get 256 grey levels, the screen
>frequency to use on the Apple LaserWriter is 300/16=19.
>
>If you want to try this on your Apple LaserWriter, add the following
>line to your PostScript program to set the screen frequency to 19:
>
>currentscreen 3 -1 roll pop 19 3 1 roll setscreen

Sorry, it's not that simple.  The default screen on the laserwriter
is not at 90 degrees, so it's not a simple 5x5 box.  In fact, the
default screen is at 45 degrees, and has 32 dots, so you get 33 gray
levels.

To get more gray levels, you need a coarser screen.  To make matters
more complicated, you don't generally get the exact frequency and
screen angle you request, because it has to take the closest values
which are possible given the limited resolution of the printer.  For a
good discussion of this, see chapter 10 of "Real Word PostScript",
Stephen F. Roth, ed., Addison-Wesley, 1988.

Stew Rubenstein
Cambridge Scientific Computing, Inc.
UUCPnet:    seismo!harvard!rubenstein            CompuServe: 76525,421
Internet:   rubenstein@harvard.harvard.edu       MCIMail:    CSC