Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!hc!lll-winken!uunet!mcvax!kth!draken!jmr
From: jmr@nada.kth.se (Jan Michael Rynning)
Newsgroups: comp.lang.postscript
Subject: Re: Getting 255 Grey Levels
Keywords: Apple LaserWriter, image operator, setscreen
Message-ID: <854@draken.nada.kth.se>
Date: 23 Mar 89 11:30:48 GMT
References: <1597@murdu.oz>
Reply-To: jmr@nada.kth.se (Jan Michael Rynning)
Organization: Royal Institute of Technology, Stockholm, Sweden
Lines: 34

In article <1597@murdu.oz> rab@murdu.oz (Richard Alan Brown) writes:
>Firstly, according to the PostScript book (reference manual), the
>LaserWriter should produce 255 grey levels with the image operator if the
>correct parameters have been supplied. With the default settings, it
>seems that in fact you can only get 32 grey levels, yet inspection of the
>output suggests that the printer should be able to do much better than
>this. Reading the red book, it seems that the actual rendering of grey
>levels is done via the setscreen operator, which determines how pixels
>are filled in to acheive various levels from black to white.
>
>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

Jan Michael Rynning,			jmr@nada.kth.se
Department of Numerical Analysis	If you can't fully handle domains:
  and Computing Science,		ARPA: jmr%nada.kth.se@uunet.uu.net
Royal Institute of Technology,		UUCP: {uunet,mcvax,...}!nada.kth.se!jmr
S-100 44 Stockholm,			BITNET: jmr@sekth
Sweden.					Phone: +46-8-7906288