Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!rutgers!ucsd!ames!lll-winken!uunet!tektronix!sequent!ccssrv!ccsrd11!rob
From: rob@ccsrd11.UUCP (Robert Sleator)
Newsgroups: comp.lang.postscript
Subject: Re: Getting 255 Grey Levels and EOF problems
Keywords: Apple LaserWriter, image operator, setscreen, EOF problems
Message-ID: <292@ccsrd11.UUCP>
Date: 24 Mar 89 19:57:17 GMT
References: <1597@murdu.oz>
Reply-To: rob@ccssrv.UUCP (Robert Sleator)
Organization: Control-C Software, Inc., Beaverton, OR
Lines: 28


In article <1597@murdu.oz> rab@murdu.oz (Richard Alan Brown) writes:
>We have been having a couple of problems with our Apple LaserWriter Plus
>and I was wondering if anyone out there has had the same difficulties:
>
>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.


The number of gray levels you can actually achieve through the halftone screen
mechanism is limited by the number of device pixels that fit in each halftone
cell.  Assume a screen with 60 cells per inch and zero rotation.  On a 300 DPI 
printer this would give 25 device pixels in a cell (300/60 squared), resulting
in 26 possible gray levels ( 0 through 25 pixels turned on).  If the screen is
rotated, the actual cell dimensions are altered slightly, for reasons too
complicated to explain here, and you end up with a few more pixels in the cell
and thus a few more gray levels. 

There are two ways to increase the number of achievable gray levels: increase the
cell size or increase the device resolution.  You can increase the cell size with
the frequency operand of the setscreen operator, the lower the frequency the larger
the cell.  You can increase device resolution by buying a Linotronic.