Path: news.cs.umb.edu!hsdndev!rutgers!usc!rpi!news.columbia.edu!cunixa.cc.columbia.edu!gmw1
   From: gmw1@cunixa.cc.columbia.edu (Gabe M Wiener)
   Newsgroups: comp.sys.mac.hardware
   Date: 16 Aug 92 00:38:52 GMT
   Sender: usenet@news.columbia.edu (The Network News)
   Organization: Columbia University
   Lines: 30
   Nntp-Posting-Host: cunixa.cc.columbia.edu

   Never having gone from laser to offset before, I'm curious as to the way
   photographs are handled.

   Generally, if one were to take a regular b&w photograph and insert it into
   a paste-up for offset, it would need to be halftoned first.

   My assumption is that if you were to scan an image on the mac and insert
   it into your favorite DTP package and then output it to a LaserWriter, the
   very process of printing gray on the LW, even if you scanned the image in
   grayscale, would essentially do the same as halftoning, since the LW cannot
   print true gray to begin with.

   Several questions:

   1. Is my above assumption correct?

Yes, but your printing house will go ballistic. See below.

   2. What is the effect of using the "Photograde" output of a IIf or IIg?
      Or outputting to a Lino?  I assume the output process in all cases is
      such that the output needs no further processing.
   3. Assuming this is true, then at what resolution would an offset press
      no longer be able to reproduce a dithered gray?  (i.e. how fine is
      too fine?)  

The halftone screening implementable on a 300 dpi laser printer
typically has a screen frequency of about 60 lines/inch.  This is
generally too coarse for reasonable reproduction by offset printing,
where screens around 125 lpi and higher are common.  Unfortunately,
typesetter output is not quite the solution: no matter what the
resolution you supply, the printer's re-screening is likely to produce
objectionable Moire due to the overlap of the two screens.  Hence, the
best results for digital images can be obtained by using a typesetter,
screening at the desired resolution, and have the result shot as line
art, i.e., not halftoned at all. This will require close cooperation
with the production department at your publisher, who, if not
experienced in these matters will tell you to give them the best image
you can---until they see it has been screened. If you have original
photographs, letting them deal with those is even better.

Although aimed at color, an excellent introduction to the Moire issues
can be found in "The Moire Phenomenon in Color Separation", by Isaac
Amidror, in Raster Imaging and Digital Typography II (Proceedings or
RIDT91), Robert A. Morris and Jacque Andre, eds., Cambridge
University Press, ISBN 0-521-41764-3, 1991.

The canonical book on the entire subject is "Digital Halftoning", by
Robert Ulichney, MIT Press, 1987. However, it will not assist you
directly in understanding the needs of the printing industry.

The currentscreen operator will tell you what the default screen
frequency and angle are for your PostScript printer is. If your
spooling system reports printer standard output to you, queing this
PostScript will get it for you.

	%!
	currentscreen pop == ==

The first number returned will be the screen angle, the second the
screen frequency.

If you can only get the physical printed output from your printer,
printing this job will print the screen paramaters for you:

	%!
	/Courier findfont 10 scalefont
	72 72 moveto
	currentscreen pop
	show
	36 0 rmoveto
	show
	showpage

The default screen frequency for a Linotronic PostScript printer is, I
believe 150 lpi. Our LaserWriter IINT and TI microLaser Turbo both
report 60 lpi. I suppose this is the same for all 300 dpi PostScript
printers, or at least those with write-black engines.

Bob Morris
ram@cs.umb.edu