Path: utzoo!attcan!uunet!mcsun!cernvax!chx400!sgzh!root
From: root@sgzh.uucp (Bruno Pape)
Newsgroups: comp.sys.sgi
Subject: Re: How Many Gray Scales On Laserwriter II NTX?
Message-ID: <1990Mar28.170643.2251@sgzh.uucp>
Date: 28 Mar 90 17:06:43 GMT
References: <5890597@um.cc.umich.edu> <1990Mar26.134330.12298@sgzh.uucp> <1465@dftsrv.gsfc.nasa.gov>
Reply-To: root@sgzh.UUCP (Bruno Pape)
Organization: Silicon Graphics S.A., Zuerich, Switzerland
Lines: 43

In article <1465@dftsrv.gsfc.nasa.gov> buck@drax.gsfc.nasa.gov (Loren (Buck) Buchanan) writes:
>In article <1990Mar26.134330.12298@sgzh.uucp> root@sgzh.UUCP (Bruno Pape) writes:
>>In article <5890597@um.cc.umich.edu> Tim_Buxton@UM.CC.UMICH.EDU writes:
>>>
>[[[about limited number of gray levels when printing]]]
>
>The answer is you can change the number of gray levels by using the
>setscreen command.  The trade off is that to get more gray levels you
>have to increase the spot size.  For a 300 dpi printer, I believe the
>default screen setting gives 37 shades of grey (36 pixels per spot).
>

I was wrong about the spot function.  The spot function determines in what
order the pixels are changed from black to white when stepping through the
shades.  The PS Language Refenece Manual, page 85.

The problem with using 256 gray shades is the size of the spot required.
Using:
		currentscreen 3 -1 roll pop 18.75 3 1 roll setscreen

You can change the frequency to 18.75 which gives 257 gray shades.
( 16 dots x 16 dots = 256 dots = 257 shades, 300 dpi / 16 dots = 18.75 )
Which is almost 1/16 of an inch per cell.  It's a big cell.

A cell that is 1/18 of an inch will not look good in detail images, like
fractals, but it looks ok if you use the whole page to shade from black
to white.

On a 300 dpi QMS PS 810 the default frequency is 60.0, or a 5x5 spot size
for 26 shades of gray. 

And a couple questions for my friends in Mountain View.

	Why doesn't the above have any affect in psh?
		Your not really doing half-toning?

	If your not doing half-toning, because you can get all the
	shades you need by varing the intensity of every pixel, then
	why does psh only give about 10-20 shades of gray?

Thanks,

Bruno