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From: gregd@hpgrla.gr.hp.com (Greg Degi)
Newsgroups: comp.text.desktop
Subject: Re: Greyscale images on Postscript devices
Message-ID: <4160001@hpgrla.gr.hp.com>
Date: 20 May 91 14:47:49 GMT
References: <1991May16.124711.1384@aucs.AcadiaU.ca>
Organization: Hewlett-Packard, Greeley, CO
Lines: 19


> My question is how would the output look if the greyscale images
> were done directly by the Postscript typesetter? My scanner can
> do 300DPI with 16 grey levels so I recognize that an upgrade might
> be required at that end.

You won't be happy with a 4bit (16 level) greyscale image rendered on
a typesetter.  Because of the large change in density (darkness) as a
level changes by one count, pictures (particularly large areas of nearly
the same density like faces) start to look like topographic maps. This
"feature" is called contouring.  For good results you need at least 64
levels of grey but in practice the next availible size is 256 levels.
300 dpi is plenty for having a typesetter generate the half-tone from
greyscale information.  You only really need about 1.4 times the screen
size you are using (120 screen is pretty fine) so 300dpi is more than 
enough.

Greg Degi
Hewlett Packard