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From: kent@xanth.UUCP (Kent Paul Dolan)
Newsgroups: comp.graphics
Subject: Re: Thining out a bitmap image.
Message-ID: <1649@xanth.UUCP>
Date: Thu, 23-Jul-87 00:57:38 EDT
Article-I.D.: xanth.1649
Posted: Thu Jul 23 00:57:38 1987
Date-Received: Sat, 25-Jul-87 06:40:00 EDT
References: <275@uvicctr.UUCP>
Reply-To: kent@xanth.UUCP (Kent Paul Dolan)
Organization: Old Dominion University, Norfolk Va.
Lines: 35
Keywords: don't bother to look at the surrounding input
Summary: sounds like a halftone screen


John,

	Having read your original and followup posting, and several
	replies, I thought I'd stick my oar in, too.  ;-)

	Back when I used to be involved with the creation of nautical
	charts, we worked with an 800 dots per inch laser onto film
	plotter, and had a similar problem, turning black areas into
	grey areas.

	Our solution depended on the original data being "blobby", rather
	than "speckled"; i.e., the original had no isolated dots.

	We used a physical solution, a half tone screen, just because it
	was _much_ cheaper than trying to process our data after the
	potential for run length coding was destroyed (our graphics format
	was 1.6 billion pixels, 7 bits deep, so we had a bit of a storage
	nightmare without run length encoding).

	Anyway, for your purposes, and if it is appropriate, just develop
	a mask pattern that achieves the separation you want, and then go
	through and turn all the masked out pixels to white, whatever their
	original color was, and only allow the data through where the mask
	allows.  This saves lots of computation, since you don't really
	have to consider more than one pixel at a time.

	If you get in the color business later on, there is an analogous
	use of half tone screens for color.  As I remember, the trick is
	to rotate the mask a number of degrees that avoids moire fringes.

	Consult your local tech library's color press printing literature
	for more detail than I am competent to give.

Kent, the man from xanth.