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From: meekins@anaconda.cis.ohio-state.edu (Tim Meekins)
Newsgroups: comp.sys.apple2
Subject: Re: Color Graphics Printing
Message-ID: <115322@tut.cis.ohio-state.edu>
Date: 2 May 91 00:32:20 GMT
References: <1991May1.173724.13975@nevada.edu>
Sender: news@tut.cis.ohio-state.edu
Organization: The Ohio State University, Department of Computer and Information Science
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In article <1991May1.173724.13975@nevada.edu> alfter@nevada.edu (SCOTT ALFTER) writes:

>red, yellow, blue, and black.  What formulas should be applied to
>generate the color separations?  After that, is there an efficient way

C: Cyan or blue
M: Magenta or red
Y: Yellor or (you guessed it) yellow.

First RGB -> CMY

Since CMY is the inverse of RGB, it's as easy as:

C = 1-R
M = 1-G
Y = 1-B

Now, to take advantage of the black ribbon:

CMY -> CMYK, where K is black

K = min(C,M,Y) from above
C = C - K
M = M - K
Y = Y - K


>to generate a halfway-decent range of halftones?  A way to make 256
>different shades would be especially nice, but I'll probably have to
>settle for 64 shades (8x8 matrix for halftoning at 72x72 dpi).  Maybe
>the GS system software gurus can help; they have to do the same stuff
>in the Imagewriter II drivers, but any code examples must be usable on
>8-bit machines (I'm writing the programs on my IIe).
>

Hmmm. Now dithering is a whole new ballgame. With a little work, I'm
sure you can figure out ordered dithering. But, common use in printing
is the Floyd-Steinberg error diffusion. Not something I can really
go into in a post like this. Also, dithering is a lot more complicated
when working with several colors, namely CMYK. Most algorithms are for
gray scale dithering, not several-color dithering.
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