Path: utzoo!mnetor!uunet!oddjob!ncar!ames!eos!labrea!polya!kaufman
From: kaufman@polya.STANFORD.EDU (Marc T. Kaufman)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Choosing closest-color-by-blending
Message-ID: <2578@polya.STANFORD.EDU>
Date: 9 Apr 88 02:21:50 GMT
References: <3184@coherent.com>
Reply-To: kaufman@polya.stanford.edu (Marc T. Kaufman)
Distribution: comp
Organization: Stanford University
Lines: 41
Keywords: RGB blending PixPat

In article <3184@coherent.com> dplatt@coherent.com (Dave Platt) writes:
.I'm working on a Mac II application that draws on the screen in color.
.Currently, the colors are program-specified, but I plan to add a Color
.Picker routine to permit the user to choose any of the colors supported
.by Color QuickDraw.

.I'd like to be able to generate reasonably-accurate hardcopy printouts
.of the screen, using an ImageWriter II with a color ribbon.  The IW II
.supports the old-style Macintosh color model (8 specific colors,
.including black and white), but the printer driver doesn't know how to
.map Color QuickDraw 48-bit color specifications into the 8-color
.model.

.                          The Mac II's Color QuickDraw supports a
.technique like this for drawing within a color window;  one supplies an
.(R,G,B) color as input, and receives in return a colored "brush" (a
.pattern of 8*8 pixels).  Within the "brush", each 2*2 square of pixels
.can combine up to 4 of the colors available in the window's color
.table, with the "average" color falling reasonably close to the (R,G,B)
.input color.

.Unfortunately, this capability necessitates that one have a window
.available whose color table contains only those colors actually
.available for drawing.  Printing isn't performed via a color window...
.it uses an old-style 1-bit-deep GrafPort, and the Color QuickDraw
.routines won't work with it.  So, I'll need to construct such a colored
.brush myself.

Try the following: Draw into an offscreen PixMap, using 8 colors (the
imagewriter CLUT) in a 4-bit deep Map.  While the offscreen device is
active, let Quickdraw generate the color pixpat, using the CLUT you have
provided.  This should yield 4-bit pixels with one bit each of R,G,B (and
one bit unused).

Save this PixMap, and scan it to the Imagewriter driver, doing a SetForeColor
every time the pixel value changes. (grubby, but it probably will work).

[I posted this to mac.programmer only, as the comp.graphics people probably
have a better way :-)]

Marc Kaufman (kaufman@Polya.stanford.edu)