Xref: utzoo comp.sys.amiga.tech:17423 comp.graphics:15214
Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!radius!pierce
From: pierce@radius.com (Pierce T. Wetter III)
Newsgroups: comp.sys.amiga.tech,comp.graphics
Subject: Re: QMS plotmaster colors & pantone colors
Message-ID: <1390@radius.com>
Date: 5 Jan 91 01:26:37 GMT
References: <521358@neabbs.UUCP> <1990Dec28.225435.214@lavaca.uh.edu>
Followup-To: comp.sys.amiga.tech
Lines: 93

>>The problem is : monitor colors != paper colors.

  Yep. Welcome to Color NonWYSIWYG. Its even worse than that, actually,
  monitor colors today != monitor colors tomorrow.
  printer colors today != printer colors tomorrow

  My current job at radius is writing the software for a little gizmo we
 sell that calibrates monitors so that they're consistent from day to day. 

>The linear transformations needed to go from one chunk of colorspace to
>another are computationally expensive.


  Inks are nasty. Most printers work by printing various size dots of Cyan,
 Yellow, and Magenta. Cyan removes Red light, Yellow removes Green light, and
 Magenta removes Blue light.

  In theory.

  In practice, CMY aren't independent of each other, so C != (1-R).
  Also C+M+Y does not equal black, so printers add a fourth ink, black. 
 (called K for Key. Black ink is also cheaper.)

 Imagine you've got four circles of various sizes and colors that are kind
 of transparent. (four glass circles). size the circles until you get the
 color you want, remembering that you have to count both the overlapping and
 non-overlapping portions. (the color is as seen from 10' away).
 
 That's the simplest possible model.

 Now imagine that you have to ask someone else to cut the circles for you.
 Imagine that person is deaf, and doesn't quite hear you correctly, so the
 circles are sometimes a little bigger or smaller, or not quite circles.

 Got a color matched? Good, now feed the paper in crooked and slide all the
 yellow circles over 1%.


>>My question is, is there some formula to translate screencolor RGB values
>>to QMS plotmaster RGB values. If so I can write a program that reads a
>>picture calculates the paper-RGB's and prints the picture.

   There's an alogrithm:

     gamma correct your monitor.
     convert from your monitors phosphor set to CIE XYZ (a device independent
        color space based on human vision).
     Buy a spectrophotometer.

     Print out every possible combination of C,M,Y,K.
     Measure output swatches with spectrophotometer to make a XYZ-CMYK
        lookup table.

    Convert images to XYZ, then lookup CMYK.

    Print.


   Believe or not, that's about the state of the art (minus details).

><device-1> RGB != <other-device> RGB, unless you're rather lucky.  If I
>display (128,64,64) on my Sun SparcStation w/ Sony monitor, it will *not*
>look like (128,64,64) on an Amiga.  It'll be close, granted, but it won't
>be the same.  Neither one will look like (128,64,64) on <printer>.

   Yeah, but monitor RGB -> XYZ is a 3x3 multiply and some gamma correciton.

>>Another great option would be to translate PANTONE COLORS (some international
>>color numbering system) to RGB values and paper-RGB values

>Well, again, you're screwed.  Seiko has a Pantone (tm) certified
>Color PostScript printer that you might want to look at.  (We're going
>to get one.)  Apparently you can tell it "print Pantone color # x" and it
>generates that Pantone color.  The problem is, nobody makes a Pantone
>certified monitor (that I'm aware of). fyi, Pantone is a company, not an
>ISO standard.  There *are* ISO standards for color mapping, and I'd be
>surprised if somebody hasn't written a rough AmigaIFF<->ISO translator.

   Actually, radius does. Only for the macintosh, though.

   There are a lot of Pantone Certified printers, and Pantone has Pantone
 colors for the screen (using my toolkit), but you have to be a Pantone 
 licensee to get them.

  However, Pantone sells a book with CMYK seperations of the Pantone colros
 for a SWOP standard (standard set of CMYK inks) that you can buy.


Pierce
-- 
My postings are my opinions, and my opinions are my own not that of my employer.
You can get me at radius!pierce@apple.com.
(Wha'ja want? Some cute signature file? Hah! I have real work to do.