jet@karazm.math.uh.edu ("J. Eric Townsend") (12/29/90)
[I've xposted this to comp.graphics as some guru there might be able to correct my mistakes or add more information.] In article <521358@neabbs.UUCP> rrs@neabbs.UUCP (RONALD VAN EIJCK) writes: >The problem is : monitor colors != paper colors. There was a panel devoted to this at SIGGRAPH '90. Believe me, this problem exists with probably every computer sold OTC in the world. (Except MacInTrashes working in on/off bitmap mode with a LaserWriter. 1/2 :-) >The solution at the moment is that they have a lot of papers with colors >on them and find the matching color for the color on screen. they then change >the colors on screen (using the DPAINT pallet function). That's a pretty reasonable way to do things considering the level of technology you own and the bucks the average person has to spend. >The colors on screen are wrong then but if you print the picture it looks >alright. (THIS IS NOT A SOLUTION!) For what you're doing, it's probably *the* solution, barring writing a bunch of new code and buying something big to crunch all the numbers. The linear transformations needed to go from one chunk of colorspace to another are computationally expensive. (One of the SIGGRAPH panelists joked that they didn't see a problem with the linear transformation cost, they just ran it on their Convex.) Your other option is to do what I have done and buy a film recorder. The colors match up pretty damn well -- they'd better, since the idea is to take a picture of the "screen" under controlled settings. >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. <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>. You might want to try printing a series of "test pages" where you print little blocks of color with the corresponding amiga or QMS RGB values. Now, you can go back and match up all the little blocks by hand and look for a function that describes the relationship. Have fun. :-( >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. Hell, just play with the knobs on your monitor, that might fix the problem. Or turn on/off various lights in the room. See how complicated the problem of making screen == paper is? -- J. Eric Townsend Internet: jet@uh.edu Bitnet: jet@UHOU Systems Mangler - UH Dept. of Mathematics - (713) 749-2120 "If you are the system administrator and this is the first time you are logging into your system, use the login name root." -- IBM RS/6000 docs
slamont@network.ucsd.edu (Steve Lamont) (12/29/90)
In article <1990Dec28.225435.214@lavaca.uh.edu> jet@karazm.math.uh.edu ("J. Eric Townsend") writes: >In article <521358@neabbs.UUCP> rrs@neabbs.UUCP (RONALD VAN EIJCK) writes: >>The problem is : monitor colors != paper colors. > >There was a panel devoted to this at SIGGRAPH '90. Believe me, this problem >exists with probably every computer sold OTC in the world. ... You might want to look at a paper by Michael G. Lamming and Warren L. Rhodes in the current ACM Transactions on Graphics (Oct 1990, vol 9, no 4) entitled "A Simple Method for Improved Color Printing of Monitor Images." spl (the p stands for paper? Who uses *that*?) -- Steve Lamont, SciViGuy -- 1882p@cc.nps.navy.mil -- a guest on network.ucsd.edu NPS Confuser Center / Code 51 / Naval Postgraduate School / Monterey, CA 93943 What is truth and what is fable, where is Ruth and where is Mabel? - Director/producer John Amiel, heard on NPR
pierce@radius.com (Pierce T. Wetter III) (01/05/91)
>>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.