Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!amdcad!cae780!leadsv!msunix!jon From: jon@msunix.UUCP Newsgroups: sci.physics,comp.graphics,comp.sys.ibm.pc Subject: Re: Color questions Message-ID: <307@msunix.UUCP> Date: Sun, 7-Jun-87 23:20:08 EDT Article-I.D.: msunix.307 Posted: Sun Jun 7 23:20:08 1987 Date-Received: Tue, 9-Jun-87 06:41:45 EDT Organization: Via Visuals Inc. Lines: 81 Xref: utgpu sci.physics:1529 comp.graphics:698 comp.sys.ibm.pc:4087 In article <20537@sun.uucp>, falk@sun.UUCP (Ed Falk) writes: > Here's a related question, does anybody know the conversion factors to > map colors in Red-Green-Blue space to Cyan-Yellow-Magenta-Black? What > I would like to do is generate four images on a high-res laser printer > that can be used directly for printing; i.e. do the seperation in > software. Yeah, I do, but not in a way that would be considered acceptable by the Graphic Arts community. :-) Soon, though... If anyone does know how to do the RGB->CMYK conversion and has results which the Graphic Arts community has judged acceptable, please send me your resume because I have a job opening for you. (no :-)) Makers of prepress equipment don't work in RGB, they use CMYK frame buffers and some op-amp adders to generate (RS-170,-343) RGB video to drive the monitor. There is a good reason for this - their requirements are such that they can't trust the colors on the monitor. When they are picking a color to use, they look the color up in process color book and read the CMYK numbers they need to use. Besides, the color gamuts of video monitors and printed paper are different. So they just think of the monitor as "in the ballpark", but use their Pantone guides to get the *right* color. Also, ambient room light has an effect on the eye's perception of the colors on a video monitor. You can't match the colors on a monitor with those on printed paper without taking a zillion things into account, so why bother? Look the color up in the book and punch the numbers in. Then you get to worry about different types of ink and paper, along with dot gain (inks absorb light better when they land on paper, worse when they land on other ink). Also, you can have square, elliptical, or round dots, and about four different sets of screen angles. Because of all these variables, a proof (Cromalin, Matchprint) is made before the seps are taken to the printer. The proof is supposed to show you what the art is going to look like on paper, given the materials, etc. which you are going to use. Oh yeah, another thing to consider is the amount of UCR (under color removal), replacement of CMY ink with black, not to get a blacker black, but just to reduce the amount of ink used, and sometimes increase contrast and detail. So the people with extreme requirements don't even work in RGB space. Where does that leave the rest of us with RGB frame buffers? Well, I don't know what Eikonix does. I think they're still using the uvL color space. Whether they have a uvL frame buffer and do the RGB conversion on the fly, or use an RGB frame buffer is unknown to me. For people with pure RGB systems, if you can get hold of a uvL color TV analyzer, and a uvL transmission-reflection densitometer, you can come up with tables for the conversion. Make sure you can map the colors from the process book exactly (have an RGB triple in your system that maps to a CMYK quad which is a process color) to keep the Graphic Arts types happy. Or you can use the process color book, and match those to the monitor by eye. Don't laugh, I know some people who have done just that. Another way would be to model the absorption of light by the inks to come up with tables. I'm too much of a feeb at Math and Physics to figure that one out. Anyone ever come up with a model? One important thing that I forgot to mention is that the inks used in printing do not absorb light like an ideal ink. As I recall, yellow ink is the closest to being pure, and magenta is the dirtiest (I think it absorbs a lot of blue in addition to green). So there is no simple formula, despite what a lot of uninformed people think. As far as resolution goes, typical output scanning for good quality magazines (continuous tone art) is 12dots/mm (304dpi), with 255 different dot sizes. Also, each layer of dots is at a different angle, and the dots can be round, square, or elliptical. If there is a laser printer out there capable of printing 192dots/mm (that's > 4800dpi!) on a transparency, there are a lot of Hell flatbed laser scanner owners that would be interested ($500K a pop!). I find it hard to believe that a "high resolution" laser printer could produce anything except rough comps. Final separations, no way. If these numbers are making you laugh, you've never seen a Graphics Arts department manager whip out his loupe and start looking at screen angles, dot shapes, and dot sizes. dave@onfcanim.UUCP and ghn@munsell.UUCP know a lot about this field and could correct any errors in this posting. I'd be interested in hearing anything they had to say about this subject. Jonathan Hue DuPont Design Technologies/Via Visuals leadsv!msunix!jon