chris@mimsy.UUCP (Chris Torek) (05/31/87)
In article <725@bsu-cs.UUCP> dhesi@bsu-cs.UUCP (Rahul Dhesi) writes: >... Your typical printer will tell you he or she uses only three >colors, with a little black thrown in. Actually, high quality color magazine printing usually uses four colours plus black. -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690) Domain: chris@mimsy.umd.edu Path: seismo!mimsy!chris
jon@eps2.UUCP (06/02/87)
In article <6861@mimsy.UUCP>, chris@mimsy.UUCP (Chris Torek) writes: > In article <725@bsu-cs.UUCP> dhesi@bsu-cs.UUCP (Rahul Dhesi) writes: > >... Your typical printer will tell you he or she uses only three > >colors, with a little black thrown in. > Actually, high quality color magazine printing usually uses four > colours plus black. No, Rahul is correct. What you see in magazines usually begins as continuous tone art (photographs or transparencies) and are then scanned with a Hell or Crosfield laser scanner to produce four color separations (cyan, magenta, yellow and black) which look like pieces of black and white transparency film with halftone dots on them. A printer can then etch printing plates from these color seps. For the past year I have been sending images back and forth from our workstations to Scitex and Hell pre-press equipment at various color separation houses in the Bay Area and making Cromalins (proofs) so I've got some idea of what's going on here. In printing packages they often use a different technique. They use the color of ink that is actually getting printed, rather they simulate continuous tone with CMYK halftone dots. I'm looking at a "Bounce" fabric softener box and it looks like they used blue, red, and yellow ink to produce a white, yellow, orange and blue package. Jonathan Hue DuPont Design Technologies/Via Visuals leadsv!eps2!jon
chris@mimsy.UUCP (Chris Torek) (06/03/87)
>In article <6861@mimsy.UUCP> I wrote: >>Actually, high quality color magazine printing usually uses four >>colours plus black. What I neglected to mention was that this information was old; I learnt it perhaps ten years ago. The extra color (switching spelling systems will drive me crazy yet!) was brown; apparently whatever processes were used did not reproduce browns very well. In article <93@eps2.UUCP> jon@eps2.UUCP (Jonathan Hue) describes a laser scanning process that produces `four color separations (cyan, magenta, yellow and black)'. Clearly this is a newer process that has taken over now that scanning lasers are cheap. I stand corrected. (We now return to your regularly scheduled news :-) .) -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690) Domain: chris@mimsy.umd.edu Path: seismo!mimsy!chris
falk@sun.UUCP (06/06/87)
In article <93@eps2.UUCP>, jon@eps2.UUCP (Jonathan Hue) writes: > > No, Rahul is correct. What you see in magazines usually begins as > continuous tone art (photographs or transparencies) and are then > scanned with a Hell or Crosfield laser scanner to produce four > color separations (cyan, magenta, yellow and black) which look like > pieces of black and white transparency film with halftone dots on > them. 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. -ed falk, falk@sun.com, sun!falk
jon@msunix.UUCP (06/08/87)
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
amamaral@elrond.CalComp.COM (Alan Amaral) (06/11/87)
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. > > -ed falk, falk@sun.com, sun!falk It's trivial. where R=G=B, then Black = 1-R (or G or B) C=M=Y=0 else black = 0 +- -+ +- -+ +- -+ | R | | 1 | | C | | G | = | 1 | - | M | | B | | 1 | | Y | +- -+ +- -+ +- -+ -- uucp: ...decvax!elrond!amamaral I would rather be a phone: (603) 885-8075 fool than a king... us mail: Calcomp/Sanders DPD (PTP2-2D01) Hudson NH 03051-0908
jon@eps2.UUCP (06/12/87)
In article <943@elrond.CalComp.COM>, amamaral@elrond.CalComp.COM (Alan Amaral) writes: > It's trivial. Oh come on. Everyone who reads this newsgroup knows how to translate between the RGB and CMY color spaces. And I'm sure everyone could figure out how to do the crude undercolor removal... The question Ed asks is not the simple one that you think he is asking. From his posting it is obvious that he knows about how real inks used by printers work, and that they do not absorb light like ideal cyan, magenta, and yellow inks. None of the simple solutions I've posted to the net are adequate. I have a Pantone color simulator in front of me. Just looking at the front page, I can see that there is no way you can print a bright orange color on paper without using ink that is actually bright orange. I asked this same question about a year ago. One useful piece of information I received was from someone at the University of Utah. He told me that Maureen Stone at Xerox PARC has done work in this area. I found out she has published papers and she recently gave a talk out here. Ed, one thing you may want to do is buy a Pantone color simulator. You can get a feel for what the different inks look like when mixed together. The book consists of about 500 of the Pantone process colors, and then a color chip next to them showing a close approximation using CMYK ink. There are three more CMYK color chips which can be torn out next to the big chip. You can find this at most any art store. The price is a bit steep, $200. The other four Pantone books which make up the rest of the set are only $250 total. Of course the colors printed are dependent on the ink, paper, screen angles, etc, but you can get an idea of how things work from this book. I know that Eikonix has a lot of people who are experts in this area, but they're not going to divulge a lot of the information because it is proprietary. My personal opinion is that if Eikonix ever gets around to announcing a Sun-3 prepress machine (if they haven't already), one of their big features will be their expertise in color fidelity. Our systems are targeted at design, not production, so our "big features" are our tools for graphic designers. I made an error in a previous posting about halftone dot sizes. Although prepress machines store their pixels as 8-bit quantities, typically dot size is controllable on an output scanner only to 2% increments, so there are only 50 different dot sizes. The top of the line Hell scanner generates dots electronically at a maximum of 146 dots/mm. If you are doing 12 halftone dots/mm, that gives you about a 12x12 grid of dots to simulate your halftone dot, so I guess you could assume 144 dot sizes. Jonathan Hue DuPont Design Technologies/Via Visuals leadsv!eps2!jon
tkxyv@mergvax.UUCP (William Klein) (06/15/87)
In article <307@msunix.UUCP>, jon@msunix.UUCP (Jonathan Hue) writes: > ... > 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 > ... > 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 > ...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. At Linotype we make equipment for the graphics arts industry. The odd thing about this industry is that the money (in color, at least) is where the quality is. We have just evaluated our high resolution laser output printer for its suitability for color work, and it is just barely good enough to do kind of lousy color proofs (in the words of a graphics arts dept mgr). This device outputs 2540 dpi, with an error across the whole raster (for newspaper or wider widths) of <60 microns. No slouch, but nowhere near good enough for color work. Indeed, you do need ~5K dpi to get good color output. And the error must be extremely low for screen registration: the eye is amazingly good at picking up moire (beat frequencies between the different colored dot screens in the space domain) which can be caused by only a few microns of misregistration between the screens. The kind of color photographs that you see on the covers of Vogue, Life, National Geographic, etc. go through an incredible number of iterative proofs which involve one test print after the other being passed by the customer for color checks, overall impression evaluations, and all kinds of subjective judgements. Not easily automatable (yet, where are you AI and CE people!?!). Forget electronic (screen) proofing: you can't get color anywhere close (to a graphics arts person) on the screen to what the ink will look like. You can't even print proofs that look close enough for many customers without actually printing the proofs on the same press, using the same ink and paper as will be used on final output. Some customers even have a 'standard' light which will illuminate the proof with a known set of frequencies for the color evaluation. (A printed color photo will look quite different under tungsten light, flourescent, 'warm' flourescent, and daylight conditions). With all of these variables, most of what the video pre-proofing is used for is cropping, some editing, perhaps some drawing (esp in ad layout) and the like. No color adjustment. Perhaps when we get video monitors with 60K by 40K pixels, and a RFB and blit engine fast enough to draw them all in a short time, and some kind of standard colors... (:-)) -- Life is what happens to you when you are busy making other plans. Real Life: W. Ted Klein UUCP: ihnp4!motown!mergvax!ted VOICE: 516-434-2687
klm@munsell.UUCP (06/18/87)
In article <153@mergvax.UUCP> tkxyv@mergvax.UUCP (William Klein) writes: > >Forget electronic (screen) proofing: you can't get color anywhere >close (to a graphics arts person) on the screen to what the ink will look >like. I beg your pardon? Our customers seem to be very pleased with what we've done. It took a lot of color science, brain smoke, and patience, but we're very proud of what we've accomplished. >You can't even print proofs that look close enough for many customers >without actually printing the proofs on the same press, using the same ink >and paper as will be used on final output. Sure you can. I think we're the only ones who have figured out HOW, or who have bothered to do it, but it can be done. >Some customers even have a >'standard' light which will illuminate the proof with a known set of >frequencies for the color evaluation. (A printed color photo will look >quite different under tungsten light, flourescent, 'warm' flourescent, >and daylight conditions). With all of these variables, most of what the >video pre-proofing is used for is cropping, some editing, perhaps some >drawing (esp in ad layout) and the like. No color adjustment. The main selling point of our CEPS is that you *can* do color correction on the screen, and that while on many analog scanners (Hell, Crosfield, et al.) you have to adjust, proof, adjust, proof, adjust... we put a little more effort into the color science aspect of it instead of just the mechanics and we came up with a system that, when properly calibrated, produces a video proof that is nearly indistinguishable from a hardcopy proof. This is assuming of course that both are viewed under proper lighting conditions. Plus, we allow the customer the freedom of modifying the calibration tables to suit his own taste, so HE/SHE can get the results that look good to HIM/HER. Unfortunately we still have the big problem that while color can be described very accurately by science, human color perception is still a very much subjective thing, and we all know from experience that sometimes there's just no accounting for a person's taste. Stay tuned for more. The fun is just beginning. -- Kevin McBride |Disclaimer: These | harvard -\ Eikonix - A Kodak Co. | opinions are mine, | ll-xn ---adelie-----> munsell!klm 23 Crosby Dr. | not my employer's, | decvax -v talcott -v | Bedford, MA 01730 | So There! | allegra ------------encore
klm@munsell.UUCP (06/18/87)
Our ears are burning... A few of us here at Eikonix are wading through this glut of postings and we will shortly attempt to answer as many of the questions as we can. As one poster already pointed out, some of what we know to be fact is proprietary, so we'll have to gloss around it. We're also going to give references to some papers we've published on the subject, plus some other hopefully useful sources. Hold your breath until you turn a nice shade of cyan :-) BTW, I really liked the 'trivial' answer to the RGB -> CMY transform, although it really should have been posted to rec.humor. The transform from a 3-D color space (RGB, uvL, etc.) to a 4-D color space (CMYK) is not trivial and is not solvable in closed form. You gotta go backwards first and build a table that allows you to weed out the multiple choices when you go forward again. I know this explanation is rather sophomoric, but it's kind of late, I'm getting tired, and I want to be careful that I don't give out any secrets that would cause my employers to get pissed at me. I'm not quite ready to take up the job offer from the guy at DuPont (?) Stay tuned to sci.physics. That's where we'll be posting. No cross postings unless I get sufficient demand. -- Kevin McBride |Disclaimer: These | harvard -\ Eikonix - A Kodak Co. | opinions are mine, | ll-xn ---adelie-----> munsell!klm 23 Crosby Dr. | not my employer's, | decvax -v talcott -v | Bedford, MA 01730 | So There! | allegra ------------encore
cetron@utah-cs.UUCP (Edward J Cetron) (06/19/87)
In article <1080@knopfler.munsell.UUCP> klm@munsell.UUCP (Kevin McBride) writes: >Stay tuned to sci.physics. That's where we'll be posting. No cross postings >unless I get sufficient demand. demand, demand, demand.... please cross post them here, I for one would like to see them. -ed