Re: color selector with cmyk anyone?
On Wed, May 24, 2000 at 11:07:34PM -0400, Carey Bunks <[EMAIL PROTECTED]> wrote: > However, the conversion from CMYK to RGB is unique and is described by > the following formulas: While the formulas are indeed what Uwe wants, the CMYK to RGB transformation still is not unique, simply because neither "RGB" nor "CMYK" are absolute colourspaces. The RGB modul gimp uses has no relation to any physical colour model, which means that the RGB space of my machine(+monitor) is different to most other machines(+monitors). Now, if we knew the parameters of the RGB space in use of an image, and the parameters of the CMYK space in use on the printer, then the conversion would only depend on the policy, everything else is just as much as we have now (i.e. a plug-in that simply seperates RGB into CMYK). To really get profressional, the whole picture gets even more complex: there is the colourmodel of the image that has to be viewed in another colourmodel (gfxcard/monitor) and printed in a third. While, for web work, none of these are usually known, we can expect that professionals do have access to all parameters of all their hardware. -- -==- | ==-- _ | ---==---(_)__ __ __ Marc Lehmann +-- --==---/ / _ \/ // /\ \/ / [EMAIL PROTECTED] |e| -=/_/_//_/\_,_/ /_/\_\ XX11-RIPE --+ The choice of a GNU generation | |
Re: color selector with cmyk anyone?
[Sent earlier to individual instead of list by accident - remarkably similar to a subsequent mail on the list!] At 01:54 AM 5/25/00 +0200, you wrote: >Yes, I know this, but I only need an easy tool to convert this CMYK colors >to RGB. Not for an exact representation -- only for a start! > >So is there anyone with such a tool, or the basic transformation rules so >that I can write my own? If you only want an approximation, it is relatively straightforward. The C, M and Y channels of the CMY colour space are effectively just the R, G and B channels flipped. R = 1 - C G = 1 - M B = 1 - Y (assuming they are specified in the range 0 - 1). The main reason for CMYK is that the CMY inks used in printing are not pure filters corresponding to the spectra of the R, G and B channels. In order to print good blacks and greys, some proportion of C, M and Y is removed and replaced with black (K). The simplest technique for this is Grey Component Replacement (GCR), which simply removes common proportions of C, M and Y and replaces them with the same proportion of black. For example (moving to percentages now), 30C 20M 60Y would become 10C 0M 40Y 20K. There are other techniques, and many more issues than this, including the exact inks used, the printing processes, local colour calibration, printing stock absorbency and colour, whether the printing is to be back-lit and so on, but this is just a very brief overview. Real conversion seems to be based on curves which effectively map each combination of RGB to a corresponding CMYK, taking into account as many of these issues as possible. There is no real meaning in a set of CMYK values in isolation (although the same can be said about RGB...). Using this poor approximation, you could convert from CMYK to RGB using the following rules: R = 1 - (C + K) G = 1 - (M + K) B = 1 - (Y + K) Hopefully I haven't slipped up anywhwere. Hope this helps, Ian
Re: color selector with cmyk anyone?
So is there anyone with such a tool, or the basic transformation rules so that I can write my own? Hi Uwe, The map from RGB to CMYK is not unique because it depends on a printing policy that specifies the percentage black that should be in a pixel. This is often referred to as GCR (or gray componenet replacement). It is important for a variety of reasons including how much ink is needed to print an image (and thus the speed at with the print will dry) the size of printable dots on a page, etc. However, the conversion from CMYK to RGB is unique and is described by the following formulas: Presuming that C, M, Y, and K are expressed with values in [0,1] then: R = 1-(C+K) G = 1-(M+K) B = 1-(Y+K) If C, M, Y, and K are not in [0,1], normalize the equations so they are. Best regards, Carey PS You may want to read the discussion on CMYK given here: http://gimp-savvy.com/BOOK/node53.html I also recommend this section to those attending GimpCon as it describes those features needed in the GIMP for it to become CMYK capable. This would finally make the GIMP a viable tool for the pre-press world. Dr. Carey Bunks Senior Scientist BBN Technologies 70 Fawcett St, 15/2A Cambridge, MA 02138 tel: 617-873-3028 fax: 617-873-2918 email: [EMAIL PROTECTED]
Re: color selector with cmyk anyone?
You wrote on Mit, 24 Mai 2000: >On Wed, May 24, 2000 at 03:16:39AM +0200, Uwe Koloska ><[EMAIL PROTECTED]> wrote: >> colors are given as DIC numbers or CMYK. So my questions: >> >> o Is there an easy to use tool for selecting colors as CMYK and give the >> RGB? Yes, CMYK gamut != RGB gamut. > >No, both "RGB" and "CMYK" re just models, not specific colour volumes with >specified colours. "Some CMYK model's gamut" != "Some RGB model's gamut" >is the point, and this makes the question of "converting RGB to CMYK" >somewhat mood, unless you have calibrated colour volumes (in which case it >becomes easy). Yes, I know this, but I only need an easy tool to convert this CMYK colors to RGB. Not for an exact representation -- only for a start! So is there anyone with such a tool, or the basic transformation rules so that I can write my own? Uwe -- mailto:[EMAIL PROTECTED] http://rcswww.urz.tu-dresden.de/~koloska/ ---- right now the web page is in german only but this will change as time goes by ;-)
Re: color selector with cmyk anyone?
On Wed, May 24, 2000 at 03:16:39AM +0200, Uwe Koloska <[EMAIL PROTECTED]> wrote: > colors are given as DIC numbers or CMYK. So my questions: > > o Is there an easy to use tool for selecting colors as CMYK and give the > RGB? Yes, CMYK gamut != RGB gamut. No, both "RGB" and "CMYK" re just models, not specific colour volumes with specified colours. "Some CMYK model's gamut" != "Some RGB model's gamut" is the point, and this makes the question of "converting RGB to CMYK" somewhat mood, unless you have calibrated colour volumes (in which case it becomes easy). -- -==- | ==-- _ | ---==---(_)__ __ __ Marc Lehmann +-- --==---/ / _ \/ // /\ \/ / [EMAIL PROTECTED] |e| -=/_/_//_/\_,_/ /_/\_\ XX11-RIPE --+ The choice of a GNU generation | |
color selector with cmyk anyone?
Hello, I finally found some books about color harmony (that's exactly the title of the series -- brandnew in europe, but originally from 1997). But all the colors are given as DIC numbers or CMYK. So my questions: o Is there an easy to use tool for selecting colors as CMYK and give the RGB? Yes, CMYK gamut != RGB gamut. I am only looking for a starting point so that I don't have to startup photoshop for looking what RGB I have to choose ... o is there a DIC palette / color chooser for gimp? For the long run I wanna try to make something like a matching color browser. Yours Uwe Koloska -- mailto:[EMAIL PROTECTED] http://rcswww.urz.tu-dresden.de/~koloska/ ---- right now the web page is in german only but this will change as time goes by ;-)
