Re: [NTG-context] Metapost and CMYK

Hans Hagen wrote:

Peter Rolf wrote:

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luigi scarso wrote:

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On 2/2/07, Peter Rolf wrote:

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Hi all,

I have some questions about cmyk in combination with Metapost. Just by using

\setupcolors[state=start, cmyk=yes, mpcmyk=yes, rgb=no, ]

I get cmy(k=0) graphics, although all used colors are still defined in the rgb color space. This is really a nice feature and the automatic color conversion works relative good. Only (so far I have seen) gray tones have a noticeable tendency to brown (Acroreader 7.09). I have tested one of those gray

beware, without black component, on paper one indeed get brownish

colors. acrobat reader tries to mimick paper (can be configured) so this is why you don't get gray on screen

Yes, I remember playing around with this feature some time ago. Excuse my ignorance as I have no experience with other color space than RGB. ;) But the problem here is, that the conversion is not true RGBtoCMYK, but only RGBtoCMY (k is always sero). All of my graphics are in RGB, but for printing I surely need CMYK. A solution would be to define all used colors in the CMYK color space (although the origin of these colors is RGB). But to my surprise ConTeXt automatically converts my RGB colors (even the MP defined ones), except that the final colors are in the CMY instead of CMYK color space. All I want is a true RGBtoCMYK conversion. I know about the calculating limitations in TeX, so I can wait for an implemetation of that macro in LuaTeX. Best, Peter

when converting colors to gray context does weighted conversion (as with b/w televisions) so 1,0,0 and 0,1,0 give different grays

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tones by defining it as cmyk and the result looks ok.

RGB(223,223,227) CMY(0.12548, 0.12547, 0.10980) CMYK(0.01762, 0.01760, 0, 0.10980)

So is there a way to get cymk instead of cmy?

hmm, can you post an example ?

Mojca has pointed me to the definition of

\convertRGBtoCMYK (colo-ini.tex)

and (if I understand it right) this is only a RGB->CMY conversion.

The algorithm for the missing CMY->CMYK conversion is simple (see http://www.easyrgb.com/math.php?MATH=M13#text13 ), but "exact" calculation in TeX is no fun at all (at least not for me). I have tried to implement it, but yet without success. In the worst case I have to wait for luatex, which will solve all calculation problems. So think twice, before you waste your time... ;)

Maybe I should add this color conversion to the feature request list, so that it will not be forgotten?

Peter

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