Hi! If you want to do it right you'll have to convert the RGB values using the source colorprofile to a universal color- space (such as LAB) and then convert using the dest. color- profile to CMYK.
Otherwise you could convert to CMY and drop K or find a formula that also calculates K. Greets, Christian > -----Original Message----- > From: lijun zou [mailto:[EMAIL PROTECTED] > Sent: Friday, October 10, 2003 5:07 AM > To: [EMAIL PROTECTED] > Subject: RE: CMKY > > > Another question, > How can I mathematically convert RGB to CMYK? > Theoreically it's 1- RGB, but what about K value? > > LZ > > --- "Beer, Christian" <[EMAIL PROTECTED]> wrote: > > > I think CMYK support in tools used early in the > > publishing chain > > > is somewhat overrated. > > > > Right, but FOP _MUST_ not be early in the publishing > > chain! It could > > in fact be the only and therefore even the last step > > in a publishing > > chain... > > > > > > > I can only think of the following use cases for > > XSLFO: > > > - Avoid round-off errors and other numerical > > problems in the > > > conversion CMYK->sRGB->CMYK, perhaps in order to > > match exactly > > > colors of embedded CMYK bitmap images in text, > > borders and > > > background. > > > > Conversion CMYK -> RGB is nearly always lossy. And > > for print sRGB > > is also not that perfect (because of it's small > > gammut (nearly the > > smalest gammut of all RGB colorspaces)). > > > > > > > - Get color values into the document which can't > > be expressed > > > as RGB. I don't know whether this can even > > happen. > > > > If you create PDFs for print, you nearly always want > > CMYK! Okay, > > RGB and colorspace would be great but nowaday not > > all print- > > companys now how to handle colorspaces (for example > > PDF/X-3). > > > > > > > > > > As far as round-off goes, AFAIK the transformation > > from CMYK to > > > RGB and vice versa is linear and numerically well > > behaved with > > > the exception of some extreme saturated colors, in > > particular > > > very dark colors. I've never heard of anyone > > noticing these > > > kind of problem without deliberately using a > > somewhat braindamaged > > > tool for colorspace conversion which rounded all > > values to two > > > significant digits *and* both a quite expensive > > printing kit and > > > a well trained eye. > > > > 100% c 83% c > > 0% m 5% m > > 0% y --> sRGB --> 2% y > > 0% k 0% k > > > > So in fact, CMYK -> sRGB -> CMYK is not linear. If > > you convert the > > result to sRGB and again back the values will even > > be more bad. > > > > > > > > J.Pietschmann > > > > > > > > > > Christian Beer > > > > > > > > > > > > --------------------------------------------------------------------- > > To unsubscribe, e-mail: > > [EMAIL PROTECTED] > > For additional commands, e-mail: > > [EMAIL PROTECTED] > > > > > __________________________________ > Do you Yahoo!? > The New Yahoo! Shopping - with improved product search > http://shopping.yahoo.com > --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
