[cairo] Subtractive API, part 0

Chris Murphy lists at colorremedies.com
Fri Feb 5 08:41:53 PST 2010

On Feb 4, 2010, at 4:57 PM, ecir hana wrote:
> I'm  still trying to connect the dots and struggling with this one:
> I read your post above as if you were suggesting that the better
> alternative to untagged CMYK PDF would be tagged one because the
> printer at the other location could easily just swap the profile and
> print it on different paper, say.

It's for two reasons:
1.) to confirm/deny the intended output condition for the PDF. The PDF job is created with a unique flavor of CMYK, and that flavor needs to be defined in order to ensure that the intended appearance is preserved both in proofs and in the final product.

2. to allow for repurposing. This is somewhat less of a factor because most printing companies don't have the skills to use device-link profiles to repurpose CMYK to CMYK.

> But that means going to LAB again, and back to CMYK again.

This is not necessarily a literal conversion. Some CMMs only use the PCS to create a virtual device-link in memory (Apple calls this a color world) and then that device-link is used such that you get a direct CMYK to CMYK conversion (or direct RGB to CMYK) conversion. This is done for performance reasons.

> And I just
> did a small experiment, trying to convert 40% 30% 30% 100% CMYK SWOP
> to Euroscale Coated and it completely messed-up the colors. And from
> the little I read about ICC profiles it seems (?) that there is no
> official way (intent) of how to preserve such blacks.

a.) Qualify "messed up the colors". 40/30/30/100 in SWOP has a specific L*a*b* value, and that is usually what we are trying to preserve. So long as that color is preserved in some other printing process, we shouldn't care what the CMYK values are.

b.) Device-link profiles can do things like preserve black channel L* and purity (or any other channel), to help with some of the known problems with converting from CMYK to CMYK, which aren't so much about color but about preserving an expected behavior.

>> Untagged CMYK in a PDF is OK, but I would then require that an outputintent is set for the PDF, so that we know what the intended CMYK output condition is, in case the PDF needs to be repurposed.
> So wouldn't it better solution to just use untagged PDF and then name
> the file something like "new_movie-poster_final-cmyk_uncoated.pdf"
> (that's only because I'm not yet familiar with PDF specs on
> "outputintent", yet)?

No, this is a huge violation of filename domain, just as are filename extensions that denote the file's format. Filenames are user domain, and we should not be putting non-user metadata in the file name. It screws up both the filename, and it risks losing important metadata about the file. This data absolutely needs to be embedded in the PDF. All the PDF/X specs require that the output intent and/or output condition be set.

Chris Murphy
Color Remedies (TM)
New York, NY
Co-author "Real World Color Management, 2nd Ed"

More information about the cairo mailing list