Re: Lab values for cmyk colours exceeding the total ink limit
Re: Lab values for cmyk colours exceeding the total ink limit
- Subject: Re: Lab values for cmyk colours exceeding the total ink limit
- From: Marco Ugolini <email@hidden>
- Date: Sun, 30 Apr 2006 21:29:05 -0700
In a message dated 4/30/06 2:40 PM, Giordano Galli wrote:
> I'm trying to understand how the software (eg Photoshop) calculates
> Lab values for cmyk colours exceeding a profile's total ink limit.
>
> Are the relevant data included in AToBxTag tags? IOW, are LUT-based
> profiles required or allowed to implement a complete Device to PCS
> mapping? If so, I can't think of how this could be achieved in some
> situations, eg when mapping [90 70 70 100] for a newsprint profile,
> as this would involve printing 'forbidden' patches.
Salve, Giordano, from a fellow countryman!
The way I see it, using those values [C 90, M 70, Y 70, K 100] inside a file
that has a newspaper profile embedded in it should just be avoided, ideally.
The way I would bypass the problem would be to determine the appearance of
those CMYK values in an environment in which they have a definable meaning
(e.g., US Sheetfed Coated SWOP, where a TAC of 330 is within bounds), then
make a conversion from that space to your newspaper profile, and then use
the resultant CMYK values when you add that color in your newspaper-bound
file.
That would be my workaround. That, or using the "high" CMYK values in the
file, then converting from the newspaper output profile to Lab, then back to
the same profile from which you started, using absolute colorimetric in both
directions. That way any excessive ink coverage would be trimmed.
(That manual conversion route would be necessary, because a conversion from
a profile back to itself would otherwise be "null", which would leave the
file unchanged.)
Buona fortuna!
--------------
Marco Ugolini
Mill Valley, CA
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