[argyllcms] Re: Conversion omitting/locking device colorant

Good description Roger,

On Thu, Jan 15, 2009 at 02:30, Roger wrote:
> First, create a CMY profile from characterization data.
> Second, convert the following Lab values through that CMY profile to find
> out the corresponding device values:
>
> 56,0,0 -> % cyan, % magenta, % yellow
> 73,0,0 -> % cyan, % magenta, % yellow
> 39,0,0 -> % cyan, % magenta, % yellow
>
> That, in a nutshell, is what Heidelberg's technique is all about.
>
> Now there are a few other Source Lab data points that also need converting
> to CMY values. There is a whole tone scale that extends from 3c2m2y through
> 100c100m100y.

Just a note: "It is STRONGLY RECOMMENDED that only the 50% block is
used to calculate gray balance curves for offset presses, or any other
device on which gray balance is inconsistent or hard to measure",
Calibrating, Printing and Proofing by the G7 Method v6.


> So, suppose I make a CMY profile with Argyll, the next step is finding out
> which device values I need to emit on that devive to match 56,0,0 and 73,0,0
> and so on.
>
> Suppose Lab = 56,0,0 in AbsCol call for 49%c 42%m and 35%y.
>
> The next step is to figure out what "corrections" are required on linear
> plates to achieve the target Lab values in order that 56,0,0 ends up as 50%c
> 40%m 40%y on press, 73,0,0 ends up as 25%c 19%m 19%y, 39,0,0 ends up as 75%c
> 66%m 66%y. And so on for the rest of the defined CMY tone scale which should
> achieve a smooth transition between 0%c 0%m 0% and 100%c 100%m 100%y.
>
> Coming back to the above example, knowing that the characterization data
> that led to the CMY profile was obtained through linear plates on press (50%
> at 50% and so on), you would subtract 1% out of cyan at 50%c, add 2% of
> magenta at 40%m and subtract 5% of yellow at 40%y so that, when the 50%c
> 40%m and 40%y are imaged onto plates, the dot hitting the substrate is
> actually 50%c 42%m and 35%y.
>
> If everything is held constant, a CIE Lab match should be obtained which
> correspond to a certain normative definition of gray. That's what the job of
> the CMY profile is all about.

Martin Weberg

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