[argyllcms] Re: Conversion omitting/locking device colorant
- From: Roger <graxx@xxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Wed, 14 Jan 2009 20:30:31 -0500
Graeme,
> It's not clear to me what sort of curves you are after...
Probably not very complicated.
As you may have heard, the whole world of printing in North America has
adopted a normative colorimetric definition of gray balance. It's assorted
with a method to derive printing plates curves that, at given levels of ink
densities, should yield a set of CIELab values for a given set of CMY
combinations.
I'm far from being privvy to the math behind inversing an A2B1 table but, to
me, if Argyll can come up with CMY ICC profiles, it should not be too
difficult to come up CMY dot percentages that meet a certain set of Source
Lab values and turn those into plate corrections factors given an assumption
of plates starting values.
Heidelberg has shown that this can be done easily by looking up the dot
percentages values into a CMY profile. Here's a rundown of Heidelberg's
technique.
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.
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.
That's the challenge, Graeme. That's what everybody in the printing industry
is after. The colorimetric targets may change to suit different substrate a*
and b* values but the goal remains the same.
Best / Roger
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