Re: Photoshop Gamut warning vs ColorThink
Re: Photoshop Gamut warning vs ColorThink
- Subject: Re: Photoshop Gamut warning vs ColorThink
- From: Klaus Karcher <email@hidden>
- Date: Sun, 24 Feb 2008 22:01:39 +0100
Matthew Larmour wrote:
I believe in many instances the quesion of, "is this (CIELab) colour
within the gamut of this device" is rooted in a very practical,
rather than theortical, need: attempting to reproduce that colour on
the target device. The A2B gamut would tell you if theoretically the
gamut is large enough to encompass such a colour, but doesn't provide
a means to determine the device values necessary to do so. For this,
we need a conversion from CIELab to device values, provided by the
B2A tables.
not necessarily: argyllcms -fif uses the /inverted/ A2B table to obtain
the device values. Take a look at this example:
xicclu -fif -ia -l330 -a ISOcoated_v2_eci.icc
93
0 50
93.000000 0.000000 50.000000 [Lab] -> Lut -> 0.000000 0.035278 0.595226
0.000000 [CMYK] Lim 0.630504 (clip)
[Actual 89.454360 -2.704798 49.788440, deltaE 4.464555]
i.e. when you print CMYK 0/3.5/59.5/0 you will get the closest match to
Lab 93.0/0.0/50.0. You will get Lab 89.5/-2.7/49.8, that's Delta E = 4.5
off the mark.
On a related note, what is the best way to determine device values
that will match (within some tolerance) a CIELab colour? Simply an
Abs Col conversion from CIELab value to device values, or Rel Col, or
are there better methods?
It depends. In general I'd say abs., but there are exceptions: when you
are looking for the best Newspaper match to 10% Yellow in Sheetfed, you
will get essentially 0/0/0/0 using abs. col. -- that's colorimetrical
correct, but probably not what you want ;-)
A very useful tool to find the best device values interactively is
X-Rite/GMB ColorPicker (part of ProfileMaker): You start with the
"standard" B2A conversion, then you can minimize the Delta E in
consideration of additional conditions like maximum number of channels
or minimum ink coverage.
Klaus Karcher
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