Re: Photoshop Gamut warning vs ColorThink
Re: Photoshop Gamut warning vs ColorThink
- Subject: Re: Photoshop Gamut warning vs ColorThink
- From: email@hidden
- Date: Sun, 24 Feb 2008 13:16:49 -0800
Matthew Larmour writes on Sun, 24 Feb 2008 09:44:31 -0800
> Harold Boll writes:
>> The person who started this thread (Mike Eddington, Date: Thu,
>> 21 Feb 2008
>> 10:39:31 -0500), asked the following question:
>>
>> > ... if I want to determine a group of Pantone colors that are
>> within a particular gamut ...
> I believe in many instances the quesion of, "is this (CIELab)
>colour within the gamut of this device" is rooted in a very
>that colour on the target device.
Of course - you'll note that Mike's question was indeed very practical.
> 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.
But it answers the very specific question asked by Mike.
> For this, we need a conversion
>from CIELab to device values, provided by the B2A tables.
No doubt Mike may someday ask the question of how does one go about getting
device values for these Spot colors. And the B2A will indeed give you those
device values but you'd be taking an awful chance if you use it. As has
been pointed out, the B2A table does not fully exploit the entire physical
gamut and you could get device values that are not as colorful as they
could be (especially for darker colors). Secondly, 3D luts often suffer
from round off error especially near the gamut boundary so this uncertainty
would just add to the previous effect. If its Recipes for Spot colors
you're looking to get, then the B2A table is a poor man's solution. There
are applications which provide Recipes for Spot colors which optimally do
not use a B2A table. Rather they employ the machinery that populates a B2A
table in the first place. More about that in the answer to the question
you pose below.
>So this "effective" or "useable" gamut, produced by a
>combination of the device's physical gamut, profile,
>choice of rendering intent, etc, is all we can access
>unless we work in device space - which in this example
>isn't possible.
Not sure I follow your reasoning here.
>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?
Not a simple answer I'm afraid. First you star with characterization data;
then you must build a Forward Model which links device values as input and
yields color (Lab) as output values. However, when building a B2A table or
trying to determine a Recipe, you start with an Lab value and are trying to
find the inking. Unfortunately the Forward model is from ink to color, not
vice-versa which would have made your life easier. You need to invert the
Forward Model by using external mathematical machinery like Newton-Raphson
or Simplex optimization. If your printer is CMY, then life is easy because
there will in principle be one CMY for each Lab (presuming that Lab is
in-gamut; out of gamut colors require gamut mapping). But more likely your
printer is CMYK which adds more complexity to the mix, since now there is a
many to one relationship - many CMYK's will print to a single Lab (that's
GCR). So you have to figure out which K solution to use and do it in a way
such that you maintain smoothness and continuity in all neighboring
inkings. Good luck.
> Matthew Larmour
Harold
TOSHIBA AMERICA BUSINESS SOLUTIONS 2 Musick, Irvine, CA 92618
Harold Boll | Color Scientist | 1.781.856.5174 |
email@hidden
" The sunlights differ, but there is only one darkness. "
Ursula K. Leguin
This message (and any attached files) is secret, confidential and proprietary and is intended solely for specific addressee(s). If the reader of this message is not the intended recipient, please notify the sender immediately by reply email and delete it and all backup copies immediately.
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Colorsync-users mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden