Re: Photoshop Gamut warning vs ColorThink
Re: Photoshop Gamut warning vs ColorThink
- Subject: Re: Photoshop Gamut warning vs ColorThink
- From: email@hidden
- Date: Thu, 21 Feb 2008 17:53:31 -0500
Chris Cox writes on Thu, 21 Feb 2008 12:33:01 -0800
> ..but with the information available, that's the best you can get
(AFAIK).
I would agree. A gamut determined in this fashion (A2Bx/B2Ax round trips)
reflects the constraints imposed by TAC, max inkings, K strategy, etc. and
is the gamut of interest to anyone who wants gamut alarm information.
A gamut determined purely by the A2Bx transform would in all probablility
not reflect these constraints. The ICC spec requires that all Rendering
Intent transforms (A2Bx & B2Ax) be inverses of one another, i.e. round
trippable. If this requirement was rigorously adhered to, then in
principle the A2B1 derived gamut would indeed be comparable to the gamut
mapped out by the B2A1 transform. But for the Relative Colorimetric
pairing (A2B1/B2A1) this is rarely if ever the case. Why? Because one of
the most important and common quantitative tests of profile quality is how
well the Absolute A2B3 transform replicates the training data (Absolute &
Relative differ only by a scaling factor). Rigorously adhering to the ICC
spec would degrade these fit statistics. Consequently, the Rel Col A2B1
transform is usually in actuality the global Forward Model which
comprehensively describes the colorimetric relationship between ink and
color An implication of this is that physical colors mapped to by the
A2B1 transform will be out of gamut with respect to its B2A1 cousin
transform.
Case in point, lets look at following inking for a high quality profile:
USsheetfedCoated. CMYK = [0 0 1 1]. These inks obviously combine to a
physical color but no quality profile would ever contain this inking
because once Yellow has hit 100 % you want to darken it with CMK and not
just K alone as is reflected by this inking.
1) CMYK: [0 0 1 1] -> A2B1 -> Lab: [15.0 -1.64 14.01]
2) Lab: [15.0 -1.64 14.01] -> B2A1 -> CMYK: [ .76 .66 .98 .67 ]
3) CMYK: [ .76 .66 .98 .67] -> A2B1 -> Lab: [16.7 -0.8 8.9]
4) delta-Eab = 5.4
Anyone using the A2B1 transform from this profile to derive a gamut would
be computing the maximum physical gamut of this inkset without any of the
constraints mentioned above. A gamut derived by round tripping (A2Bx/B2Ax)
would reflect these contraints and is the useful information which is
sought by anyone wanting a gamut alarm.
Harold
TOSHIBA AMERICA BUSINESS SOLUTIONS 2 Musick, Irvine, CA 92618
Harold Boll | 1.781.856.5174 | email@hidden
" The sunlights differ, but there is only one darkness. "
Ursula K. Leguin
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