Re: Photoshop Gamut warning vs ColorThink
Re: Photoshop Gamut warning vs ColorThink
- Subject: Re: Photoshop Gamut warning vs ColorThink
- From: Graeme Gill <email@hidden>
- Date: Sun, 24 Feb 2008 01:18:12 +1100
Lars Borg wrote:
That approach has problems. A2B does not represent the printable gamut
or the TAC limit.
Something I acknowledged in my explanation.
In my view, the A2B tag is useless as a gamut indicator.
There are no other alternatives in an ICC profile that provide
accurate information on the range of colors the device is
capable of reproducing, even if it forms only one boundary
to the actual usable gamut. Generally all other information
in a profile (such as B2A tables etc.) are lower accuracy,
and derived from the colorimetric A2B table. I've long
noted the lack of a reasonable device gamut limit tag
in the ICC profile format. (ie. something that would
encompassing TAC + other device or operator imposed limits
on the device space gamut).
It seems there's a lack of agreement on what "gamut warning" means.
Here's my interpretation. Please let me know how you disagree.
I took it that the outcome of the discussion was a realization that
a gamut warning limit is not necessarily the same thing as the original
image colors exceeding the destination devices reproductive gamut,
since it overlooks any gamut mapping implicit in certain
color workflows such as the use of non-colorimetric B2A tables.
A "gamut warning" indicates whether a specified source color can be
"reproduced accurately" when converted to a target color space.
If by "reproduced accurately" you mean "not clipped" then I would agree.
"Gamut warning" is used only in the context of converting from a source
color space to a target color space.
"Gamut warning" is not used for color values already in the target
space, such as 400% CMYK exceeding a TAC of 240%. Is there a commonly
used term for this case?
Perhaps few things operate this way, but I think they should.
I see the practical color gamut being imposed by two elements,
one being the range of colors the device will reproduce given
no other constraints, and the other being imposed by those
other, usually device value based constraints. The most common
is usually a total ink limit or some other per channel or smaller
number channel combination limit. Another might be a gamut
boundary imposed by the smoothness criteria in combination with the
separation algorithm.
The actual usable color gamut will then be the intersection of
the limits of these two elements.
The B2A tag also reveals the gamut limits imposed by various GCR
methods. If B2A tags have no or very low GCR, then many dark colors are
out of reach, and thus out of gamut. This means that different profiles
for the same press and with the same A2B tags, but with different GCRs,
have different reachable gamuts. A high-GCR profile usually can deliver
a higher number of unique colors in the dark area than a low-GCR
profile. It is then quite appropriate that these profiles also provide
different gamut Warnings.
The way I'd put it is that the separation chosen may influence the
practical gamut when it's combined with the imposition of a smoothness
criteria.
Given that the gamut warning indicates the device's, the profile's and
the **conversion's** ability to reproduce a source color, the gamut
warning should change when the conversion changes.
Within the context of a workflow that uses A2B and B2A tables I agree.
- Reproduction errors are, for some profiles, difficult to separate from
intentional color shifts.
It seems that to actually generate more accurate gamut warning
it would be necessary to separate out the gamut mapping from the
PCS to device conversion, rather than having it hidden in the B2A table.
The PCS to device conversion is then colorimetric, and it's somewhat
easier to be precise about when this clips.
Graeme Gill.
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