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: Sat, 23 Feb 2008 11:03:10 +0100
Graeme Gill wrote:
There is a more accurate method, but it isn't "better" for checking
a raster image, since it's too slow and needs extra information.
The method is to directly invert the colorimetric A2B table, since
it is the closest definition of the device response.
("invert" meaning "locate the CMYK color in the A2B table that
produces the target PCS value. If no combination of CMYK can
create it, it's out of gamut.")
Lars Borg wrote:
That approach has problems. A2B does not represent the printable gamut
or the TAC limit. [...]
In my view, the A2B tag is useless as a gamut indicator.
IMHO the problem is not the inverted A2B table, but the missing TAC
information in the profile -- fortunately the ICC recognized the issue
and is working on a "printingCondition tag".
Phil Green wrote to email@hidden:
this approach is very similar to that
used in a current ICC proposal for a 'printingCondition' tag. If
adopted, a tag will be defined which has sub-tags for various aspects of
the printing condition, including colorimetric aim, process standard and
media, colorant set, TVI, TAC, black generation, and halftone screen.
As Graeme mentioned, using the B2A tag for gamut evaluation is a source
of inaccuracy. I guess his method is more precise even if he has to
estimate the TAC since the errors introduced by estimating the TAC are
smaller than those introduced by using the less precise (and possibly
badly inverted) A2B tag. Moreover the TAC of a reasonable profile won't
limit the device gamut essentially.
Lars Borg wrote:
It seems there's a lack of agreement on what "gamut warning" means.
Here's my interpretation. Please let me know how you disagree.
A "gamut warning" indicates whether a specified source color can be
"reproduced accurately" when converted to a target color space.
That's what I prefere to call "clipping warning" and it's indeed
depending on the rendering intent and BPC.
"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?
"ink limit warning" or "TAC warning" I'd suggest.
"Gamut warning" is not used to indicate or measure the size of a
device's gamut.
I have to contradict: from my point of view the gamut of a device is a
fixed, measurable quantity -- how much of it will be used and which
source colors will be clipped depends on the source image, the rendering
intent and the gamut mapping strategy.
Klaus Karcher
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