Re: CMYKOG profiles and GCR
Re: CMYKOG profiles and GCR
- Subject: Re: CMYKOG profiles and GCR
- From: Graeme Gill <email@hidden>
- Date: Wed, 07 Jan 2004 21:57:53 +1100
- Organization: Color Technology Solutions Pty. Ltd.
John Wickham wrote:
i too read this article recently, which prompted my question on GCR. i
understand the definition of GCR and how in offset it can be beneficial
in reducing CMY ink and in Ink-Jet
how a more pronounced or vigorous black replacement can help neutralize
gray scale when you are stuck with a poor device calibration.
however when an Ink-Jet printer has been brought into fine tune, ink
limiting,dot pattern,linearization and the resultant neutral grayscale
before a profile is made, what use is
UCR, GCR, theoretically ? and how does it affect gamut ?
Black generation is a degree of freedom in reproducing a color. This
degree of freedom allows making tradeoffs in the final image.
Typical inkjet tradeoffs, even in a well behaved, well tuned printer are:
Dot visibility. Black has the highest visible contrast. Using more
black makes dots more visible.
Neutral robustness. Even a well balanced CMY neutral has a bumpy
spectrum, making its neutral balance sensitive to the shape of the
illuminant spectrum. Black generally has a very flat spectrum, making
it's neutrality insensitive to the illuminant.
Banding. Using a dominant single channel that has the highest visible
contrast to reproduce non-saturated colors makes the overall image
sensitive to patterns and other inconsistencies of that channel.
Ink limiting. Most printer/paper combination can't take over 250% total
ink, so substituting black for CMY allows a larger gamut while staying
within the total ink limit.
With some ink/media combinations, the darkest blacks can only be
reproduced using CMY and black ink. How black the blacks are
is gamut.
If black generation is being done algorithmically in device
space (which is where this discussion started out), then the
degree of freedom is eliminated (that's the idea). But since
this is done algorithmically, it may eliminate combinations of
CMYK that have colors that are on the total CMYK gamut surface,
hence the choice of GCR/UCR can (if done algorithmically in
device space) limit the gamut achievable. If the black
generation is done using the full CMYK colorimetric information,
then all gamut surface colors are candidates for being used, and
the gamut is not generally restricted by the black generation.
i understand go make a series of a print with all the different GCR
strategies, see what they look like, and make up your own mind.
just born curious.
Certainly part of the process of setting the black generation on
a device, is trying various settings, and examining the result
in the light of the above tradeoffs.
Graeme Gill.
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