UCR/GCR revisited
UCR/GCR revisited
- Subject: UCR/GCR revisited
- From: email@hidden
- Date: Sun, 8 Jun 2008 18:33:42 -0700
> Phil Cruse writes on Sun, 8 Jun 2008 09:58:39 +0100
> Under Colour Removal was even around back in the days of
> Camera separations (using photographic masking)!
Thanks for the historical perspective on UCR and for that insight
of the term GCR being coined during the days of Analog Scanners.
In my view, these terms have outlived their usefulness and should
be replaced by a single term: Black Generation Strategy.
As Phil points out UCR has its origins in the early days when
separation methods were based on photographic techniques and wet chemistry
The following is a textbook definition of UCR taken from From
"Electronic Color Separation" by R.K. Molla (Chapter 12 on UCR)
"One way to use black to the best advantage is to remove some
of the C M & Y inks from the shadow neutral areas ... to make
room for a strong and rich black. The process is called under
color removal ..."
Implicit in the above definition is the notion that utilization
of UCR increases black density thus increasing the tonal range of
an image. In this context, replacement of K by CMY yields a lower
L* value at the black point.
The following excerpt from ”The GATF Practical Guide to Color Management”;
(pg 31).
“ A related process, GCR, is used to remove the gray component
(equal CMY) in any area where all three are present.”
In contrast to UCR, GCR is taken to mean that an “equivalent” amount
of CMY is replaced by K so that colorimetrically the inked color will
have the same Lab value.
These terms had real utility in those early days of four color
(CMYK) printing when the extent of editing possible via film masks
or complicated scanner dials was limited and the degree of control
quite coarse. In today’s digital age fine control of black generation
is now possible and spans the entire extent of colorspace. Requirements
of smoothness and continuity in all the separations require any black
generation strategy to be global in scope In our current digital era there
is
no sharp demarcation between UCR and GCR. One affects the other to some
degree.
Sometimes these terms are curiously referred to as though they were
mutually incompatible. I’ve heard the terms GCR and UCR used in an
either/or context, as though a choice had to be made. The point to
remember is that one (UCR) pertains to neutral axis K behavior; the
other (GCR) pertains to off-axis K behavior, and the necessity of
smoothness and continuity in all the separations requires that the
two to be inextricably linked. In short, both terms describe separate
but linked aspects of a profile.
Today's robust Profile Generation Apps give the user complete control
in defining neutral axis inkings (i. K start; ii) max K; iii) TAC;
iv. K curve shape) as well as control of off-axis K generation
(typically a slider between "Light" and "Heavy"). These controls
completely define and embody an overall Black Generation Strategy.
There is no need to invoke terms whose days have come and gone.
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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