5.4 The Subtractive Color Systems CMY
and CMYK
A color monitor emits light and consequently relies on the RGB
colorspace, which is an additive color system. Ink on paper, however,
is a totally different story. Ink absorbs light and the color we see
from it is the light that was reflected and not absorbed. If we shine
a white light on an area and the color seen is cyan, it is because the
red component of the white light was absorbed leaving only cyan to be
reflected. Similarly, magenta absorbs green, and yellow absorbs blue.
Thus, the cyan, magenta, and yellow colors make a subtractive color
space, which is the anti-space of RGB. This colorspace is called
CMY for cyan, magenta, and yellow.
For a pixel that has R, G, and B components in the RGB colorspace, the
corresponding C, M, and Y components are just (255-R), (255-G), and
(255-B).
Due to the physical nature of colored inks, the CMY colorspace is
perfect for working with printed images. Or almost. In principle,
the addition of equal amounts of cyan, magenta, and yellow is like
subtracting equal amounts of red, green, and blue. We know that equal
amounts of R, G, and B make a neutral color, so equal amounts of C, M,
and Y do too. Thus, adding large and equal amounts of cyan, magenta,
and yellow makes black, and adding very small amounts makes a color
that is close to white. At least this is true when CMY is used to
print on white paper.
In practice, due to imperfections in inks, adding equal amounts of
cyan, magenta, and yellow does not make a deep dark black. Rather, it
produces a muddy brown. To remedy this problem, printers subtract out
some of the cyan, magenta, and yellow from an image and replace it
with black. This new colorspace is called
CMYK, and it significantly improves
the depth and tonal range of printed images. Here, the K in CMYK
stands for black.
How much cyan, magenta, and yellow can be subtracted out and replaced
by black? Because neutral colors require equal amounts of cyan,
magenta, and yellow, the most that can be subtracted out for a pixel
is the minimum of its C, M, and Y components. Thus,
is the maximum black that can be extracted, and the resulting new
values of C, M, and Y are just the old values less this value of K.
Although choosing K to be the minimum of C, M, and Y is reasonable, it
is not necessary; it can also be chosen to be less than this value.
In addition to CMYK's natural relationship to printing because its
subtractive qualities mirrors that of inks, it also has another
important, more pragmatic advantage. Replacing equal amounts of three
colored inks with a single black one can significantly reduce the
amount of ink printed on the paper. This means that the ink will dry
faster and the printing presses can produce more copy in the same
amount of time.
There are also significant advantages to CMYK when doing color
correction on an image that will end up in a printed format (see, for
example, [6]). The most significant is that the range of
colors, known as the color gamut, that can be produced with inks is
significantly smaller than what can be created on a color monitor.
Thus, it isn't sensible to invest a lot of time working on an RGB
image only to convert it to CMYK for printing at the end. This will
more than likely produce many out of gamut colors. When the final
result is a printed image it is more sensible to convert to CMYK
before performing any color transformations.
Unfortunately, the GIMP provides very little support for working in
CMYK. There is the Decompose
function, found in the Image:Image/Mode menu, which offers a
CMYK option in its dialog. This is not terribly useful, though. To
make the GIMP truly pre-press capable, the following is needed:
- The ability to set K in the range 0 to
.
- The ability to convert RGB to CMYK while applying color gamut
limitations.
- The ability to work in CMYK space just like in RGB space. This means
a Channels dialog with C, M, Y, and K components and color editing
functions and tools like Curves, Levels, Color
Picker, and so on, which work in CMYK.
There is also the perception that spot colors are needed to make an
effective pre-press image manipulation tool. Although there is much
pageantry about the colorimetry, device calibration, and Pantone ink
systems available for Adobe's Photoshop, these are somewhat of a red
herring. Making these systems work is difficult and unreliable.
Physical printing devices like presses tend to produce different
results from day to day and even from run to run. In the printing
process, there are many problems that can change the color balance so
carefully calibrated on the monitor. The amounts of ink the printer
uses, the capability of the press to apply the specified percentages
of ink uniformly across the printing surface, and many other
environmental and press health factors make for significant
variability in the final result. No amount of calibration and
proprietary ink systems can fix that.
Thus, the main hurdle to cross before the GIMP can become a viable
pre-press tool is the functionality given in the preceding bulleted list.
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