Converting RGB to CMYK
The most common conversion of color spaces is between RGB
(red~~green-blue additive primaries) and CMYK (cyan-magenta-yellow-black
subtractive primaries) color spaces. The reason for the popularity
of this conversion is that all scanners scan in RGB, and once
an image is scanned, the colors must be converted to the four-color
space used on the printing press.
The Kodak PCD Professional Imaging Workstation's scanner scans
in RGB, then converts into a generous color space called YCC.
The YCC space can ultimately be converted back into RGB, CIELAB,
or CMYK spaces (or others) for display, print, or storage purposes.
The RGB display color space is a somewhat truncated color space
compared to the RGB scan color space, accounting for a slight
gamut reduction in the display. In other words, the monitor cannot
display all the values of red, green and blue that a scanner
sees when it digitizes the film, so some information is lost
in the translation to the RGB display. YCC color space records
the scanned image, and can later convert the image to the RGB
space that a monitor can display.
RGB display space is described by a triangle on the CIE chromaticity
chart with its strong points at the red, green and blue corners
of the trangle. There is no allocation within this triangle for
strong cyan, yellow or red all of which fall outside the perimeter
of the RGB triangle. In order to convert from one to the other,
a color coordinate system rotation must occur. Mathematically,
this is a simple task, but it runs headlong into a photomechanical
barrier which prevents it from being completely successful when
producing color separations for printing.
This barrier is created by the fact that we cannot make perfect
block absorbers (pigments) which transmit perfectly outside the
range of colors we want to absorb. No pigments can be made as
pure as we would like, and as a result we get a certain amount
of contamination in the printing of CMY images.
The theory of mixing three pure primary pigments to
get black does not work in the real world. Because of
contamination in the minerals and synthetics used to make printing
inks, the result of a three-color primary CMY mix is deep purple-brown,
a color which is far from clean black. So the printing industry
adds actual black ink to overcome the failure of the chemical
process of printing ink pigments.
Pigment impurity, and the compensation that must take place to
correct for it, requires the mathematics of color conversion
to take a turn here and there to make the colors come out in
the right places. The 120-degree rotation is made, but then a
series of rule-based distortions of colors must occur to build
the black separation from the known (or predicted) failures of
the purity of the pigments, and to get increased density in dark
areas.
Another necessary operation in the color separation process is
the calculation of gray component replacement (GCR)and under
color removal (UCR). These calculations are based on the colors
of sets of pixels in the same position in the matrix The calculations
are needed to prevent excessive ink layers from being created
and for the simplified balance of neutral gray in printing.
Rich, saturated reds, greens and blues become less brilliant
than the same colors viewed on an emissive device (an RGB monitored)
while yellows magenta and cyan colors become more dominant The
separation process requires understanding of the effect of color
conversion Color separations are made worldwide that look beautiful
and have brilliant colors that take advantage of the four pigment
primaries used on the printing press Strong yellows magentas
and cyans can rule the scene
