Graphics Programs Reference
In-Depth Information
process. We are actually subtracting red, green, and blue from the white
light striking the paper. When white light strikes cyan pigment, green
and blue is reflected and red is absorbed. When white light strikes
magenta pigment, red and blue is reflected and green is absorbed. When
white light strikes yellow pigment, red and green is reflected and blue is
absorbed. This CMY process is how color printing works. As we subtract
certain proportions of red, green, and blue, we simulate colors just as we
do using RGB. Printing the maximum of all three colors subtracts all light,
thus we get black—at least in theory. The inverse of red is cyan. The oppo-
site of green is magenta and the opposite of blue is yellow. You can see
how CMY/RGB have a bit of a ying yang kind of relationship.
In the theory of subtractive color, maximum amounts of CMY create
black. However, in the real world, with most printing processes, instead
of black, we end up with a muddy brown. This is due to the impurity of
CMY colorants. For this reason we add a black colorant. CMYK is CMY
with the addition of K or black colorant. K is also known as
Key
or,
depending on whom you ask, assigned to describe black because the
acronym or letter “B” was used to describe blue (remember RGB?). In
any case, CMYK is the color process used in offset printing and usually
referred to as four-color process.
Color Models and Color Spaces
Since color is a sensation that ultimately happens in our brain, how can
we quantify something like red? What is red other than a word we use
to describe what is essentially a sensation of light? I mentioned that com-
puters understand only numbers; however, the necessity of describing
colors numerically goes well beyond computers. Using numbers to define
a color is useful. Having a specific set of values to describe the light itself
is much more accurate than an ambiguous word like red. The terms color
space and color model are used often in both color management and
computer imaging. It's necessary for us to define and understand what
they both mean.
A
color model
is a method of grouping numeric values by a set of pri-
maries. Most color models have three primary components (e.g., RGB,
CMY, LCH, HSV, L*a*b*). Some application-specific color models use
more components, for example, CMYK. Photoshop and other image
editing applications work with multiple color models. Some scientific
color models such as x,y,Y or L*a*b* encompass all of human vision and
have a defined scale such that a particular color will always have the
same set of values. It's quite useful to be able to assign a numeric value
to a color based upon how humans perceive colors. Other color models
such as RGB or CMYK have no standard defined scale or reference. These
color models are abstractions and cannot describe a specific color without
first defining the scale or reference. Having an RGB value such as
R10/G30/B50 does not tell us how to reproduce that color; the values
