Graphics Programs Reference
In-Depth Information
provided are simply a ratio of the primary components (in this case RGB)
without a necessary scale. In this example we know there's more blue
than red, more green than red, but not as much green as blue. Without
a scale to tell us the actual amounts we can't reproduce this color. This
is where a color space comes into play. A color space provides this
additional and necessary information.
Think for a minute about the term color space . The first word, color, is
fairly obvious. Let's look at the term space in context. I have three sets
of numbers to define red, green, and blue (R10/G130/B50) but how red
or green or blue are these values? What is the scale? It is possible to plot
any three primary values in three dimensions by treating the primaries
as coordinates in space. This creates a solid or volume that represents all
possible colors in that color space. This is a color space! The color space
exists within the larger universe of human vision. Each color space is in
a different position relative to this universe. Thus the same RGB values
in each color space will be in a different location.
Going back to the chocolate chip cookie analogy, suppose a color
model is a cookie recipe with only three ingredients. I give you this recipe,
which simply calls for 1-flour, 8-butter, and 2-chocolate chips. You don't
have enough information to make the cookies. However if I provide you
the recipe with a specific scale—1 cup of flour, 8 tablespoons of butter, and
2 cups of chocolate chips—I've provided the necessary information, the
scale, to make a dozen chocolate chip cookies. If I provide additional
information such as the brand of chocolate chips, you can reproduce
exactly the same cookies I made. I can give you the cookie recipe in the
metric scale such as liters and grams and you can still makes the same
cookies even though the numbers are different. A color space is a color
model that has a known reference and scale, in this case primaries (the
ingredients) and scale (specific quantities of these ingredients).
Let's use an RGB color model as an example, but keep in mind that
this could be true for CMYK or other color models. Suppose I specify a
color as R10/G130/B50 and specify a color reference by saying the color
space is Adobe RGB (1998), which defines the scale of the RGB primar-
ies; the color coordinates of this color space. The R10/G130/B50 set of
numbers can now reproduce a color by anyone with the proper tools
since the reference and scale have been defined. Different RGB color
spaces use a different scale of red, green, and blue primaries. Adobe RGB
(1998) and sRGB (two color spaces that will be discussed in great detail)
are different color spaces, however both are based on the RGB color
model using RGB primaries. Although each color space uses the same
three primary ingredients (R, G, and B), the specific colorimetric scale of
each color space is different. The maximum of red, green, and blue are
more saturated in the Adobe RGB (1998) color space than the sRGB color
space. Even though R0/G255/B0 is the greenest green ingredient in both
Adobe RGB (1998) and sRGB, knowing that the scale is different in both
color spaces explains why this green value is more saturated in Adobe
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