CHAPTER 4
264
Graphics
Example 4.10
[ /Indexed
/DeviceRGB
255
< 000000 FF0000 00FF00 0000FF B57342 … >
]
The example shows only the first five color values in the
lookup
string; in all, there
should be 256 color values and the string should be 768 bytes long. Having
established this color space, the program can now specify colors as single-compo-
nent values in the range 0 to 255. For example, a color value of 4 selects an
RGB
color whose components are coded as the hexadecimal integers
B5
,
73
, and
42
.
Dividing these by 255 and scaling the results to the range 0.0 to 1.0 yields a color
with red, green, and blue components of 0.710, 0.451, and 0.259, respectively.
Although an
Indexed
color space is useful mainly for images, index values can
also be used with the color selection operators
SC
,
SCN
,
sc
, and
scn
. For example:
123 sc
selects the same color as does an image sample value of 123. The index value
should be an integer in the range 0 to
hival
. If the value is a real number, it is
rounded to the nearest integer; if it is outside the range 0 to
hival
, it is adjusted to
the nearest value within that range.
Separation Color Spaces
Color output devices produce full color by combining
primary
or
process
colorants
in varying amounts. On an additive color device such as a display, the
primary colorants consist of red, green, and blue phosphors; on a subtractive de-
vice such as a printer, they typically consist of cyan, magenta, yellow, and some-
times black inks. In addition, some devices can apply special colorants, often
called
spot colorants,
to produce effects that cannot be achieved with the standard
process colorants alone. Examples include metallic and fluorescent colors and
special textures.
When printing a page, most devices produce a single
composite
page on which all
process colorants (and spot colorants, if any) are combined. However, some de-
vices, such as imagesetters, produce a separate, monochromatic rendition of the
page, called a
separation,
for each colorant. When the separations are later com-