SECTION 4.5
241
Color Spaces
4.5.3 Device Color Spaces
The device color spaces enable a page description to specify color values that are
directly related to their representation on an output device. Color values in these
spaces map directly (or by simple conversions) to the application of device colo-
rants, such as quantities of ink or intensities of display phosphors. This enables a
PDF document to control colors precisely for a particular device, but the results
may not be consistent from one device to another.
Output devices form colors either by adding light sources together or by subtract-
ing light from an illuminating source. Computer displays and film recorders typi-
cally add colors; printing inks typically subtract them. These two ways of forming
colors give rise to two complementary methods of color specification, called
ad-
ditive
and
subtractive
color (see Plate 1). The most widely used forms of these two
types of color specification are known as
RGB
and
CMYK,
respectively, for the
names of the primary colors on which they are based. They correspond to the fol-
lowing device color spaces:
•
DeviceGray
controls the intensity of achromatic light, on a scale from black to
white.
•
DeviceRGB
controls the intensities of red, green, and blue light, the three addi-
tive primary colors used in displays.
•
DeviceCMYK
controls the concentrations of cyan, magenta, yellow, and black
inks, the four subtractive process colors used in printing.
Although the notion of explicit color spaces is a PDF 1.1 feature, the operators for
specifying colors in the device color spaces—
G
,
g
,
RG
,
rg
,
K
, and
k
—are available
in all versions of PDF. Beginning with PDF 1.2, colors specified in device color
spaces can optionally be remapped systematically into other color spaces; see
Note:
In the transparent imaging model
(PDF 1.4),
the use of device color spaces is
subject to special treatment within a transparency group whose group color space is
CIE-based (see Sections 7.3, “Transparency Groups,” and 7.5.5, “Transparency
only if device color spaces have been remapped to CIE-based spaces by means of the
default color space mechanism. Otherwise, the results are implementation-
dependent and unpredictable.