SECTION 6.2
481
Conversions among Device Color Spaces
6.2.1 Conversion between DeviceGray and DeviceRGB
Black, white, and intermediate shades of gray can be considered special cases of
RGB
color. A grayscale value is described by a single number: 0.0 corresponds to
black, 1.0 to white, and intermediate values to different gray levels.
A gray level is equivalent to an
RGB
value with all three components the same. In
other words, the
RGB
color value equivalent to a specific gray value is simply
red
=
gray
green
=
gray
blue
=
gray
The gray value for a given
RGB
value is computed according to the NTSC video
standard, which determines how a color television signal is rendered on a black-
and-white television set:
gray
=
0.3
×
red
+
0.59
×
green
+
0.11
×
blue
6.2.2 Conversion between DeviceGray and DeviceCMYK
Nominally, a gray level is the complement of the black component of
CMYK.
Therefore, the
CMYK
color value equivalent to a specific gray level is simply
cyan
magenta
yellow
black
=
=
=
=
0.0
0.0
0.0
1.0
–
gray
To obtain the equivalent gray level for a given
CMYK
value, the contributions of
all components must be taken into account:
·
gray
=
1.0
–
min(1.0
,
0.3
×
cyan
+
0.59
×
magenta
+
0.11
×
yellow
+
black
)
The interactions between the black component and the other three are elaborated
below.
6.2.3 Conversion from DeviceRGB to DeviceCMYK
Conversion of a color value from
RGB
to
CMYK
is a two-step process. The first
step is to convert the red-green-blue value to equivalent cyan, magenta, and yel-