CHAPTER 6
484
Rendering
6.2.4 Conversion from DeviceCMYK to DeviceRGB
Conversion of a color value from
CMYK
to
RGB
is a simple operation that does
not involve black generation or undercolor removal:
red
=
1.0
–
min
(
1.0
,
cyan
+
black
)
green
=
1.0
–
min
(
1.0
,
magenta
+
black
)
blue
=
1.0
–
min
(
1.0
,
yellow
+
black
)
In other words, the black component is simply added to each of the other compo-
nents, which are then converted to their complementary colors by subtracting
them each from 1.0.
6.3 Transfer Functions
In PDF 1.2, a
transfer function
adjusts the values of color components to compen-
sate for nonlinear response in an output device and in the human eye. Each com-
ponent of a device color space—for example, the red component of the
DeviceRGB
space—is intended to represent the perceived lightness or intensity of
that color component in proportion to the component’s numeric value. Many de-
vices do not actually behave this way, however; the purpose of a transfer function
is to compensate for the device’s actual behavior. This operation is sometimes
called
gamma correction
(not to be confused with the
CIE-based gamut mapping
function
performed as part of CIE-based color rendering).
In the sequence of steps for processing colors, the consumer application applies
the transfer function
after
performing any needed conversions between color
spaces, but
before
applying a halftone function, if necessary. Each color compo-
nent has its own separate transfer function; there is no interaction between com-
ponents.
Transfer functions always operate in the native color space of the output device,
regardless of the color space in which colors were originally specified. (For exam-
ple, for a
CMYK
device, the transfer functions apply to the device’s cyan, magen-
ta, yellow, and black color components, even if the colors were originally
specified in, for example, a
DeviceRGB
or
CalRGB
color space.) The transfer func-
tion is called with a numeric operand in the range 0.0 to 1.0 and must return a
number in the same range. The input is the value of a color component in the de-
vice’s native color space, either specified directly or produced by conversion from