Image Processing Reference
In-Depth Information
produces gray level with
d
255
L*
¼
100
100,
a*
¼
b*
¼
0
(
8
:
5
)
The TRC for the black channel is also constructed as a second step to perform
channel-wise linearization. The main reason for the adaptation of the gray-balance
calibration for the CMY channels is the fact that the human visual system is very
sensitive to colors near the neutral axis. Small color differences near the neutral are
more noticeable than larger color differences far from it. Once the gray balance is
achieved with TRCs and the black channel is linearized, the printer will tend to make
other colors containing combinations of CMYK lot more pleasing and closer to what
is required by the press (see the images in Figure 8.5 with and without gray balance).
However, the choice of gray-balance reference or target values matters a lot with
respect to the neutral appearance [1]. If the gray-balance reference is altered, then the
color balance will also be altered and may result in an error in color.
In system theoretic terms, the outcome of the gray-balance calibration can be
thought of as a way of generating inverse maps to linearize the printer along
the chosen reference
target axis. For example, when the target values are selected
on the neutral axis, the printer with a gray-balanced inverse map will be linear to
=
FIGURE 8.5 (See color insert following page 428.)
CMYK test images with and without
gray-balanced print engine.
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