Image Processing Reference
In-Depth Information
TABLE 8.7
Round-Trip Accuracy Numbers for 1-D
and 2-D Calibrations
Mean
D
E
a
*
95th Percentile
D
E
a
*
Calibration Type
One-dimensional gray-balance calibration
5.13
8.23
One-dimensional channel-independent
calibration
3.12
9.06
Two-dimensional calibration
1.75
5.32
L
target
r
=
a
target
b
target
u
(
k
)
x
(
k
)
V
(
k
)
Sensor
K
Integrator
Printer
+
_
+
+
Nominal
CMY
values
FIGURE 8.12
Gray-balance control loop.
eliminate the need for operator intervention. Otherwise, it can be a semiautomated
system.
We have shown the above control loop for one color patch. The nominal CMY
values are constructed using any of the following procedure:
(1) When a printer model is not available, use C
¼
M
¼
Y
¼
d and K
¼
0 where
d is computed from Equation 8.16 by knowing the gray-balance target for
the chosen patch (i.e., L*, a*, and b* values):
d
¼
(
L*
)
100
*255
;
100
a*
¼
b*
¼ gray-balance target (or 0 for equivalent neutral)
(
8
:
16
)
(2) When a printer model is available, compute CMY for the L*a*b* gray-
balance target using the ICI algorithm.
To construct three TRCs, we can select N number of patches using the nominal
CMY values obtained for each of the patch gray-balance targets. As a result of this,
we would have N number of control loops running in parallel. If an actual printer is
used, these loops will be iterating on the printer. If a printer model is available, then
the control iterations are carried out as described in Section 7.5.2.2.
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