Image Processing Reference
In-Depth Information
Calibrated
C'M'Y'K'
L
*
a
*
b
*
CMYK
sRGB
CMYK
Calibration
Printer
With smoothing
algorithm
Δ
E
2000
sRGB
Δ
E
2000
computation
Calibrated
C'M'Y'K'
L
*
a
*
b
*
CMYK
sRGB
CMYK
Calibration
Printer
Without smoothing
algorithm
FIGURE 6.22
Image reproduction with LUTs.
6.6.3 A
PPLICATION TO
P
RINTING
S
YSTEMS
As an example, consider the image reproduction system shown in Figure 6.22. In this
example, the input sRGB image is mapped to the CMYK color space by a multi-
dimensional pro
le LUT, which is a GCR-constrained inverse of the printer (Chapter 7).
The CMYK pixels can be processed through 1-D calibration tone reproduction curves
to generate the printer C
0
M
0
Y
0
K
0
. In this case, the pro
ling LUT is a mapping from the
3-D device-dependent sRGB to the 4-D device-dependent CMYK color space. Since
color measurement is a noisy process, the
final LUT may not be a smooth 3-D function.
To produce a smooth characterization LUT, we insert a 3-D smoothing algorithm.
The results of simulations of the Lena image thorough the reproduction system
of Figure 6.22 with and without a smoothing algorithm for a noisy sRGB to CMYK
LUT are shown in Table 6.17.
D
E
2000
is used as a measure of the performance. The
lower the
D
E
2000
, the better the color balance and image quality become. It is
computed by converting the input sRGB image to L*a*b* and comparing the
L*a*b* to the printer output L*a*b*. The smoothing parameter
is changed from
0 to 2. The results shown in Table 6.18 correspond to the optimal choice of
a
a¼
0.5.
As can be seen, there is a 50% improvement in the 95th percentile.
The original and reproduced images with and without smoothing are shown in
Figures 6.23 through 6.25. The results of the
D
E
2000
accuracy in terms of maximum,
mean, and 95th percentile
D
E
2000
for different values of
a
are shown in Figure 6.26.
TABLE 6.17
Simulation Results of 3-D Pro
ling
Mean
D
E
2000
Maximum
D
E
2000
95th Percentile
No noise LUT
0.465
6.954
1.664
Noise and no smoothing
2.891
12.999
6.005
Noise and smoothing
1.609
7.419
2.923
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