Image Processing Reference
In-Depth Information
Measurement guidelines, such as same paper, same white point, same stock backing,
averaging with multiple sets, etc., have to be strictly followed when a spectrophotom-
eter is used. Measurement procedures adopted during the printer characterization
should match those during the pro
le validation stage. Otherwise, validation will
not be accurate. For determining round trip accuracy, use the measured L*a*b* values
as inputs to the B2A tag. Create patches for the output of B2A tag and print and
measure. If this procedure is dif
cult to implement, then create patches using the
measured L*a*b* values and print them through the ICC pro
le and measure
the printed patches. This procedure will eventually result in a second set of L*a*b*
values. Compare the gamuts between the second and the
first set of L*a*b* values.
Also obtain the round trip accuracy statistics. A vector plot of these L*a*b* values will
give additional information about the rendering accuracy, which is particularly useful
to accessing the spot roundtrip inaccuracy in localized regions of the color space.
7.7.2 G
AMUT
C
ORNER
P
LOTS AND
N
EUTRAL
R
ESPONSE
Gamut corner plots can give additional measurement of the quality of inversion
and gamut-mapping algorithms. They can be used to identify unsmooth color
gradients and help ensure smoothness in CMYK formulations in critical regions of
the transformation.
A generic CMYK printer gamut has at least 14 distinct corners, 24 edges, and 12
faces. The corner CMYK and corresponding L*a*b* values for a representative printer
are shown in Table 7.14. Figure 7.64a and b show these corners when viewed from the
top and bottom, respectively, and Figure 7.64c shows all 14 lines emanating through
the corners until they touch the sides of the L*a*b* cube. Figures 7.65(a to d) and 7.66
TABLE 7.14
CMYK and L*a*b* Values for a Typical EP Production Printer
Gamut Corners
Corners
C MY
K
L*
a*
b*
Cyan
255
0
0
0
50.05
29.98
57.79
Magenta
0
255
0
0
50.83
84.90
6.47
Yellow
0
0
255
0
95.21
7.63
111.12
White
0
0
0
0
100.00
0.00
0.01
Red
0
255
255
0
51.93
72.34
47.80
63.37
Green
255
0
255
0
50.34
35.09
Blue
255
255
0
0
20.89
43.68
51.07
Dark cyan
255
0
0
255
17.51
10.08
21.40
Dark magenta
0
255
0
255
17.34
24.71
0.70
Dark yellow
0
0
255
255
27.78
3.11
27.80
Dark black
0
0
0
255
12.64
1.11
1.00
Dark blue
255
255
0
255
14.70
21.17
24.02
Dark green
255
0
255
255
27.01
33.17
16.33
Dark red
0
255
255
255
23.41
30.65
17.67
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