Image Processing Reference
In-Depth Information
TABLE 8.1
G7 Gray-Balance Chart for a Commercial-Coated Paper
CMY (%)
Nominal CIELab
C
M
Y
a*
b*
2
0
0
0
0
2
12.5
9
9
0
25.1
18.8
18.8
0
2
37.3
29
29
0
2
49.8
40
40
0
2
62.7
52.9
52.9
0
1.5
75.3
66.3
66.3
0
1
color of a 50% black ink tint on paper. To ensure the best chance of matching the
output of devices with different colored papers or black ink, GRACoL Standards
Organization [1] de
nes neutral gray balance in colorimetric terms with non-equal
CMY as [50 C, 40 M, 40 Y]
¼
[a*
¼
0.0, b*
¼
2] with a tolerance of
0.5 for a*
and
1.0 for b*. Since most people prefer a slightly bluer gray, b* is set close to
ed by GRACoL
standards, as shown in Table 8.1 for other CMY area coverages on a commercial-
coated paper, where paper is near neutral with a*
2.0 instead of 0, as in neutral color. A similar de
nition is speci
nition
of gray balance for different paper types is still under discussion by the Standards
Organization. However, in Ref. [1], for gray balance, target a* and b* values are
shown for a subset of nonstandard paper.
If we disregard the effects of paper, then a good gray has zero chroma (i.e.,
¼
0.0 and b*
¼
2. The de
¼
b*
¼
a*
0.0). When equal amounts of cyan, magenta, and yellow are printed on
white paper, a well-balanced printer should produce an equivalent neutral gray.
However, a brownish color rather than a neutral gray often occurs due to paper
effects. As in GRACoL standards, the CMY values for one or all three of the colors
may need readjustment to properly reproduce the gray scale within standards.
Rather than trying to develop a gray-balance approach for various de
nitions,
we show a general approach in which the gray-balance calibration is performed
to make the engine produce equivalent neutral gray with equal amounts of CMY colors
[2,3]. For example,
the equivalent neutral gray means that
input digital count
40 should produce gray with L
*
100
of C
¼
M
¼
Y
¼
¼
100
255
40
¼
84
:
3 and
a*
nition. To have a gray-
balanced device, the input CMY digital counts have to be passed through a
transformation LUTs to produce device C
0
M
0
Y
0
, which is then used to make prints.
The three 1-D transformations C
0
¼
f
c
(C), M
0
¼
f
c
(M), and Y
0
¼
f
c
(Y) that map the input
CMY
¼
¼
b*
¼
0. The K separation does not come into this de
[C
0
M
0
Y
0
]
T
for every digital count (or area
coverage) are called tone reproduction curves. In practice, these three TRCs are given
by three 1-D LUTs. Each LUT has 256 entries for input digital counts of 0
[CMY]
T
to the device CMY
¼
255. This
should not to be confused with channel-wise linearization, as in the paper-based
level 3 controls described in Chapter 9 and later in this chapter. Such 1-D TRCs
-
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