Image Processing Reference
In-Depth Information
neutral colors, but is not linear for other regions of the color axes in the 3-D CMY
color space. If the reference values are off-neutral, then colors are adjusted by
changing CMY to follow the off-neutral axis, effectively making the printer linear
to the off-neutral reference axis. To perform gray-balance calibration, we take the
following steps:
(1) Generate a set of CMY color patches in and around the neutral axis (our
reference axis). Print and measure the corresponding L*a*b* values and
form a printer forward map. This can be achieved either by creating M
3
uniformly spaced CMY patches to form a LUT or by measuring fewer
patches to
fit a model and then use the model to create the desired LUT
(i.e., uniformly spaced CMY
!
L*a*b* forward map).
(2) Utilize interpolation (ICI or tetrahedral) or a regression method to compute
N CMY values that yield a neutral measurement (a*
¼
b*
¼
0) for the L*
(L
1
, L
2
,
, L
N
).
values spanning the dynamic range of
the printer
...
¼
b*
¼
If the CMY gamut coverage is such that
0 axis is not
within the CMY gamut, then create smooth off-neutral reference
values. In particular, this may be required for dark regions of the color
space. Let the CMY values corresponding to L*i
i
the a*
be CMY
i
¼
[C
i
M
i
Y
i
].
Now compute
255
100
D
i
¼
100
L
i
for
i
¼
1, 2,
...
, N
(
8
:
6
)
(3) Create the three TRCs using the following functions:
(a) Cyan TRC:
where
f
1
:
D
i
!
C
i
C
out
¼
f
1
C
i
ðÞ
,
(
8
:
7
)
(b) Magenta TRC:
where
f
2
:
D
i
!
M
i
M
out
¼
f
2
M
i
ðÞ
,
(
8
:
8
)
(c) Yellow TRC:
where
f
3
:
D
i
!
Y
i
Y
out
¼
f
3
Y
i
ðÞ
,
(
8
:
9
)
ned for N input digital counts.
Linear or cubic interpolation is used to obtain all the values from 0 to 255, in steps of
one digital count. Also, TRCs should be smooth and monotonically increasing
functions of their arguments. If required, 1-D smoothing techniques described in
Section 6.6.2.1 are used to generate smooth TRCs for all three channels.
Note that the above three TRCs are only de
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