Image Processing Reference
In-Depth Information
250
SmoothTRC
Control patches
200
150
100
50
0
0
50
100
150
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250
K
in
FIGURE 8.24
Black channel-wise linearized TRC with highlight and shadow corrections
for K
in
.
8.6.4 P
REDICTIVE
G
RAY
B
ALANCE
In the conventional gray-balance control strategy, N gray patches are used resulting
in N control loops running in parallel. In principle, it is desirable to use a large
number of target patches (i.e., large N) to better capture the printer nonlinearity.
It is also desirable to use large-size patches or multiple patches (at different
locations on the paper) for the same CMY input to reduce the process and sensor
noise by averaging measurements over a large patch or over multiple patches with
same CMY input. Due to cost constraints and competitive pressure, large N or
multiple patches or oversized patches are not allowed. Use of lower number of
control patches is preferred in practice to save cost. Typically, the dimension and
the layout of these target N patches are designed to
fit a single page to save printing
and measurement time as well as material (paper and toner). For a typical printer,
10
11 patches with one being pure white to extract media white can be used to
construct CMY TRCs and another 10
-
11 patches for building black K TRC. By
using predictive gray-balance techniques, we can reduce the total number of control
patches by a factor of two or more that require printing and measurement. There-
fore, the predictive calibration method becomes especially useful for long print runs
(e.g., 20 h) with single
-
=
multiple jobs where calibration pages are interspersed
=
within
between print jobs. For a 20 h long run on a cut sheet digital press, with
two test pages for calibration printed every 30 min, we would end up wasting a
total of 80 pages for calibration only. Hence, any savings in wasted paper is
considered useful.
For illustration, let us assume we decided to use N
ΒΌ
10 control patches for
printer gray-balance calibration. Using the normal control-based technique with
state-feedback approach (Section 8.6), we would print 10 patches per each iteration.
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