Image Processing Reference
In-Depth Information
the gamut may be handled incorrectly by the gamut-mapping algorithm, which can
lead to unacceptable artifacts (e.g., contours) and color errors when images are
rendered through those multidimensional LUTs. Also, for spot colors, if the out-
of-gamut colors are wrongly assigned as
, then the consequence of this
decision might lead to mapping the colors to a wrong region of the destination
gamut. In such a case, the reproduction of the mapped color can be
in-gamut
''
''
from
the original. Such mistakes can easily be avoided by not mapping the wrongly
assigned in-gamut color. We present below a control-based method to decide
whether node colors are inside or outside a printable gamut.
Consider a ray (modeled by a line), expressed in L*a*b* space by Equation 7.95
with start and end points given by x
0
and x
c
, where x
0
and x
c
have coordinates
{L
0
way off
''
''
a
0
b
0
} and {L
c
a
c
b
c
}, respectively:
x
¼
x
0
þ
mi
(
7
:
95
)
where
2
3
2
3
2
3
L
a
b
L
0
a
0
b
0
L
c
L
0
a
c
a
0
b
c
b
0
1
N
4
5
,
4
5
, m
¼
4
5
x
¼
x
0
¼
(
7
:
96
)
-
x
c
is the node color whose in
out determination has to be made. x
0
is inside the
gamut (e.g., a centroid of the gamut). In Equation 7.95, i is the index that is
incremented from 0 to a convenient integer until the node color x
c
. When i
¼
0, the
line will start at x
¼
x
0
. When i
¼
N, the line will end on the node color, where N is
equal to the number of points along the line. Figure 7.56 shows schematically a
chroma plane and a line starting from point x
0
to the node color x
c
. For
i
¼
0toN,
there will be a total of N
þ
1 colors whose values are given by x along the ray
between x
0
and x
c
(including extreme points). Now, 4-to-3 control iterations, as in
Figure 7.41, are carried out using x value as the target for i
¼
0toN. A GCR LUT can
b
127
x
c
x
b
a
x
0
-128
127
Gamut in 2-D
chroma plane
-128
FIGURE 7.56
A line between start and end points.
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