Image Processing Reference
In-Depth Information
x
c
x
b
Soft gamut
mapping curve
x
0
x
c
x
b
Colors along the ray
FIGURE 7.59
Soft gamut mapping.
A soft gamut-mapping curve can be constructed using a piecewise linear com-
pression function shown below followed by a 1-D smoothing algorithm described in
Section 6.5.3.1.
h
i
h
i
x
b
x
0
N
x
c
x
0
N
x
¼ (
1
b)
x
0
þ
i
þ b
x
0
þ
i
for
0
i
i
b
(
7
:
97
)
h
i
x
b
x
0
N
x
¼ (
1
b)
x
0
þ
i
þ b
x
b
for
i
b
i
N
(
7
:
98
)
where 0
1 is the compression parameter, i is the index incremented between 0
to a convenient integer value until x
c
. Figure 7.60 shows L*, a*, b* curves with
respect to color indices i for three different values of
b
b
.
0, the interval {x
0
to x
c
} is mapped linearly into {x
0
to x
b
}. A ray-
based control model is run for
When
b¼
0 (representing color x
0
)toi
¼
N
(representing color x
c
which is now mapped to x
b
) to obtain the CMYK values
corresponding to the colors along the ray. If the out-of-gamut node color happens
to fall anywhere between x
b
and x
c
then it is mapped according to the rules described
below. With
b¼
0 and i
¼
0, all colors including the in-gamut colors are moved inward except
for the node color x
c
which is clipped to the boundary. As
b¼
b
increases to a value
greater than 0, the in-gamut colors are compressed less. For
1, all in-gamut
colors are passed through the mapping without any compression. All colors greater
than x
b
are mapped to x
b
. Smoother compression can be achieved using the 1-D
b¼
filtering of the function shown in Equations 7.97 and 7.98. In Figure 7.60, we used
the 1-D
filtering algorithm described in Section 6.6.2 with
a
set equal to 100,000.
Both
contribute to the overall smoothness of the soft gamut-mapping curves
and can be customized differently to each region of the gamut.
A conventional gamut-mapping approach is described below that preserves the
lightness and hue angle.
b
and
a
Search WWH ::
Custom Search