Image Processing Reference
In-Depth Information
Fig. 4.5
Gamma-mapping
curves for different gammas
positive
γ
as
f(x,y)
γ
=
g(x,y)
(4.5)
Some gamma-mapping curves are illustrated in Fig.
4.5
.For
γ
=
1 we get the iden-
tity mapping. For 0
<γ <
1 we increase the dynamics in the dark areas by in-
creasing the mid-levels. For
γ>
1 we increase the dynamics in the bright areas by
decreasing the mid-levels. The gamma mapping is defined so that the input and out-
put pixel values are in the range
. It is therefore necessary to first transform the
input pixel values by dividing each pixel value with 255 before the gamma trans-
formation. The output values should also be scaled from
[
0
,
1
]
[
0
,
1
]
to
[
0
,
255
]
after the
gamma transformation.
A concrete example is given. A pixel in a gray-scale image with value
v
in
=
120
is gamma mapped with
γ
=
2
.
22. Initially, the pixel value is transformed into the
interval
[
0
,
1
]
by dividing with 255,
v
1
=
120
/
255
=
0
.
4706. Secondly, the gamma
0
.
4706
2
.
22
mapping is performed
v
2
=
=
0
.
1876. Finally, it is mapped back to the
interval
[
0
,
255
]
giving the result
v
out
=
0
.
1876
·
255
=
47. Examples are illustrated
in Fig.
4.6
.
Fig. 4.6
Gamma mapping to the
left
with
γ
=
0
.
45 and to the
right
with
γ
=
2
.
22. In the
middle
the original image
