62
2: Raghuveer M. Rao, Manoj K. Arora
intensity values above 230 are set to 255. Intensity value transformations are
then applied as
y == 3. 5(x - 125) for 125 ~ x ~ 175
and
16
Y == -(x - 175) + 175 for 175 ~ x ~ 230 .
11
(2.6)
(2.7)
The effect of these transformations is to map values between 125 and 175
to 0 and 175, and to map values between 175 and 230 to 175 and 255. The
value of 175 itself is chosen because of it being the third mode as shown in the
histogram. The piecewise linear mapping corresponding to these operations
is also shown in Fig 2.6b. The operation can be seen as a form of stretching
in the sense that values between 125 and 230 are stretched to a new set of
values.
To the extent that a pointwise operator depends only on the local pixel value,
it can be implemented as a lookup table. Any such lookup table is by definition a pointwise operator and for gray values in the range 0 to 255, there are
256 factorial such tables. However, only a few will lead to meaningful results.
Apart from the histogram equalization or the linear stretching for contrast
enhancement, which have been illustrated above and are dependent on the
a
b
Fig. 2.7. a Original image. b Image after nonlinear contrast enhancement
2: Raghuveer M. Rao, Manoj K. Arora
intensity values above 230 are set to 255. Intensity value transformations are
then applied as
y == 3. 5(x - 125) for 125 ~ x ~ 175
and
16
Y == -(x - 175) + 175 for 175 ~ x ~ 230 .
11
(2.6)
(2.7)
The effect of these transformations is to map values between 125 and 175
to 0 and 175, and to map values between 175 and 230 to 175 and 255. The
value of 175 itself is chosen because of it being the third mode as shown in the
histogram. The piecewise linear mapping corresponding to these operations
is also shown in Fig 2.6b. The operation can be seen as a form of stretching
in the sense that values between 125 and 230 are stretched to a new set of
values.
To the extent that a pointwise operator depends only on the local pixel value,
it can be implemented as a lookup table. Any such lookup table is by definition a pointwise operator and for gray values in the range 0 to 255, there are
256 factorial such tables. However, only a few will lead to meaningful results.
Apart from the histogram equalization or the linear stretching for contrast
enhancement, which have been illustrated above and are dependent on the
a
b
Fig. 2.7. a Original image. b Image after nonlinear contrast enhancement