Image Processing Reference
In-Depth Information
The basis of the implementation of point operators was given earlier, for addition in
Code
1.3
. The stretching process can be displayed as a mapping between the input and
output ranges, according to the specified relationship, as in Figure
3.3
. Figure
3.3
(a) is a
mapping where the output is a direct copy of the input (this relationship is the dotted line
in Figures
3.3
(c) and (d)); Figure
3.3
(b) is the mapping for brightness
inversion
where dark
parts in an image become bright and vice versa. Figure
3.3
(c) is the mapping for
addition
and Figure
3.3
(d) is the mapping for
multiplication
(or
division
,
if the slope was
less than
that of the input). In these mappings, if the mapping produces values that are smaller than
the expected minimum (say negative when zero represents black), or larger than a specified
maximum, then a
clipping
process can be used to set the output values to a chosen level.
For example, if the relationship between input and output aims to produce output points
with intensity value greater than 255, as used for white, the output value can be set to white
for these points, as it is in Figure
3.3
(c).
Output brightness
Output brightness
White
White
Black
Black
Input brightness
Input brightness
Black
White
Black
White
(a) Copy
(b) Brightness inversion
Output brightness
Output brightness
White
White
Black
Black
Input brightness
Input brightness
Black
White
Black
White
(c) Brightness addition
(d) Brightness scaling by multiplication
Figure 3.3
Intensity mappings
The
sawtooth
operator is an alternative form of the linear operator and uses a repeated
form of the linear operator for chosen intervals in the brightness range. The sawtooth
operator is actually used to emphasise local contrast change (as in images where regions
