Image Processing Reference
In-Depth Information
of interest can be light or dark). This is illustrated in Figure 3.4 where the range of
brightness levels is mapped into four linear regions by the sawtooth operator, Figure
3.4 (b). This remaps the intensity in the eye image to highlight local intensity variation, as
opposed to global variation, in Figure 3.4 (a). The image is now presented in regions, where
the region selection is controlled by its pixels' intensities.
50
Saw _Tooth bright
0
100 200
Bright
(a) Image of 'sawn' eye
(b) Sawtooth operator
Figure 3.4
Applying the sawtooth operator
Finally, rather than simple multiplication we can use arithmetic functions such as logarithm
to reduce the range or exponent to increase it. This can be used, say, to equalise the
response of a camera, or to compress the range of displayed brightness levels. If the camera
has a known exponential performance, and outputs a value for brightness which is proportional
to the exponential of the brightness of the corresponding point in the scene of view, the
application of a logarithmic point operator will restore the original range of brightness
levels. The effect of replacing brightness by a scaled version of its natural logarithm
(implemented as N x,y = 20 ln(100 O x,y )) is shown in Figure 3.5 (a); the effect of a scaled
version of the exponent (implemented as N x,y = 20 exp( O x,y /100)) is shown in Figure
(a) Logarithmic compression
(b) Exponential expansion
Figure 3.5
Applying exponential and logarithmic point operators
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