Image Processing Reference

In-Depth Information

Fig. 4.15
The effect of histogram stretching and histogram equalization on an input image with

both very high and very low pixel values

Fig. 4.16
An example of thresholding. Notice that it is impossible to define a perfect silhouette

with the thresholding algorithm. This is in general the case

4.4

Thresholding

One of the most fundamental point processing operations is
thresholding
. Thresh-

olding is the special case when
f
1
=
f
2
in Eq.
4.11
. Mathematically this is unde-

fined, but in practice it simply means that all input values below
f
1
are mapped to

zero in the output and all input values above
f
1
are mapped to 255 in the output.

This means that we will only have completely black and completely white pixel

values in the output image. Such an image is denoted a
binary image
, see Fig.
4.16
,

and this representation of an object is denoted the
silhouette
of the object.