Neighborhood Processing - Introduction to Video and Image Processing - page 71

Fig. 5.1 A part of the giraffe-image has been enlarged to show the edge which humans easily
perceive. Using methods described in this chapter the computer will also be able to tell where the
edge is

Fig. 5.2 The principle of neighborhood processing. To calculate a pixel in the output image, a
pixel from the input image and its neighbors are processed

205 + 204 + 204 + 206 + 0 + 208 + 201 + 199 + 205

Mean value =

9

= 181 . 3 181 (5.1)

This results in the noise pixel being replaced by 181, which is more similar to
the neighbors. However, the value still stands out and therefore the median is often
used instead. The median value of a group of numbers is found by ordering the
numbers in increasing order and picking the middle value. Say we use the eight
nearest neighbors for the first pixel infested by noise in Fig. 5.3 . The ordering yields

Ordering :[ 0 , 199 , 201 , 204 , 204 , 205 , 205 , 206 , 208 ]

Median = 204 (5.2)

and the middle value is 204, hence the median is 204. The noise pixel is now re-
placed by 204, which does not stand out.