14.10.2 Nonlinear Filters
Nonlinear filters can also be implemented by bx, y, zðÞ¼fax, y, zðÞðÞ,as
earlier, except that f is no longer a linear function. They are more general and
can be designed to preserve edge information and remove impulse noise. Some
representative nonlinear 3-D filters are discussed next.
14.10.2.1 Median Filter
A median filter belongs to the class of order-statistic filters, i.e., filters based on
a rank ordering of the input voxel values. Most implementations use a square
kernel of neighborhood pixels (e.g., 3 3, 5 5) as the filter window. Consider
the voxels X
1
,X
2
, ..., X
W
, where W is an odd number, within a filter window of
size W ¼ L M N. Then the rank-ordered voxel values can be expressed as
X
1
< X
2
< < X
i
< X
W
, and