The third subunit is the Frequency Domain Filtering.
Again two possibilities are available. Either filtering with
frequency filters defined in the spatial domain and then
transferred to the frequency domain by FFT (demonstration
of the Prewitt frequency filter is in Fig. 4a) or filters defined
directly in the frequency domain. Filtering in frequency
domain is useful for example when the original
image has a line-structure. That means that every even (or
every odd) line of the picture is missing (all pixels of the
line are black – Fig. 4b). Image like that can be obtained by
taking a picture of the CRT screen. The result of filtering
with gaussian frequency filter from Fig. 4c) is in Fig. 4d).
The line-structure is corrected but the output image is
slightly blurred.