12.5.2 Correction Using the Recorded Image
In the first approach, the recorded digital image cm, n½is used to estimate the
background shading pattern. Three different methods can be used to estimate
the shading pattern. In the first, low-pass filtering is applied to smooth cm, n
½
,
where the smoothing effect is larger than the size of objects in the image.
The choice of the low-pass filter requires that the spatial frequencies where
the shading persists be known. The corrected image,
^
am, n½, is then estimated
as follows [19]
^
am, n½¼cm, n½LowPass cm, n½
fg
þ constant (12:11)
Second, instead of low-pass filtering cm, n½, smoothing can be performed by
using morphological filtering (Chapter 8). Morphological filtering is used when
the background variation ...