Parent Topic: ALGORITHM

ENHANCED FROST FILTER

The enhanced Frost filter is similar to the enhanced Lee filter in that it considers three different types of image areas separately: homogeneous areas, heterogeneous area, and isolated point targets. In the heterogeneous case, the filter output is obtained from convolving the image with a circular kernel.

The filter output is:

       R = Im                  for Ci <= Cu
       R = Rf                  for Cu < Ci < Cmax
       R = Ic                  for Ci >= Cmax

Where Rf is the result of convolving the image with a circularly symmetric filter whose weighting values M for each pixel is:

       M = exp(- A * T)

where:

       A   =   DAMP * (Ci-Cu)/(Cmax-Ci)
       Ci  =   S / Im
       Cu  =   SQRT(1/NLOOK)
       Cmax =  SQRT(1+2/NLOOK)         
       Im  =   mean value of intensity within window
       S   =   standard deviation of intensity within window
       T   =   the absolute value of the pixel distance
               from the centre pixel to its neighbours in 
               the filter window       

       DAMP =  exponential damping factor

The resulting grey-level value Rf for the smoothed pixel is:

       Rf = (P1*M1 + P2*M2 + ... +  Pn*Mn) / (M1 + M2 + ... + Mn)

where:

       P1 .. Pn   are grey levels of each pixel in filter window

       M1 .. Mn   are weights (as defined above) for each pixel

Parent Topic: ALGORITHM
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