FFROST

Description

Parameters

Parameter descriptions

Input: Input detected SAR image

Specifies the name of the file that contains the image layers to be filtered.

Mask: Area mask

Optionally specifies the bitmap that defines the area to be processed within the input raster. If this parameter is not specified, the entire layer is used by default. For a bitmap mask, you must specify the bitmap segment that you want to use. All of the pixels within the specified segment, having a pixel value of 1, define the area to be processed.

Only the area under the mask is written to the output.

Output: Output filtered image

Specifies the output channel(s) to receive the filtered results. If the specified output file already exists, the filtered channels will be appended to the existing file. If the output file does not already exist, a new file is created.

Filter X Size

Optionally specifies the horizontal size of the filter, in pixel units. This value must be an odd integer between 1 and 33. The default value is 7 pixels.

Filter Y Size

Optionally specifies the vertical size of the filter, in pixel units. This value must be an odd integer between 1 and 33. The default value is 7 pixels.

Damping Factor

Optionally specifies the damping constant for the adaptive filter. This constant specifies the extent of the damping effect of the filtering. The default value of 1.0 is sufficient for most SAR images.

Image Type

Optionally specifies the image format of the SAR image, which defines how the correction factor should be computed. By default, the image is assumed to be in amplitude units.

Image units can be "Amplitude" (or equivalently "AMP") or "Power" (or equivalently "POW").

Details

FFROST consists of an adaptive filter used to suppress speckle where the result is further refined (or smoothed, or dampened) by applying a used-defined exponential weight to the initial filter output.

The Frost filter differs from the Lee and Kuan filters in that the scene reflectivity is estimated by convolving the observed image with the impulse response of the SAR system. The impulse response of the SAR system is obtained by minimizing the mean square error between the observed image and the scene reflectivity model.

The damping factor for adaptive filtering depends on the non-filtered image and may require trial-and-error experiments to determine the best value. The value of the damping factor defines the extent of the exponential damping (the smaller the value, the smaller the damping effect). The default damping factor is 1.

The Mask parameter specifies the area within the input channel to be processed. Only the area under the mask is filtered; the rest of the image remains unchanged. This parameter points to a bitmap segment that defines the area to be filtered. If the Mask parameter is not specified, the entire image is processed.

Algorithm

The implementation of this filter consists of defining a circularly symmetric filter with a set of weighting values (M) for each pixel:

M = exp(- A * T)

where:

A = DAMP * (V / I**2)

The resulting gray-level value (R) for the smoothed pixel is:

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

All pixels are filtered. To filter pixels located near the edges of the image, edge pixels values are replicated to provide sufficient data.

          Image before filtering            Image after filtering

          8 8  8 9 9 9 7 6 6 6   6 6
          8 8  8 9 9 9 7 6 6 6   6 6
          +----------------+           +----------------+
          8 8 |8 9 9 9 7 6 6 6 | 6 6       |8 8 8 8 7 7 6 6 |
          8 8 |8 9 9 7 6 6 6 6 | 6 6       |8 8 8 7 7 6 6 6 |
          9 9 |9 8 8 6 6 6 6 6 | 6 6       |8 8 8 7 7 6 6 6 |
          9 9 |9 8 7 7 6 5 6 6 | 6 6       |8 7 7 7 6 6 6 6 |
          7 7 |7 7 7 6 6 6 6 6 | 6 6       |7 7 7 6 6 6 6 6 |
          6 6 |6 6 6 6 6 6 6 6 | 6 6       |7 7 6 6 6 6 6 6 |
          6 6 |6 6 6 6 6 6 6 5 | 5 5       |6 6 6 6 6 6 6 6 |
          6 6 |6 6 6 6 6 6 6 6 | 6 6       |6 6 6 6 6 6 6 6 |
          +----------------+           +----------------+
          6 6  6 6 6 6 6 6 6 6   6 6
          6 6  6 6 6 6 6 6 6 6   6 6
        

In this example, the filter does not have a full window. To rectify the situation, the edge pixels are replicated, hence the extra rows and columns of data outside the box in the Image before filtering section.

References

V.S. Frost, J.A. Stiles, K.S. Shanmugan, and J.C. Holtzman, "A model for radar images and its application to adaptive digital filtering of multiplicative noise," IEEE Trans. Pattern Analysis and Machine Intelligence, vol. 4, no. 2, pp. 157-166, March 1982.