FFROST -- Frost SAR Speckle Filter

Applies a Frost speckle filter on a SAR image. This filter is primarily used on radar data to remove high frequency noise (speckle), while preserving high frequency features (edges).

See Also: FSPEC, FAV, FED, FGA, FME, FMO, FSHARP, FSOBEL, FPR, FPRE

INPUT PORT(S)

Input

Type:	Raster
Connection:	Mandatory
Minimum Layers:	1
Maximum Layers:	1024

Contains the input image layer(s), consisting of the data to be filtered.

Mask

Type:	Bitmap
Connection:	Optional
Minimum Layers:	0
Maximum Layers:	1

Contains the input area mask layer, consisting of a bitmap indicating which area in the input image layer(s) should be processed. If no bitmap is provided the entire layer is processed.

OUTPUT PORT(S)

Output

Type:	Raster
Connection:	Optional
Minimum Layers:	0
Maximum Layers:	1024

Contains the output layer(s) that will receive the filtered image data.

Only the area under Mask is written to Output.

INPUT PARAMETER(S)

FFROST is controlled by the following parameter(s):

Image Type

Name:	ImageType
Type:	Text
Valid Values:	AMP, POW
Default:	AMP
Requirement:	Mandatory

Specifies the image format of the radar image, which can be supplied in either the amplitude or power (intensity) format.

 AMP   | image is in amplitude format
 POW   | image is in power format

Amplitude is the square root of power. Most radar images are supplied in the amplitude format to preserve the values.

Filter X Size (Pixels)

Name:	FiltPixels
Type:	Integer
Valid Values:	1 <= x <= 33
Default:	3
Requirement:	Mandatory

Specifies the filter's X size (pixels). The filter need not be square. Acceptable values are odd integers from 1 to 33, inclusive.

Filter Y Size (Lines)

Name:	FiltLines
Type:	Integer
Valid Values:	3 <= x <= 33
Default:	3
Requirement:	Mandatory

Specifies the filter's Y size (lines). The filter need not be square. Acceptable values are odd integers from 3 to 33, inclusive.

Number of Looks

Name:	NumberLooks
Type:	Integer
Valid Values:	0 <= x <= 100
Default:	1
Requirement:	Mandatory

Specifies the (effective) number of looks of the image. This is used to derive noise variance. By adjusting NumberLooks, the user can control the amount of smoothing applied to the image.

Damping Factor

Name:	DampFactor
Type:	Integer
Valid Values:	0 <= x <= 10
Default:	1
Requirement:	Mandatory

Specifies the damping constant for the adaptive filter. This constant specifies the extent of the damping effect of adaptive filtering. The default value of 1 is sufficient for most SAR images.

The use of large values for DampFactor, allows for better preservation of sharp edges, but reduces the smoothing effect.
The use of small values for DampFactor, increases the smoothing effect, but does not preserve sharp edges well.
If DampFactor is 0, the results are identical to a smoothing (or averaging) filter (see the FAV module), where the weights of each pixel in the filter window are equal.

DETAILS

The Frost filter is used primarily to filter speckled radar data. It is designed to smooth out noise while retaining edges or shape features in the image.

The filter size can be specified through the FilterSize parameters. Different filter sizes will greatly affect the quality of processed images. If the filter is too small, the noise filtering algorithm is not effective. If the filter is too large, subtle details of the image will be lost in the filtering process. A 7x7 filter usually gives the best results.

The NumberLooks parameter is used to estimate noise variance and it effectively controls the amount of smoothing applied to the image by the filter. Theoretically, the correct value for NumberLooks should be the effective number of looks of the radar image. It should be close to the actual number of looks, but may be different if the image has undergone resampling. The user may experimentally adjust the NumberLooks value so as to control the effect of the filter. A smaller NumberLooks value leads to more smoothing; a larger NumberLooks value preserves more image features.

A damping factor (DampFactor) is required by the Frost filter. The value of DampFactor defines the extent of exponential damping (the smaller the value, the smaller the damping effect). It depends on the non-filtered image and may require trial-and-error experiments to determine the best value. The default value for DampFactor is 1.

FFROST performs spatial filtering on each individual pixel in an image using the grey level values in a window surrounding each pixel. The dimensions of the filter must be odd, and must be at least 1x3.

All pixels are filtered. In order to filter pixels located near the edges of the image, edge-pixel values are replicated to give sufficient data.

A bitmap specifies the area within the input layer which will be processed. Only this area will be filtered and the rest of the image will be unchanged. If no bitmap is connected, the entire database is processed.

The Frost filter model requires that the signal represents power. If the input image is in amplitude format, each grey level will be squared to derive power and finally square root will be applied to the filtered result.

ALGORITHM

Implementation of the speckle filters are based on the following papers, and especially the review paper by Shi and Fung.

 Jong-Sen Lee, "Digital Image Enhancement and Noise Filtering
 by Use of Local Statistics", IEEE Transactions on Pattern
 Analysis and Machine Intelligence, Vol. PAM1-2, No. 2, March, 1980.

 J.S.Lee, "Refined Filtering of Image Noise Using Local Statistics"
 Computer Graphic and Image Processing 15, 380-389 (1981)

 D.T. Kuan, A.A. Sawchuk, T.C. Strand, and P. Chavel, 
 "Adaptive restoration of images with speckle," IEEE Trans. ASSP.,
 Vol. 35, no. 3, pp. 373-383, March 1987.

 A. Lopes, R. Touzi and E. Nezry, "Adaptive speckle filters and
 Scene heterogeneity", IEEE Transaction on Geoscience and Remote
 Sensing, Vol. 28, No. 6, pp. 992-1000, Nov. 1990.

 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. 

 A. Lopes, E. Nezry, R. Touzi, and H. Laur, "Structure detection
 and statistical adaptive speckle filtering in SAR images",
 International Journal of Remote Sensing, Vol. 14, No. 9, 
 pp. 1735-1758, 1993.

 A. Lopes, R. Touzi and E. Nezry, "Adaptive speckle filters and
 Scene heterogeneity", IEEE Transaction on Geoscience and Remote
 Sensing, Vol. 28, No. 6, pp. 992-1000, Nov. 1990.

 Zhenghao Shi and Ko B. Fung, 1994, "A Comparison of Digital
 Speckle Filters", Proceedings of IGRASS 94, August 8-12, 1994.

FROST FILTER

The Frost filter is an adaptive filtering algorithm. It uses an exponentially damped convolution kernel which adapts itself to features based on local statistics. The Frost filter differs from the Lee and Kuan filters with respect 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 which is assumed to be an autoregressive process.

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   =   DampFactor * Ci^2
       Ci  =   S / Im
       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       

       DampFactor = exponential damping factor

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

         R = (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

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