FELEE

Description

Parameters

Parameter descriptions

Input: Input detected SAR image

Specifies 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.

Number of Looks

Optionally specifies the effective number of looks of the SAR image. This is used to derive noise variance. Over homogeneous areas, the effective number of looks can be computed as the mean value squared divided by the variance (for amplitude data) or the mean divided by the standard deviation (for power data). By adjusting this parameter value, you can control the amount of smoothing applied to the image.

Acceptable values are 1 to 100. The default value of 1 is appropriate for single-look data and specifies maximum smoothing correction to the image.

Instead of 1, the default value of this parameter will be the file level metadata value for 'NumLooks' if this metadata exists.

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 Units

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

FELEE is used primarily to filter speckled radar data. The enhanced Lee filter smoothes the image data, without removing edges or sharp features in the images.

The enhanced Lee filter divides an image into areas of three classes. The first class corresponds to the homogeneous areas in which the speckles may be eliminated simply by applying a low pass filter (or equivalently, averaging, multi-look processing). The second class corresponds to the heterogeneous areas in which the speckles are to be reduced while preserving texture. The third class includes areas containing isolated point targets, in which the filter should preserve the observed value.

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.

Algorithm

FELEE performs spatial filtering on each individual pixel in an image using the gray-level values in a square window surrounding each pixel. The dimensions of the filter must be odd, and can be from 3x3 to 11x11 pixels. All pixels are filtered. To filter pixels located near edges of the image, edge-pixels are replicated to give sufficient data.

        +----------+
        | a1 a2 a3 |
        | a4 a5 a6 |     <---  Filter window 3 X 3
        | a7 a8 a9 |
        +----------+
      

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

        R = I                 for Ci less than or equal to Cu
        R = I*W + CP*(1-W)    for Cu < Ci < Cmax
        R = CP                for Ci greater than or equal to Cmax
      

For amplitude images, each gray level will be squared and the square root will be applied to the final result.

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

To compare different radar filtering methods, refer to the second paper listed in References.

References

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 IGARSS 94, August 8-12, 1994.