FKUAN

Description

FKUAN performs Kuan filtering on image data. The Kuan filter is primarily used on SAR data to remove high-frequency noise (speckle) while preserving high-frequency features (edges).

Parameters

Name	Type	Length	Value range
Input: Input detected SAR image *	Raster port	1 -
Mask: Area mask	Bitmap port	0 - 1
Output: Output filtered image *	Raster port	1 - 1
Filter X Size	Integer	0 - 1	1 | 3 | 5 | 7 | 9 | 11 | 13 | 15 | 17 | 19 | 21 | 23 | 25 | 27 | 29 | 31 | 33 Default: 7
Filter Y Size	Integer	0 - 1	1 | 3 | 5 | 7 | 9 | 11 | 13 | 15 | 17 | 19 | 21 | 23 | 25 | 27 | 29 | 31 | 33 Default: 7
Number of Looks	Float	0 - 1	1 - 100 Default: 1
Image Units	String	0 - 1	Amplitude | Power Default: Amplitude

* Required parameter

Parameter descriptions

Input: Input detected SAR image

Specifies 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.

To define 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. Pixels that are outside the mask but within the filter size window will be used to determine the output pixel values.

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.

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

FKUAN is used primarily to filter speckled radar data. The Kuan filter smoothes the image data without removing edges or sharp features in the images. It is only applicable for radar-intensity images.

The Kuan filter first transforms the multiplicative noise model into a signal-dependent additive noise model. Then the minimum mean square error criteria is applied to the model. The resulting filter has the same form as the Lee filter but with a different weighting function. Because the Kuan filter makes no approximation to the original model, it can be considered superior to the Lee filter.

The filter size can be specified through the Filter X Size and Filter Y Size 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.

Theoretically, the correct value for the Number of looks parameter 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.

Radar images can either be in amplitude or power (intensity) format. Amplitude is the square root of power. Most radar images are stored in amplitude format.

Algorithm

FKUAN performs spatial filtering on each individual pixel in an image using the gray-level values in a square window surrounding each pixel. The values for the filter size must be odd integers, and can be 1 to 33 pixels. All pixels are filtered. To filter pixels located near edges of the image, edge-pixels are replicated to provide 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 = CP*W + I*(1-W)
      

References

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.

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