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

fkuan(file, dbic, dboc, flsz, mask, nlook, imagefmt)

Name	Type	Caption	Length	Value range
FILE *	str	Input detected SAR image	1 -
DBIC *	List[int]	Input raster channel	1 - 1
DBOC *	List[int]	Output filtered image	1 - 1
FLSZ	List[int]	Filter size	0 - 2	1 | 3 | 5 | 7 | 9 | 11 | 13 | 15 | 17 | 19 | 21 | 23 | 25 | 27 | 29 | 31 | 33 Default: 7,7
MASK	List[int]	Area mask	0 - 4
NLOOK	List[float]	Number of looks	0 - 1	1 - 100 Default: 1
IMAGEFMT	str	Image units	0 - 9	Amplitude | Power Default: Amplitude

* Required parameter

Parameter descriptions

FILE

Specifies the name of the file that contains the image data to be filtered and to receive the output.

DBIC

Specifies the channel in the input detected SAR image to be processed.

DBOC

Specifies the output channel to receive the filtered results. This channel must already exist within the input file (FILE). If this parameter specifies the same channel as the one defined as the input (DBIC), DBOC overwrites the input channel.

FLSZ

Optionally specifies the horizontal and vertical dimensions of the filter, in pixel units. Acceptable values are odd integers from 1 to 33, inclusively. The filter need not be square. Minimum filter sizes are 1x3 or 3x1; the default size is 7x7.

MASK

Optionally specifies the window or bitmap that defines the area to be processed within the input raster. If this parameter is not specified, the entire channel is processed.

If a single value is specified, that value represents the channel number of the bitmap segment in the input file. All the pixels within the specified segment, having a pixel value of 1, define the area to be processed.

If four values are specified, they define the X,Y offsets and X,Y dimensions of a rectangular window identifying the area to process. Xoffset, Yoffset define the upper-left starting pixel coordinates of the window. Xsize is the number of pixels that define the window width. Ysize is the number of lines that define the window height.

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.

NLOOK

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.

IMAGEFMT

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 FLSZ parameter. 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.

Example

Remove the noise from a subarea of the radar image "gatineau.pix" with a NLOOK value of 1. Filter size is 7x7.

from pci.fkuan import fkuan

file	=	"gatineau.pix"
dbic	=	[1]	# channel to be filtered
dboc	=	[2]	# filtered results
flsz	=	[7,7]	# 7x7 filter size
mask	=	[]	# filter entire image
nlook	=	[1.0]	# number of looks
imagefmt	=	'AMP'	# Amplitude image format

fkuan( file, dbic, dboc, flsz, mask, nlook, imagefmt )
      

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.