The Radar Kuan resampling option determines the gray level for each pixel by replacing the center pixel with a weighted average of the central pixel and the mean of the values in a square kernel surrounding the pixel. To filter pixels located near the edges of the image, edge pixel values are replicated to produce sufficient data.
This filter is used primarily to suppress speckle. It smooths image data without removing edges or sharp features in the images while minimizing the loss of radiometric and textural information. 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 gray-level value (R) for the smoothed pixel is:
R = Ic * W + Im * (1 - W)
The Number of Looks and the Image Format of the radar image are usually recorded on the CD jacket or magnetic tape label or in the format specifications provided by the data vendor.
Filter Size:
The kernel must be square with its width and length in odd numbers. You control the size of the kernel with the Filter Size option by typing the number of pixels (width) in the X box and the number of lines (length) in the Y box. Different filter sizes greatly affect the quality of the 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 are lost in the filtering process. The minimum size for the kernel is 3-by-3 pixels. A 7-by-7 kernel usually gives the best results.
Image Format:
The radar images are supplied in one of two Image Formats: Power or Amplitude. Power is the sum of the squares of the real and imaginary values of the complex pixel values in the radar image. Amplitude is the square root of Power. Most radar images are supplied in the Amplitude format to preserve the values. You identify the format used with your images in the Image Format list.
Number of Looks:
You use the Number of Looks to estimate noise variance and to control the amount of smoothing applied to the image. In theory, the correct value for the Number of Looks should be the effective number of looks of the radar image, or close to the actual number, but it may be different if the image was resampled. Using a smaller value for the Number of Looks leads to more smoothing, and a larger value preserves more image features. In the No. of Looks list, select the Number of Looks that you want to apply to the image.
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