Yunham Dong 
  
artificial unfiltered image having as the approximately same speckle level as in the filtered images. It has been noticed 
that even though the filtered and the unfiltered images have the same mean and standard deviation, the appearance of 
the filtered images look coarser than the unfiltered image. That is the data distribution in the co-occurrence matrix of 
the filtered images is wider than that of unfiltered image. In general, among all the filters tested including the mean, 
median, Lee, Kuan, Frost, and proposed filters, the median filter gives the worst result in terms of the second-order 
texture restoration, which is true at least for the image used in the test. As known, the median filter assigns the median 
value to the central pixel in the moving window. As a result, it has been found that, quite a few of pixels are assigned as 
the same values as their adjacent pixels, causing a dramatic distortion in the co-occurrence matrix compared to the co- 
occurrence matrix of the unfiltered image. As far as uniform areas are concern, the mean filter shows the best result of 
restoration of the intrinsic texture. However, the mean filter also gives the poorest result in edge areas. Other adaptive 
filters perform better in edge areas, but seem not as well as the mean filter does for uniform areas. In general the texture 
of filtered images has been dramatically improved to a level very close to what the unfiltered image has. Details of 
texture restoration of filters can be found elsewhere (Dong and Milne, 2000). 
    
(a) Original (b) Lee 
  
   
  
(c) Frost (d) Proposed 
Figure 7. Three-dimensional view of an edge crossing area of 40 x 40 pixels. Figures shown in (a), (b), (c) and (d) 
correspond to the original data, and the data processed by Lee, Frost and the proposed filters, respectively. 
4 CONCLUSIONS 
A speckle filtering algorithm for edge sharpening and enhancement has been proposed. Its performance in terms of edge 
enhancement as well as mean preservation and standard deviation reduction has been analyzed. The preservation 
(restoration) of the intrinsic texture of the filter is also tested, and found to be comparable to other adaptive filters. The 
algorithm can be applied in conjunction with other speckle filters in order to sharpen edges which might be smeared. 
Even for images with unsmeared edges it may be still worth implementing edge enhancement, as the response of a pulse 
(a sharp edge) by SAR systems is a sinc function (a smeared edge with local oscillations) due to systems' limited 
bandwidths. 
  
94 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part Bl. Amsterdam 2000. 
ACI 
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