A REFINED GAMMA-MAP-SAR SPECKLE FILTER WITH IMPROVED GEOMETRICAL ADAPTIVITY

A modified version of the Refined Gamma Maximum-A-Posteriori (RGMAP) speckle filter, which is found in the literature, is presented in this paper. The traditional RGMAP speckle filter first detects contours belonging to step edges and thin linear structures, then applies the GMAP filter to local statistics extracted from rectangular masks that do not cross image contours. The proposed Modified RGMAP (MRGMAP) filter first exploits local operators belonging to the odd-symmetric filter category employed by RGMAP to detect image segments, then it computes local statistics over areas that are not necessarily rectangular, but are subsets of the image segments having any possible shape. Therefore, MRGMAP enhances the RGMAP ability in exploiting shape adaptive windowing near image contours, where speckle is not fully developed. The MRGMAP computation time is estimated to be of the same magnitude of that of the original RGMAP, the latter depending on the number of filter categories being employed. The qualitative and quantitative results of the MRGMAP filter applied to real SAR images are satisfactory as the filter seems to be effective in speckle removal whereas it retains edge sharpness and subtle details. However, tests on simulated SAR images must still be performed in order to provide definitive evidence supporting MRGMAP effectiveness. Since MRGMAP typically removes image structures featuring a constant reflectivity gradient, this filter is not particularly suitable for image enhancement in human photo-interpretation. MRGMAP can be rather employed as a preprocessing module in a computer-based SAR image classification procedure based on segment mean value analysis.