Nonlocal Filtering for Polarimetric SAR Data: A Pretest Approach

A pretest approach based on the complex Wishart distribution in polarimetric synthetic aperture radar (POLSAR) speckle filtering is proposed in this paper. The main principle is to select homogeneous pixels in a large-scale area in the filtering process, which is called pretesting. To preserve details and fine structures while despeckling, the homogeneous pixels are selected by comparing their 3 x 3 neighboring windows. A test statistic based on the complex Wishart distribution is used to decide the selection of homogeneous pixels. Speckle filtering is processed by summing up the homogeneous pixels with weights according to the values of their test statistics. To accelerate the pretest filter, we further propose a refined algorithm, which eliminates redundant operations without deteriorating the performance. We demonstrate the performance of the proposed algorithm by using both simulated and real airborne POLSAR data.