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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _filter_meshes(self):
 """ Apply a 2D median filter to the low-resolution 2D mesh, including only pixels inside the image at the borders. """


        from scipy.ndimage import generic_filter
        try:
            nanmedian_func = np.nanmedian    # numpy >= 1.9
        except AttributeError:    # pragma: no cover
            from scipy.stats import nanmedian
            nanmedian_func = nanmedian

        if self.filter_threshold is None:
            # filter the entire arrays
            self.background_mesh = generic_filter(
                self.background_mesh, nanmedian_func, size=self.filter_size,
                mode='constant', cval=np.nan)
            self.background_rms_mesh = generic_filter(
                self.background_rms_mesh, nanmedian_func,
                size=self.filter_size, mode='constant', cval=np.nan)
        else:
            # selectively filter
            indices = np.nonzero(self.background_mesh > self.filter_threshold)
            self.background_mesh = self._selective_filter(
                self.background_mesh, indices)
            self.background_rms_mesh = self._selective_filter(
                self.background_rms_mesh, indices)

        return