This filter is found in
It is used to remove small defects due to dust, or scratches, on a scanned image, and also moiré effect on image scanned from a magazine. You ought to select isolated defects before applying filter.
Options
Parameter setting results are interactively displayed in preview.
Adaptive: Adapts radius to image or selection content (using Histogram). If this option is checked, radius slider is not efficient. It renders a result smoother than with radius alone.
Recursive: Repeats filter action which gets stronger.
Radius: Sets size of action window from 1 (3x3 pixels) to 20 (41x41). This window moves over image, and the color in it is smoothed, so imperfections are removed.
Black level: Removes pixels darker than set value (0-255).
White level: Removes pixels whiter than set value (0-255).