def matte_fill_to_border(x, y)
      f = copy
      f.opacity = Magick::OpaqueOpacity unless f.alpha?
      f.matte_flood_fill(border_color, TransparentOpacity,
                         x, y, FillToBorderMethod)
    end
def matte_fill_to_border(x, y)
      f = copy
      f.opacity = Magick::OpaqueOpacity unless f.alpha?
      f.matte_flood_fill(border_color, TransparentOpacity,
                         x, y, FillToBorderMethod)
    end
Make transparent any neighbor pixel that is not the border color.