Python-Fuレイヤーの不透明部分の縁を透明化

#!/usr/bin/env python
# coding: utf8
from gimpfu import *
from array import array
from time import time
def plugin_main(image, layer, grow_step):
    # レイヤーの幅と高さを取得
    w = layer.width
    h = layer.height
    # 不透明部分を選択範囲に
    pdb.gimp_image_select_item(image, 2, layer)
    # 選択範囲の反転
    pdb.gimp_selection_invert(image)
    # 範囲選択を拡大
    if grow_step > 0:
        pdb.gimp_selection_grow(image, grow_step)
    # 色を透明度に
    pdb.plug_in_colortoalpha(image, layer, (255,255,255,1.0))
    # レイヤーの更新
    layer.flush()
    layer.update(0,0,w,h)
register("FuchiToumei", "", "", "", "", "",
    "縁を透明化", 
    "*",
    [
    (PF_IMAGE, "image", "Input image", None),
    (PF_DRAWABLE, "drawable", "Drawable", None),
    (PF_INT32, "grow_step", "領域拡大するサイズ", 2),
    ],
    [],
    plugin_main,
    menu = "<Image>/Filters")
main()