help needed with Python plugin

["lode leroy" <lode_leroy@xxxxxxxxxxx>]

This plugin calculates the "optimal" thresholding value for converting a 
graymap to a pixelmap
(i.e grey to b/w).

I think it works, but I do not manage to write pixels to the drawable.
Could someone give a hint on how to accomplish this?

#!/usr/bin/env python

import struct
import math

import gimp
from gimpfu import *

def python_otsu_threshold(img, drawable):
   width = drawable.width
   height = drawable.height
   bpp = drawable.bpp

   pr = drawable.get_pixel_rgn(0, 0, width, height, True, False)

   gimp.progress_init("Thresholding...")
   maxval = 255

   hist = []
   for i in range(0,maxval+1):
       hist.append(0)

   for y in range(0,height):
       row = pr[0:width, y]
       for x in range(0, width):
           pixel = ord(row[x])
           hist[pixel] += 1

       gimp.progress_update(y / float(height))

   chist = []
   chist.append(hist[0])
   cmom = []
   cmom.append(0 * hist[0])
   hist_max = hist[0]
   for i in range(1,maxval+1):
       chist.append ( chist[i-1] + hist[i] )
       cmom.append ( cmom[i-1] + ( i * hist[i] ) )
       if (hist[i] > hist_max):
           hist_max = hist[i]

   chist_max = chist[maxval]
   cmom_max = float(cmom[maxval])

   bvar_max = 0
   threshold = 0
   re_size = width * height
   bvars = []
   for i in range(0,maxval):
       if ( (chist[i] > 0) and (chist[i] < re_size) ):
           bvar = float(cmom[i]) / chist[i];
           bvar -= ( cmom_max - cmom[i]) / ( re_size - chist[i]);
           bvar *= bvar; bvar *= chist[i]; bvar *= ( re_size - chist[i] );
           bvars.append(bvar)
           if (bvar > bvar_max):
               bvar_max = bvar
               threshold = i
       else:
           bvars.append(bvar_max)

   for y in range(0,height):
       row = pr[0:width, y]
       for x in range(0, width):
           pixel = ord(row[x])
           if pixel < threshold:
               row[x] = chr(0)
           else:
               row[x] = chr(255)


   drawable.merge_shadow()

register(
       "python_fu_threshold",
       "Optimal Binary Threshold on region using Otsu's method",
       "Optimal Binary Threshold on region using Otsu's method (adapted 
from code in GOCR-0.14)",
       "Lode Leroy",
       "Lode Leroy",
       "2006",
       "<Image>/Filters/Misc/_Threshold...",
       "*",
       [],
       [],
       python_otsu_threshold)

main()


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