Wait until images are loaded

imageConversion.py

 # coding: utf-8
 
 # This file is part of the Adblock Plus build tools,
 # Copyright (C) 2006-2013 Eyeo GmbH
 #
 # Adblock Plus is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 3 as
 # published by the Free Software Foundation.
 #
 # Adblock Plus is distributed in the hope that it will be useful,
@@ skipping 20 matching lines @@
     return image.split()[image.getbands().index('A')]
 
   if image.mode == 'P':
     transparency = image.info.get('transparency')
 
     if transparency is not None:
       table = [255] * 256
       table[transparency] = 0
 
       return image.point(table, 'L')
+
+def load_image(path):
+  image = Image.open(path)
+  # Make sure the image is loaded, some versions of PIL load images lazily.
+  image.load()
+  return image
 
 def ensure_same_mode(im1, im2):
   # if both images already have the same mode (and palette, in
   # case of mode P), don't convert anything. Images with mode P,
   # and a different palette, are the only case where images
   # using the same mode, will be incompatible with each other.
   if im1.mode == im2.mode and (im1.mode != 'P' or im1.getpalette() == im2.getpalette()):
     return (im1, im2)
 
   # if any given image has a mode that supports colors convert both
@@ skipping 30 matching lines @@
   alpha.paste(image, mask=get_alpha(image))
   alpha = ImageOps.invert(alpha)
   alpha = ImageOps.autocontrast(alpha)
 
   return Image.merge('LA', [Image.new('L', image.size), alpha])
 
 def filter_blend(image, baseDir, *args):
   if len(args) == 2:
     filename, opacity = args
 
-    overlay = Image.open(os.path.join(
+    overlay = load_image(os.path.join(
       baseDir,
       *filename.split('/')
     ))
   else:
     red, green, blue, opacity = args
 
     overlay = Image.new('RGB', image.size, (
       int(red),
       int(green),
       int(blue),
     ))
 
     # if the background image has an alpha channel copy it to
     # the overlay, so that transparent areas stay transparent.
     alpha = get_alpha(image)
     if alpha:
       overlay.putalpha(alpha)
 
   image, overlay = ensure_same_mode(image, overlay)
   return Image.blend(image, overlay, float(opacity))
 
 def convertImages(params, files):
   metadata = params['metadata']
 
   for filename, chain in metadata.items('convert_img'):
     baseDir = os.path.dirname(metadata.option_source('convert_img', filename))
     steps = re.split(r'\s*->\s*', chain)
-    image = Image.open(os.path.join(baseDir, *steps.pop(0).split('/')))
-    image.load()
+    image = load_image(os.path.join(baseDir, *steps.pop(0).split('/')))
 
     for step in steps:
       filter, args = re.match(r'([^(]+)(?:\((.*)\))?', step).groups()
       args = re.split(r'\s*,\s*', args) if args else ()
       image = globals()['filter_' + filter](image, baseDir, *args)
 
     f = StringIO()
     f.name = filename
     image.save(f)
     files[filename] = f.getvalue()