Issue 2038 - Convert palette images with transparency to RGBA before pasting

imageConversion.py

 # coding: utf-8
 
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 import os
 import re
 
 try:
   from PIL import Image
   from PIL import ImageOps
 except ImportError:
   import Image
   import ImageOps
 
 from imageCompression import image_to_file
 
-def get_alpha(image):
-  if image.mode in ('RGBA', 'LA'):
-    return image.split()[image.getbands().index('A')]
-
-  # In order to generate an alpha channel for images using a palette, we
-  # convert the image to RGBA. It's important to use RGBA, not LA (grayscale+alpha),
-  # since PIL can't reliably convert P to LA. Also initially, we created an
-  # alpha channel by replacing opaque pixels with a high mark and transparent
-  # pixels with a low mark. However, it turned out that you can't rely on the
-  # value of Image.info['transparency'] since in some cases it might be an
-  # unparsed string instead an int indicating the value of transparent pixels.
-  if image.mode == 'P' and 'transparency' in image.info:
-    return image.convert('RGBA').split()[3]
-
 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
@@ skipping 15 matching lines @@
 
   if any(im.mode in ('RGBA', 'LA', 'P') for im in (im1, im2)):
     mode += 'A'
 
   return (
     im1 if im1.mode == mode else im1.convert(mode),
     im2 if im2.mode == mode else im2.convert(mode),
   )
 
 def filter_contrastToAlpha(image, baseDir):
+  # In order to generate an alpha channel for images using a palette, we must
+  # convert the image to RGBA. It's important to use RGBA, not LA (grayscale+alpha),
+  # since PIL can't reliably convert P to LA. Also initially, we created an
+  # alpha channel by replacing opaque pixels with a high mark and transparent
+  # pixels with a low mark. However, it turned out that you can't rely on the
+  # value of Image.info['transparency'] since in case the transparency is
+  # specified in bytes (pngout loves to do that), we can't associate that value
+  # with transparent pixels. Moreover, some versions of PIL raise a warning
+  # when such images are pasted into another image.
+  if image.mode == 'P' and 'transparency' in image.info:
+    image = image.convert('RGBA')
+
+  # Image.paste() ignores the alpha channel of the pasted image, but expects
+  # the mask to be passed as separate argument. So we have to extract the alpha
+  # channel (if any) from the image we are going to paste.
+  if image.mode in ('RGBA', 'LA'):
+    mask = image.split()[image.getbands().index('A')]
+  else:
+    mask = None
+
   alpha = Image.new('L', image.size, 255)
-  alpha.paste(image, mask=get_alpha(image))
+  alpha.paste(image, mask=mask)
   alpha = ImageOps.invert(alpha)
   alpha = ImageOps.autocontrast(alpha)
 
   return Image.merge('LA', [Image.new('L', image.size), alpha])
 
 def filter_blend(image, baseDir, filename, opacity):
   image, overlay = ensure_same_mode(
     image,
     load_image(os.path.join(
       baseDir,
@@ skipping 14 matching lines @@
     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)
 
     file = image_to_file(image, filename)
     try:
       files[filename] = file.read()
     finally:
       file.close()