Issue 1897 - Compress auto-generated images with pngout

imageCompression.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 subprocess
 import threading
 import errno
@@ skipping 20 matching lines @@
     elif Image.getmodebase(image.mode) == 'L':
       args.append('-c0')  # grayscale
 
     self._pngout = subprocess.Popen(args, stdin=subprocess.PIPE, stdout=subprocess.PIPE)
 
     # Writing will block when the buffer is full until we read more data
     # from the output. Reading the output will block when the input isn't
     # complete yet. So we have to use threads to do both at the same time.
     self._thread = threading.Thread(target=self._run_thread, args=(image,))
     self._thread.daemon = True
     self._thread.start()

[Wladimir Palant, 2015/02/10 18:47:59]

Why not use subprocess.communicate()? The files aren't large enough to care
about in-memory buffering.

[Sebastian Noack, 2015/02/10 19:23:56]

So you mean I should save the image into a StringIO, get its value to pass it to
communicate()? Sounds kinda like a hack, and not really simpler.

[Wladimir Palant, 2015/02/10 20:49:18]

Seems a lot simpler than running a thread IMHO, especially if image_to_file
returns a string rather than a file object (the caller will call read() on it
anyway). Sure, the function needs to be renamed then. But converting the image
to string will always be the first step - whether with compression or without.

 
   # This is supposed to be a file-like object, reading the compressed PNG file.
   # So proxy methods like read() to the stdout of the underlying subprocess.
   def __getattr__(self, name):
     return getattr(self._pngout.stdout, name)
 
   def _run_thread(self, image):
     image.save(self._pngout.stdin, 'PNG')
     self._pngout.stdin.close()
 
   def close(self):
     self._thread.join()
     self._pngout.stdout.close()
     self._pngout.wait()
 
 class ImageCompressor:
   use_pngout = True
 
   def make_uncompressed_file(self, image, filename):
     file = StringIO()
-    file.name = filename  # Set the 'name' attribute, that PIL can determine

[kzar, 2015/02/10 09:29:54]

Nitpick: Comment should read "...so that PIL can..."

[Sebastian Noack, 2015/02/10 09:57:38]

Done.

+    file.name = filename  # Set the 'name' attribute, so that PIL can determine
                           # the correct image type based on the file extension
     image.save(file)
     file.seek(0)
 
     return file
 
   def make_file(self, image, filename):
     if self.use_pngout and os.path.splitext(filename)[1].lower() == '.png':
       try:
         return Pngout(image)
       except OSError, e:
         if e.errno != errno.ENOENT:
           raise
 
         logging.warning("Couldn't find 'pngout', can't compress images")
         self.use_pngout = False
 
     return self.make_uncompressed_file(image, filename)
 
 image_to_file = ImageCompressor().make_file