Add thumbnailing support for animated GIFs

docs/ref/animated_formats.rst

@@ -21,9 +21,25 @@ PNGs.
 There have been issues with conversion from GIF to WEBP, so it's currently not recommended to
 enable this specific conversion for animated images.
 
+Animated GIF
+============
+
+Thumbnailing animated GIFs requires extra processing. To avoid this, you can enable the
+`RGB_ALWAYS` loading strategy for the GifImagePlugin by adding this to your project:
+
+.. code-block:: python
+
+    from PIL import GifImagePlugin
+    GifImagePlugin.LOADING_STRATEGY = GifImagePlugin.LoadingStrategy.RGB_ALWAYS
+
+This setting is an optional optimization because it changes how all GIFs are loaded by
+Pillow, not just animated GIFs. The `Pillow GIF docs
+<https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html#gif/>`_
+explain how this settings works and you can decide if it's the right choice for
+your project.
 
 Remark
 ======
 
 In the future, Easy Thumbnails might preserve animated images by default, and/or provide the
-option to enable/disable animations for each generated thumbnail.
+option to enable/disable animations for each generated thumbnail.

easy_thumbnails/processors.py

@@ -3,7 +3,7 @@
 from functools import partial
 from io import BytesIO
 
-from PIL import Image, ImageChops, ImageFilter
+from PIL import Image, ImageChops, ImageFilter, GifImagePlugin
 from easy_thumbnails import utils
 
 
@@ -55,7 +55,20 @@ def apply_to_frames(self, method, *args, **kwargs):
         new_frames[0].save(
             write_to, format=self.im.format, save_all=True, append_images=new_frames[1:]
         )
-        return Image.open(write_to)
+
+        to_return = Image.open(write_to)
+        # Animated GIFs are always opened in palette mode (P). We need seek through the
+        # frames to ensure that to_return has the correct mode.
+        # Background information:
+        # https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html#gif
+        if (
+            to_return.format == "GIF"
+            and to_return.is_animated
+            and GifImagePlugin.LOADING_STRATEGY != GifImagePlugin.LoadingStrategy.RGB_ALWAYS
+        ):
+            for i in range(to_return.n_frames):
+                to_return.seek(i)
+        return to_return
 
     def __getattr__(self, key):
         method = getattr(self.im, key)

easy_thumbnails/tests/test_animated_formats.py

@@ -1,7 +1,9 @@
 from io import BytesIO
-from PIL import Image, ImageChops, ImageDraw
+from PIL import Image, ImageDraw, GifImagePlugin
 from easy_thumbnails import processors
-from unittest import TestCase
+from unittest import TestCase, mock
+
+from easy_thumbnails.files import get_thumbnailer
 
 
 def create_animated_image(mode='RGB', format="gif", size=(1000, 1000), no_frames=6):
@@ -81,3 +83,23 @@ def test_background(self):
         # indeed processed?
         self.assertEqual(frames_count, processed_frames_count)
         self.assertEqual(processed.size, (1000, 1800))
+
+    def test_gif_with_mode_p(self):
+        image_path = "easy_thumbnails/tests/files/animated_mode_p.gif"
+        with open(image_path, "rb") as im:
+            t = get_thumbnailer(im, image_path)
+            # Should not fail because of wrong mode and should still be animated.
+            # https://github.com/SmileyChris/easy-thumbnails/issues/653
+            thumbnail = t.get_thumbnail({'size': (500, 50), 'crop': True})
+            self.assertTrue(thumbnail.image.is_animated)
+
+    @mock.patch("PIL.GifImagePlugin.LOADING_STRATEGY", GifImagePlugin.LoadingStrategy.RGB_ALWAYS)
+    def test_gif_with_mode_p__gif_plug_loading_strategy_rgb_always(self):
+        print("m",GifImagePlugin.LOADING_STRATEGY)
+        image_path = "easy_thumbnails/tests/files/animated_mode_p.gif"
+        with open(image_path, "rb") as im:
+            t = get_thumbnailer(im, image_path)
+            # Should not fail because of wrong mode and should still be animated.
+            # https://github.com/SmileyChris/easy-thumbnails/issues/653
+            thumbnail = t.get_thumbnail({'size': (500, 50), 'crop': True})
+            self.assertTrue(thumbnail.image.is_animated)