[preview] Add Animated GIF support

url_image_preview/resize_gif.py

@@ -0,0 +1,86 @@
+from io import BytesIO
+from PIL import Image
+
+
+def resize_gif(mem, path, resize_to):
+    frames = extract_and_resize_frames(mem, resize_to)
+
+    if len(frames) == 1:
+        frames[0].save(path, optimize=True)
+    else:
+        frames[0].save(path,
+                       optimize=True,
+                       save_all=True,
+                       append_images=frames[1:],
+                       loop=1000)
+
+
+def analyse_image(mem):
+    """
+    Pre-process pass over the image to determine the mode (full or additive).
+    Necessary as assessing single frames isn't reliable. Need to know the mode
+    before processing all frames.
+    """
+    image = Image.open(BytesIO(mem))
+    results = {
+        'size': image.size,
+        'mode': 'full',
+    }
+    try:
+        while True:
+            if image.tile:
+                tile = image.tile[0]
+                update_region = tile[1]
+                update_region_dimensions = update_region[2:]
+                if update_region_dimensions != image.size:
+                    results['mode'] = 'partial'
+                    break
+            image.seek(image.tell() + 1)
+    except EOFError:
+        pass
+    return results
+
+
+def extract_and_resize_frames(mem, resize_to):
+    mode = analyse_image(mem)['mode']
+    image = Image.open(BytesIO(mem))
+
+    i = 0
+    palette = image.getpalette()
+    last_frame = image.convert('RGBA')
+
+    frames = []
+
+    try:
+        while True:
+            '''
+            If the GIF uses local colour tables,
+            each frame will have its own palette.
+            If not, we need to apply the global palette to the new frame.
+            '''
+            if not image.getpalette():
+                image.putpalette(palette)
+
+            new_frame = Image.new('RGBA', image.size)
+
+            '''
+            Is this file a "partial"-mode GIF where frames update a region
+            of a different size to the entire image?
+            If so, we need to construct the new frame by
+            pasting it on top of the preceding frames.
+            '''
+            if mode == 'partial':
+                new_frame.paste(last_frame)
+
+            new_frame.paste(image, (0, 0), image.convert('RGBA'))
+
+            new_frame.thumbnail(resize_to, Image.ANTIALIAS)
+            frames.append(new_frame)
+
+            i += 1
+            last_frame = new_frame
+            image.seek(image.tell() + 1)
+    except EOFError:
+        pass
+
+    return frames