BigglesZX · November 5, 2012 10:31 · Nov 5, 2012
diff --git a/gifextract.py b/gifextract.py
@@ -0,0 +1,102 @@
+import os
+from PIL import Image
+
+
+'''
+I searched high and low for solutions to the "extract animated GIF frames in Python"
+problem, and after much trial and error came up with the following solution based
+on several partial examples around the web (mostly Stack Overflow).
+
+There are two pitfalls that aren't often mentioned when dealing with animated GIFs -
+firstly that some files feature per-frame local palettes while some have one global
+palette for all frames, and secondly that some GIFs replace the entire image with
+each new frame ('full' mode in the code below), and some only update a specific
+region ('partial').
+
+This code deals with both those cases by examining the palette and redraw
+instructions of each frame. In the latter case this requires a preliminary (usually
+partial) iteration of the frames before processing, since the redraw mode needs to
+be consistently applied across all frames. I found a couple of examples of
+partial-mode GIFs containing the occasional full-frame redraw, which would result
+in bad renders of those frames if the mode assessment was only done on a
+single-frame basis.
+
+Nov 2012
+'''
+
+
+def analyseImage(path):
+    '''
+    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.
+    '''
+    im = Image.open(path)
+    results = {
+        'size': im.size,
+        'mode': 'full',
+    }
+    try:
+        while True:
+            if im.tile:
+                tile = im.tile[0]
+                update_region = tile[1]
+                update_region_dimensions = update_region[2:]
+                if update_region_dimensions != im.size:
+                    results['mode'] = 'partial'
+                    break
+            im.seek(im.tell() + 1)
+    except EOFError:
+        pass
+    return results
+
+
+def processImage(path):
+    '''
+    Iterate the GIF, extracting each frame.
+    '''
+    mode = analyseImage(path)['mode']
+
+    im = Image.open(path)
+
+    i = 0
+    p = im.getpalette()
+    last_frame = im.convert('RGBA')
+
+    try:
+        while True:
+            print "saving %s (%s) frame %d, %s %s" % (path, mode, i, im.size, im.tile)
+
+            '''
+            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 im.getpalette():
+                im.putpalette(p)
+
+            new_frame = Image.new('RGBA', im.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(im, (0,0), im.convert('RGBA'))
+            new_frame.save('%s-%d.png' % (''.join(os.path.basename(path).split('.')[:-1]), i), 'PNG')
+
+            i += 1
+            last_frame = new_frame
+            im.seek(im.tell() + 1)
+    except EOFError:
+        pass
+
+
+def main():
+    processImage('foo.gif')
+    processImage('bar.gif')
+
+
+if __name__ == "__main__":
+    main()
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