Merge pull request #765 from larsjsol/master

Fix dispose calculations for animated GIFs

PIL/GifImagePlugin.py

@@ -96,8 +96,15 @@ class GifImageFile(ImageFile.ImageFile):
             # rewind
             self.__offset = 0
             self.dispose = None
+            self.dispose_extent = [0, 0, 0, 0] #x0, y0, x1, y1
             self.__frame = -1
             self.__fp.seek(self.__rewind)
+            self._prev_im = None
+            self.disposal_method = 0
+        else:
+            # ensure that the previous frame was loaded
+            if not self.im:
+                self.load()
 
         if frame != self.__frame + 1:
             raise ValueError("cannot seek to frame %d" % frame)
@@ -114,8 +121,7 @@ class GifImageFile(ImageFile.ImageFile):
             self.__offset = 0
 
         if self.dispose:
-            self.im = self.dispose
-            self.dispose = None
+            self.im.paste(self.dispose, self.dispose_extent)
 
         from copy import copy
         self.palette = copy(self.global_palette)
@@ -140,17 +146,16 @@ class GifImageFile(ImageFile.ImageFile):
                     if flags & 1:
                         self.info["transparency"] = i8(block[3])
                     self.info["duration"] = i16(block[1:3]) * 10
-                    try:
-                        # disposal methods
-                        if flags & 8:
-                            # replace with background colour
-                            self.dispose = Image.core.fill("P", self.size,
-                                self.info["background"])
-                        elif flags & 16:
-                            # replace with previous contents
-                            self.dispose = self.im.copy()
-                    except (AttributeError, KeyError):
-                        pass
+
+                    # disposal method - find the value of bits 4 - 6
+                    dispose_bits = 0b00011100 & flags
+                    dispose_bits = dispose_bits >> 2
+                    if dispose_bits:
+                        # only set the dispose if it is not
+                        # unspecified. I'm not sure if this is
+                        # correct, but it seems to prevent the last
+                        # frame from looking odd for some animations
+                        self.disposal_method = dispose_bits
                 elif i8(s) == 255:
                     #
                     # application extension
@@ -172,6 +177,7 @@ class GifImageFile(ImageFile.ImageFile):
                 # extent
                 x0, y0 = i16(s[0:]), i16(s[2:])
                 x1, y1 = x0 + i16(s[4:]), y0 + i16(s[6:])
+                self.dispose_extent = x0, y0, x1, y1
                 flags = i8(s[8])
 
                 interlace = (flags & 64) != 0
@@ -194,6 +200,26 @@ class GifImageFile(ImageFile.ImageFile):
                 pass
                 # raise IOError, "illegal GIF tag `%x`" % i8(s)
 
+        try:
+            if self.disposal_method < 2:
+                # do not dispose or none specified
+                self.dispose = None
+            elif self.disposal_method == 2:
+                # replace with background colour
+                self.dispose = Image.core.fill("P", self.size,
+                                               self.info["background"])
+            else:
+                # replace with previous contents
+                if self.im:
+                    self.dispose = self.im.copy()
+
+            # only dispose the extent in this frame
+            if self.dispose:
+                self.dispose = self.dispose.crop(self.dispose_extent)
+        except (AttributeError, KeyError):
+            pass
+
+
         if not self.tile:
             # self.__fp = None
             raise EOFError("no more images in GIF file")
@@ -205,6 +231,18 @@ class GifImageFile(ImageFile.ImageFile):
     def tell(self):
         return self.__frame
 
+    def load_end(self):
+        ImageFile.ImageFile.load_end(self)
+
+        # if the disposal method is 'do not dispose', transparent
+        # pixels should show the content of the previous frame
+        if self._prev_im and self.disposal_method == 1:
+            # we do this by pasting the updated area onto the previous
+            # frame which we then use as the current image content
+            updated = self.im.crop(self.dispose_extent)
+            self._prev_im.paste(updated, self.dispose_extent, updated.convert('RGBA'))
+            self.im = self._prev_im
+        self._prev_im = self.im.copy()
 
 # --------------------------------------------------------------------
 # Write GIF files

Tests/test_file_gif.py

@@ -106,6 +106,50 @@ class TestFileGif(PillowTestCase):
         GifImagePlugin._save_netpbm(img, 0, tempfile)
         self.assert_image_similar(img, Image.open(tempfile).convert("L"), 0)
 
+    def test_seek(self):
+        img = Image.open("Tests/images/dispose_none.gif")
+        framecount = 0
+        try:
+            while True:
+                framecount += 1
+                img.seek(img.tell() +1)
+        except EOFError:
+            self.assertEqual(framecount, 5)
+
+    def test_dispose_none(self):
+        img = Image.open("Tests/images/dispose_none.gif")
+        try:
+            while True:
+                img.seek(img.tell() +1)
+                self.assertEqual(img.disposal_method, 1)
+        except EOFError:
+            pass
+
+    def test_dispose_background(self):
+        img = Image.open("Tests/images/dispose_bgnd.gif")
+        try:
+            while True:
+                img.seek(img.tell() +1)
+                self.assertEqual(img.disposal_method, 2)
+        except EOFError:
+            pass
+
+    def test_dispose_previous(self):
+        img = Image.open("Tests/images/dispose_prev.gif")
+        try:
+            while True:
+                img.seek(img.tell() +1)
+                self.assertEqual(img.disposal_method, 3)
+        except EOFError:
+            pass
+
+    def test_iss634(self):
+        img = Image.open("Tests/images/iss634.gif")
+        # seek to the second frame
+        img.seek(img.tell() +1)
+        # all transparent pixels should be replaced with the color from the first frame
+        self.assertEqual(img.histogram()[img.info['transparency']], 0)
+
 
 if __name__ == '__main__':
     unittest.main()