Do not presume that the background color index is 0

Tests/test_file_gif.py

@@ -319,14 +319,14 @@ class TestFileGif(PillowTestCase):
             self.assertEqual(img.disposal_method, i + 1)
 
     def test_dispose2_palette(self):
-        out = self.tempfile('temp.gif')
+        out = self.tempfile("temp.gif")
 
         # 4 backgrounds: White, Grey, Black, Red
         circles = [(255, 255, 255), (153, 153, 153), (0, 0, 0), (255, 0, 0)]
 
         im_list = []
         for circle in circles:
-            img = Image.new('RGB', (100, 100), (255, 0, 0))
+            img = Image.new("RGB", (100, 100), (255, 0, 0))
 
             # Red circle in center of each frame
             d = ImageDraw.Draw(img)
@@ -334,18 +334,13 @@ class TestFileGif(PillowTestCase):
 
             im_list.append(img)
 
-        im_list[0].save(
+        im_list[0].save(out, save_all=True, append_images=im_list[1:], disposal=2)
-            out,
-            save_all=True,
-            append_images=im_list[1:],
-            disposal=2
-            )
 
         img = Image.open(out)
 
         for i, circle in enumerate(circles):
             img.seek(i)
-            rgb_img = img.convert('RGB')
+            rgb_img = img.convert("RGB")
 
             # Check top left pixel matches background
             self.assertEqual(rgb_img.getpixel((0, 0)), (255, 0, 0))
@@ -354,20 +349,20 @@ class TestFileGif(PillowTestCase):
             self.assertEqual(rgb_img.getpixel((50, 50)), circle)
 
     def test_dispose2_diff(self):
-        out = self.tempfile('temp.gif')
+        out = self.tempfile("temp.gif")
 
         # 4 frames: red/blue, red/red, blue/blue, red/blue
         circles = [
             ((255, 0, 0, 255), (0, 0, 255, 255)),
             ((255, 0, 0, 255), (255, 0, 0, 255)),
             ((0, 0, 255, 255), (0, 0, 255, 255)),
-            ((255, 0, 0, 255), (0, 0, 255, 255))
+            ((255, 0, 0, 255), (0, 0, 255, 255)),
         ]
 
         im_list = []
         for i in range(len(circles)):
             # Transparent BG
-            img = Image.new('RGBA', (100, 100), (255, 255, 255, 0))
+            img = Image.new("RGBA", (100, 100), (255, 255, 255, 0))
 
             # Two circles per frame
             d = ImageDraw.Draw(img)
@@ -377,18 +372,14 @@ class TestFileGif(PillowTestCase):
             im_list.append(img)
 
         im_list[0].save(
-            out,
+            out, save_all=True, append_images=im_list[1:], disposal=2, transparency=0
-            save_all=True,
-            append_images=im_list[1:],
-            disposal=2,
-            transparency=0
         )
 
         img = Image.open(out)
 
         for i, colours in enumerate(circles):
             img.seek(i)
-            rgb_img = img.convert('RGBA')
+            rgb_img = img.convert("RGBA")
 
             # Check left circle is correct colour
             self.assertEqual(rgb_img.getpixel((20, 50)), colours[0])
@@ -399,6 +390,31 @@ class TestFileGif(PillowTestCase):
             # Check BG is correct colour
             self.assertEqual(rgb_img.getpixel((1, 1)), (255, 255, 255, 0))
 
+    def test_dispose2_background(self):
+        out = self.tempfile("temp.gif")
+
+        im_list = []
+
+        im = Image.new("P", (100, 100))
+        d = ImageDraw.Draw(im)
+        d.rectangle([(50, 0), (100, 100)], fill="#f00")
+        d.rectangle([(0, 0), (50, 100)], fill="#0f0")
+        im_list.append(im)
+
+        im = Image.new("P", (100, 100))
+        d = ImageDraw.Draw(im)
+        d.rectangle([(0, 0), (100, 50)], fill="#f00")
+        d.rectangle([(0, 50), (100, 100)], fill="#0f0")
+        im_list.append(im)
+
+        im_list[0].save(
+            out, save_all=True, append_images=im_list[1:], disposal=[0, 2], background=1
+        )
+
+        im = Image.open(out)
+        im.seek(1)
+        self.assertEqual(im.getpixel((0, 0)), 0)
+
     def test_iss634(self):
         img = Image.open("Tests/images/iss634.gif")
         # seek to the second frame

src/PIL/GifImagePlugin.py

@@ -426,9 +426,8 @@ def _write_multiple_frames(im, fp, palette):
 
     im_frames = []
     frame_count = 0
-    background = None
+    background_im = None
-    for imSequence in itertools.chain([im],
+    for imSequence in itertools.chain([im], im.encoderinfo.get("append_images", [])):
-                                      im.encoderinfo.get("append_images", [])):
         for im_frame in ImageSequence.Iterator(imSequence):
             # a copy is required here since seek can still mutate the image
             im_frame = _normalize_mode(im_frame.copy())
@@ -447,16 +446,23 @@ def _write_multiple_frames(im, fp, palette):
             if im_frames:
                 # delta frame
                 previous = im_frames[-1]
-                if disposal == 2:
+                if encoderinfo.get("disposal") == 2:
-                    base_image = background
+                    if background_im is None:
+                        background = _get_background(
+                            im,
+                            im.encoderinfo.get("background", im.info.get("background")),
+                        )
+                        background_im = Image.new("P", im_frame.size, background)
+                        background_im.putpalette(im_frames[0]["im"].palette)
+                    base_im = background_im
                 else:
-                    base_image = previous["im"]
+                    base_im = previous["im"]
-
+                if _get_palette_bytes(im_frame) == _get_palette_bytes(base_im):
-                if _get_palette_bytes(im_frame) == _get_palette_bytes(base_frame):
+                    delta = ImageChops.subtract_modulo(im_frame, base_im)
-                    delta = ImageChops.subtract_modulo(im_frame, base_image)
                 else:
                     delta = ImageChops.subtract_modulo(
-                        im_frame.convert("RGB"), base_image.convert("RGB"))
+                        im_frame.convert("RGB"), base_im.convert("RGB")
+                    )
                 bbox = delta.getbbox()
                 if not bbox:
                     # This frame is identical to the previous frame
@@ -465,7 +471,6 @@ def _write_multiple_frames(im, fp, palette):
                     continue
             else:
                 bbox = None
-                background = Image.new("P", im_frame.size, 0)
             im_frames.append({"im": im_frame, "bbox": bbox, "encoderinfo": encoderinfo})
 
     if len(im_frames) > 1:
@@ -726,6 +731,18 @@ def _get_palette_bytes(im):
     return im.palette.palette
 
 
+def _get_background(im, infoBackground):
+    background = 0
+    if infoBackground:
+        background = infoBackground
+        if isinstance(background, tuple):
+            # WebPImagePlugin stores an RGBA value in info["background"]
+            # So it must be converted to the same format as GifImagePlugin's
+            # info["background"] - a global color table index
+            background = im.palette.getcolor(background)
+    return background
+
+
 def _get_global_header(im, info):
     """Return a list of strings representing a GIF header"""
 
@@ -745,14 +762,7 @@ def _get_global_header(im, info):
         if im.info.get("version") == b"89a":
             version = b"89a"
 
-    background = 0
+    background = _get_background(im, info.get("background"))
-    if "background" in info:
-        background = info["background"]
-        if isinstance(background, tuple):
-            # WebPImagePlugin stores an RGBA value in info["background"]
-            # So it must be converted to the same format as GifImagePlugin's
-            # info["background"] - a global color table index
-            background = im.palette.getcolor(background)
 
     palette_bytes = _get_palette_bytes(im)
     color_table_size = _get_color_table_size(palette_bytes)