Remove disposal 2 duplicate frame exemption and add true delta test

Tests/test_file_gif.py

@@ -315,19 +315,19 @@ class TestFileGif(PillowTestCase):
             img.seek(img.tell() + 1)
             self.assertEqual(img.disposal_method, i+1)
 
-    def test_dispose2_diff(self):
+    def test_dispose2_palette(self):
         out = self.tempfile('temp.gif')
 
         # 4 backgrounds: White, Grey, Black, Red
-        backgrounds = [(255, 255, 255), (153, 153, 153), (0, 0, 0), (255, 0, 0)]
+        circles = [(255, 255, 255), (153, 153, 153), (0, 0, 0), (255, 0, 0)]
 
         im_list = []
-        for background in backgrounds:
-            img = Image.new('RGB', (100, 100), background)
+        for circle in circles:
+            img = Image.new('RGB', (100, 100), (255,0,0))
 
             # Red circle in center of each frame
             d = ImageDraw.Draw(img)
-            d.ellipse([(40, 40), (60, 60)], fill='#f00')
+            d.ellipse([(40, 40), (60, 60)], fill=circle)
 
             im_list.append(img)
 
@@ -340,15 +340,62 @@ class TestFileGif(PillowTestCase):
 
         img = Image.open(out)
 
-        for i, background in enumerate(backgrounds):
+        for i, circle in enumerate(circles):
             img.seek(i)
             rgb_img = img.convert('RGB')
 
             # Check top left pixel matches background
-            self.assertEqual(rgb_img.getpixel((0, 0)), background)
+            self.assertEqual(rgb_img.getpixel((0, 0)), (255, 0, 0))
 
             # Center remains red every frame
-            self.assertEqual(rgb_img.getpixel((50, 50)), (255, 0, 0))
+            self.assertEqual(rgb_img.getpixel((50, 50)), circle)
+
+    def test_dispose2_diff(self):
+        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))
+        ]
+
+        im_list = []
+        for i in range(len(circles)):
+            # Transparent BG
+            img = Image.new('RGBA', (100, 100), (255,255,255,0))
+
+            # Two circles per frame
+            d = ImageDraw.Draw(img)
+            d.ellipse([(0, 30), (40, 70)], fill=circles[i][0])
+            d.ellipse([(60, 30), (100, 70)], fill=circles[i][1])
+
+            im_list.append(img)
+
+        im_list[0].save(
+            out,
+            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')
+
+            # Check left circle is correct colour
+            self.assertEqual(rgb_img.getpixel((20, 50)), colours[0])
+
+            # Check right circle is correct colour
+            self.assertEqual(rgb_img.getpixel((80, 50)), colours[1])
+
+            # Check BG is correct colour
+            self.assertEqual(rgb_img.getpixel((1, 1)), (255,255,255,0))
+
 
     def test_iss634(self):
         img = Image.open("Tests/images/iss634.gif")

src/PIL/GifImagePlugin.py

@@ -457,8 +457,7 @@ def _write_multiple_frames(im, fp, palette):
                     delta = ImageChops.subtract_modulo(
                         im_frame.convert('RGB'), base_image.convert('RGB'))
                 bbox = delta.getbbox()
-                if not bbox and not ("disposal" in encoderinfo
-                   and encoderinfo["disposal"] == 2):
+                if not bbox:
                     # This frame is identical to the previous frame
                     if duration:
                         previous['encoderinfo']['duration'] += \