Use transparency behind first GIF frame and when disposing to background

Tests/test_file_gif.py

@@ -298,6 +298,12 @@ def test_eoferror():
         im.seek(n_frames - 1)
 
 
+def test_first_frame_transparency():
+    with Image.open("Tests/images/first_frame_transparency.gif") as im:
+        px = im.load()
+        assert px[0, 0] == im.info["transparency"]
+
+
 def test_dispose_none():
     with Image.open("Tests/images/dispose_none.gif") as img:
         try:
@@ -331,6 +337,16 @@ def test_dispose_background():
             pass
 
 
+def test_transparent_dispose():
+    expected_colors = [(2, 1, 2), (0, 1, 0), (2, 1, 2)]
+    with Image.open("Tests/images/transparent_dispose.gif") as img:
+        for frame in range(3):
+            img.seek(frame)
+            for x in range(3):
+                color = img.getpixel((x, 0))
+                assert color == expected_colors[frame][x]
+
+
 def test_dispose_previous():
     with Image.open("Tests/images/dispose_prev.gif") as img:
         try:

src/PIL/GifImagePlugin.py

@@ -269,9 +269,14 @@ def _seek(self, frame):
                 dispose_size = (x1 - x0, y1 - y0)
 
                 Image._decompression_bomb_check(dispose_size)
-                self.dispose = Image.core.fill(
-                    "P", dispose_size, self.info.get("background", 0)
+
+                # by convention, attempt to use transparency first
+                color = (
+                    frame_transparency
+                    if frame_transparency is not None
+                    else self.info.get("background", 0)
                 )
+                self.dispose = Image.core.fill("P", dispose_size, color)
             else:
                 # replace with previous contents
                 if self.im:
@@ -317,6 +322,12 @@ def _seek(self, frame):
         if self.palette:
             self.mode = "P"
 
+    def load_prepare(self):
+        if not self.im and "transparency" in self.info:
+            self.im = Image.core.fill(self.mode, self.size, self.info["transparency"])
+
+        super(GifImageFile, self).load_prepare()
+
     def tell(self):
         return self.__frame