Improved handling of RGBA palettes when saving GIF images

Tests/test_file_gif.py

@@ -1429,3 +1429,21 @@ def test_saving_rgba(tmp_path: Path) -> None:
     with Image.open(out) as reloaded:
         reloaded_rgba = reloaded.convert("RGBA")
         assert reloaded_rgba.load()[0, 0][3] == 0
+
+
+def test_optimizing_p_rgba(tmp_path: Path) -> None:
+    out = str(tmp_path / "temp.gif")
+
+    im1 = Image.new("P", (100, 100))
+    d = ImageDraw.Draw(im1)
+    d.ellipse([(40, 40), (60, 60)], fill=1)
+    data = [0, 0, 0, 0, 0, 0, 0, 255] + [0, 0, 0, 0] * 254
+    im1.putpalette(data, "RGBA")
+
+    im2 = Image.new("P", (100, 100))
+    im2.putpalette(data, "RGBA")
+
+    im1.save(out, save_all=True, append_images=[im2])
+
+    with Image.open(out) as reloaded:
+        assert reloaded.n_frames == 2

src/PIL/GifImagePlugin.py

@@ -553,7 +553,9 @@ def _normalize_palette(
 
     if im.mode == "P":
         if not source_palette:
-            source_palette = im.im.getpalette("RGB")[:768]
+            im_palette = im.getpalette(None)
+            assert im_palette is not None
+            source_palette = bytearray(im_palette)
     else:  # L-mode
         if not source_palette:
             source_palette = bytearray(i // 3 for i in range(768))
@@ -629,7 +631,10 @@ def _write_single_frame(
 def _getbbox(
     base_im: Image.Image, im_frame: Image.Image
 ) -> tuple[Image.Image, tuple[int, int, int, int] | None]:
-    if _get_palette_bytes(im_frame) != _get_palette_bytes(base_im):
+    palette_bytes = [
+        bytes(im.palette.palette) if im.palette else b"" for im in (base_im, im_frame)
+    ]
+    if palette_bytes[0] != palette_bytes[1]:
         im_frame = im_frame.convert("RGBA")
         base_im = base_im.convert("RGBA")
     delta = ImageChops.subtract_modulo(im_frame, base_im)
@@ -984,7 +989,13 @@ def _get_palette_bytes(im: Image.Image) -> bytes:
     :param im: Image object
     :returns: Bytes, len<=768 suitable for inclusion in gif header
     """
-    return bytes(im.palette.palette) if im.palette else b""
+    if not im.palette:
+        return b""
+
+    palette = bytes(im.palette.palette)
+    if im.palette.mode == "RGBA":
+        palette = b"".join(palette[i * 4 : i * 4 + 3] for i in range(len(palette) // 3))
+    return palette
 
 
 def _get_background(

src/PIL/Image.py

@@ -1066,7 +1066,7 @@ class Image:
                     trns_im = new(self.mode, (1, 1))
                     if self.mode == "P":
                         assert self.palette is not None
-                        trns_im.putpalette(self.palette)
+                        trns_im.putpalette(self.palette, self.palette.mode)
                         if isinstance(t, tuple):
                             err = "Couldn't allocate a palette color for transparency"
                             assert trns_im.palette is not None
@@ -2182,6 +2182,9 @@ class Image:
                 source_palette = self.im.getpalette(palette_mode, palette_mode)
             else:  # L-mode
                 source_palette = bytearray(i // 3 for i in range(768))
+        elif len(source_palette) > 768:
+            bands = 4
+            palette_mode = "RGBA"
 
         palette_bytes = b""
         new_positions = [0] * 256