Improve transparency handling when saving GIF images

Tests/test_file_gif.py

@@ -619,7 +619,8 @@ def test_dispose2_background(tmp_path):
         assert im.getpixel((0, 0)) == (255, 0, 0)
 
 
-def test_transparency_in_second_frame():
+def test_transparency_in_second_frame(tmp_path):
+    out = str(tmp_path / "temp.gif")
     with Image.open("Tests/images/different_transparency.gif") as im:
         assert im.info["transparency"] == 0
 
@@ -629,6 +630,14 @@ def test_transparency_in_second_frame():
 
         assert_image_equal_tofile(im, "Tests/images/different_transparency_merged.png")
 
+        im.save(out, save_all=True)
+
+    with Image.open(out) as reread:
+        reread.seek(reread.tell() + 1)
+        assert_image_equal_tofile(
+            reread, "Tests/images/different_transparency_merged.png"
+        )
+
 
 def test_no_transparency_in_second_frame():
     with Image.open("Tests/images/iss634.gif") as img:
@@ -640,6 +649,22 @@ def test_no_transparency_in_second_frame():
         assert img.histogram()[255] == 0
 
 
+def test_remapped_transparency(tmp_path):
+    out = str(tmp_path / "temp.gif")
+
+    im = Image.new("P", (1, 2))
+    im2 = im.copy()
+
+    # Add transparency at a higher index
+    # so that it will be optimized to a lower index
+    im.putpixel((0, 1), 5)
+    im.info["transparency"] = 5
+    im.save(out, save_all=True, append_images=[im2])
+
+    with Image.open(out) as reloaded:
+        assert reloaded.info["transparency"] == reloaded.getpixel((0, 1))
+
+
 def test_duration(tmp_path):
     duration = 1000
 

Tests/test_image.py

@@ -607,6 +607,20 @@ def test_remap_palette(self):
             with pytest.raises(ValueError):
                 im.remap_palette(None)
 
+    def test_remap_palette_transparency(self):
+        im = Image.new("P", (1, 2))
+        im.putpixel((0, 1), 1)
+        im.info["transparency"] = 0
+
+        im_remapped = im.remap_palette([1, 0])
+        assert im_remapped.info["transparency"] == 1
+
+        # Test unused transparency
+        im.info["transparency"] = 2
+
+        im_remapped = im.remap_palette([1, 0])
+        assert "transparency" not in im_remapped.info
+
     def test__new(self):
         im = hopper("RGB")
         im_p = hopper("P")

src/PIL/GifImagePlugin.py

@@ -573,10 +573,14 @@ def _write_multiple_frames(im, fp, palette):
             im_frame = _normalize_mode(im_frame.copy())
             if frame_count == 0:
                 for k, v in im_frame.info.items():
+                    if k == "transparency":
+                        continue
                     im.encoderinfo.setdefault(k, v)
-            im_frame = _normalize_palette(im_frame, palette, im.encoderinfo)
 
             encoderinfo = im.encoderinfo.copy()
+            im_frame = _normalize_palette(im_frame, palette, encoderinfo)
+            if "transparency" in im_frame.info:
+                encoderinfo.setdefault("transparency", im_frame.info["transparency"])
             if isinstance(duration, (list, tuple)):
                 encoderinfo["duration"] = duration[frame_count]
             if isinstance(disposal, (list, tuple)):

src/PIL/Image.py

@@ -1934,6 +1934,13 @@ def remap_palette(self, dest_map, source_palette=None):
         m_im.putpalette(new_palette_bytes)
         m_im.palette = ImagePalette.ImagePalette("RGB", palette=palette_bytes)
 
+        if "transparency" in self.info:
+            try:
+                m_im.info["transparency"] = dest_map.index(self.info["transparency"])
+            except ValueError:
+                if "transparency" in m_im.info:
+                    del m_im.info["transparency"]
+
         return m_im
 
     def _get_safe_box(self, size, resample, box):