From 7928e944cb73589014bfebaea15042a92bc3c68f Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Sun, 20 Mar 2022 15:09:01 +1100
Subject: [PATCH] Keep subsequent L frames without transparency as L

---
 Tests/images/no_palette.gif          | Bin 0 -> 48 bytes
 Tests/test_file_gif.py               |  10 ++++++++-
 docs/handbook/image-file-formats.rst |   5 +++--
 src/PIL/GifImagePlugin.py            |  32 +++++++++++++++------------
 4 files changed, 30 insertions(+), 17 deletions(-)
 create mode 100644 Tests/images/no_palette.gif

diff --git a/Tests/images/no_palette.gif b/Tests/images/no_palette.gif
new file mode 100644
index 0000000000000000000000000000000000000000..0432ebcb61c30a77eb385b323a0bd6c8d8577ef3
GIT binary patch
literal 48
gcmZ?wbh9u|WMp7u00JEl0cLZsFfg*PU<z0R09R}RY5)KL

literal 0
HcmV?d00001

diff --git a/Tests/test_file_gif.py b/Tests/test_file_gif.py
index 924adad9e..ba277a776 100644
--- a/Tests/test_file_gif.py
+++ b/Tests/test_file_gif.py
@@ -59,7 +59,15 @@ def test_invalid_file():
         GifImagePlugin.GifImageFile(invalid_file)
 
 
-def test_l_mode_transparency():
+def test_l_mode_subsequent_frames():
+    with Image.open("Tests/images/no_palette.gif") as im:
+        assert im.mode == "L"
+        assert im.load()[0, 0] == 0
+
+        im.seek(1)
+        assert im.mode == "L"
+        assert im.load()[0, 0] == 0
+
     with Image.open("Tests/images/no_palette_with_transparency.gif") as im:
         assert im.mode == "L"
         assert im.load()[0, 0] == 0
diff --git a/docs/handbook/image-file-formats.rst b/docs/handbook/image-file-formats.rst
index 08af189da..5db1b8c7d 100644
--- a/docs/handbook/image-file-formats.rst
+++ b/docs/handbook/image-file-formats.rst
@@ -106,8 +106,9 @@ writes run-length encoded files in GIF87a by default, unless GIF89a features
 are used or GIF89a is already in use.
 
 GIF files are initially read as grayscale (``L``) or palette mode (``P``)
-images, but seeking to later frames in an image will change the mode to either
-``RGB`` or ``RGBA``, depending on whether the first frame had transparency.
+images. Seeking to later frames in a ``P`` image will change the image to
+``RGB`` (or ``RGBA`` if the first frame had transparency). ``L`` images will
+stay in ``L`` mode (or change to ``LA`` if the first frame had transparency).
 
 The :py:meth:`~PIL.Image.open` method sets the following
 :py:attr:`~PIL.Image.Image.info` properties:
diff --git a/src/PIL/GifImagePlugin.py b/src/PIL/GifImagePlugin.py
index 9cf27b026..764eda6ee 100644
--- a/src/PIL/GifImagePlugin.py
+++ b/src/PIL/GifImagePlugin.py
@@ -174,21 +174,22 @@ class GifImageFile(ImageFile.ImageFile):
         if update_image:
             if self.__frame == 1:
                 self.pyaccess = None
-                if "transparency" in self.info:
-                    if self.mode == "P":
+                if self.mode == "P":
+                    if "transparency" in self.info:
                         self.im.putpalettealpha(self.info["transparency"], 0)
                         self.im = self.im.convert("RGBA", Image.Dither.FLOYDSTEINBERG)
                         self.mode = "RGBA"
+                        del self.info["transparency"]
                     else:
-                        self.im = self.im.convert_transparent(
-                            "LA", self.info["transparency"]
-                        )
-                        self.mode = "LA"
-
+                        self.mode = "RGB"
+                        self.im = self.im.convert("RGB", Image.Dither.FLOYDSTEINBERG)
+                elif "transparency" in self.info:
+                    self.im = self.im.convert_transparent(
+                        "LA", self.info["transparency"]
+                    )
+                    self.mode = "LA"
                     del self.info["transparency"]
-                else:
-                    self.mode = "RGB"
-                    self.im = self.im.convert("RGB", Image.Dither.FLOYDSTEINBERG)
+
             if self.dispose:
                 self.im.paste(self.dispose, self.dispose_extent)
 
@@ -387,14 +388,17 @@ class GifImageFile(ImageFile.ImageFile):
     def load_end(self):
         if self.__frame == 0:
             return
-        if self._frame_transparency is not None:
-            if self.mode == "P":
+        if self.mode == "P":
+            if self._frame_transparency is not None:
                 self.im.putpalettealpha(self._frame_transparency, 0)
                 frame_im = self.im.convert("RGBA")
             else:
-                frame_im = self.im.convert_transparent("LA", self._frame_transparency)
+                frame_im = self.im.convert("RGB")
         else:
-            frame_im = self.im.convert("RGB")
+            if self._frame_transparency is not None:
+                frame_im = self.im.convert_transparent("LA", self._frame_transparency)
+            else:
+                frame_im = self.im
         frame_im = self._crop(frame_im, self.dispose_extent)
 
         self.im = self._prev_im