Added setting to convert first GIF frame to RGB

Tests/test_file_gif.py

@@ -78,6 +78,18 @@ def test_l_mode_subsequent_frames():
         assert im.load()[0, 0] == (0, 255)
 
 
+def test_strategy():
+    with Image.open(TEST_GIF) as im:
+        expected = im.convert("RGB")
+
+    GifImagePlugin.PALETTE_TO_RGB = GifImagePlugin.ModeStrategy.ALWAYS
+    with Image.open(TEST_GIF) as im:
+        assert im.mode == "RGB"
+        assert_image_equal(im, expected)
+
+    GifImagePlugin.PALETTE_TO_RGB = GifImagePlugin.ModeStrategy.AFTER_FIRST
+
+
 def test_optimize():
     def test_grayscale(optimize):
         im = Image.new("L", (1, 1), 0)

docs/handbook/image-file-formats.rst

@@ -110,6 +110,13 @@ 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).
 
+If you would prefer the first ``P`` image frame to be ``RGB``, so that ``P``
+frames are always converted to ``RGB`` or ``RGBA`` mode, there is a setting
+available::
+
+    from PIL import GifImagePlugin
+    GifImagePlugin.PALETTE_TO_RGB = GifImagePlugin.ModeStrategy.ALWAYS
+
 The :py:meth:`~PIL.Image.open` method sets the following
 :py:attr:`~PIL.Image.Image.info` properties:
 

src/PIL/GifImagePlugin.py

@@ -28,12 +28,21 @@ import itertools
 import math
 import os
 import subprocess
+from enum import IntEnum
 
 from . import Image, ImageChops, ImageFile, ImagePalette, ImageSequence
 from ._binary import i16le as i16
 from ._binary import o8
 from ._binary import o16le as o16
 
+
+class ModeStrategy(IntEnum):
+    AFTER_FIRST = 0
+    ALWAYS = 1
+
+
+PALETTE_TO_RGB = ModeStrategy.AFTER_FIRST
+
 # --------------------------------------------------------------------
 # Identify/read GIF files
 
@@ -355,18 +364,22 @@ class GifImageFile(ImageFile.ImageFile):
                 del self.info[k]
 
         if frame == 0:
-            self.mode = "P" if frame_palette else "L"
+            if frame_palette:
+                self.mode = "RGB" if PALETTE_TO_RGB == ModeStrategy.ALWAYS else "P"
+            else:
+                self.mode = "L"
 
-            if self.mode == "P" and not palette:
+            if not palette and self.global_palette:
                 from copy import copy
 
                 palette = copy(self.global_palette)
             self.palette = palette
         else:
-            self._frame_palette = frame_palette
             self._frame_transparency = frame_transparency
+        self._frame_palette = frame_palette
 
     def load_prepare(self):
+        self.mode = "P" if self._frame_palette else "L"
         if self.__frame == 0:
             if "transparency" in self.info:
                 self.im = Image.core.fill(
@@ -375,18 +388,19 @@ class GifImageFile(ImageFile.ImageFile):
         else:
             self._prev_im = self.im
             if self._frame_palette:
-                self.mode = "P"
                 self.im = Image.core.fill("P", self.size, self._frame_transparency or 0)
                 self.im.putpalette(*self._frame_palette.getdata())
-                self._frame_palette = None
             else:
-                self.mode = "L"
                 self.im = None
+        self._frame_palette = None
 
         super().load_prepare()
 
     def load_end(self):
         if self.__frame == 0:
+            if self.mode == "P" and PALETTE_TO_RGB == ModeStrategy.ALWAYS:
+                self.mode = "RGB"
+                self.im = self.im.convert("RGB", Image.Dither.FLOYDSTEINBERG)
             return
         if self.mode == "P":
             if self._frame_transparency is not None: