From 541d2605b9235a427379e3df51c9cdd4ffe59998 Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Wed, 14 Jun 2023 14:21:07 +1000
Subject: [PATCH] Allow alpha differences to indicate different frames when
 saving GIF

---
 Tests/test_file_gif.py    | 12 ++++++++++++
 src/PIL/GifImagePlugin.py |  4 ++--
 src/PIL/PngImagePlugin.py |  2 +-
 3 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/Tests/test_file_gif.py b/Tests/test_file_gif.py
index 0e50ee1abf9..f4a17264f4a 100644
--- a/Tests/test_file_gif.py
+++ b/Tests/test_file_gif.py
@@ -1130,6 +1130,18 @@ def test_bbox(tmp_path):
         assert reread.n_frames == 2
 
 
+def test_bbox_alpha(tmp_path):
+    out = str(tmp_path / "temp.gif")
+
+    im = Image.new("RGBA", (1, 2), (255, 0, 0, 255))
+    im.putpixel((0, 1), (255, 0, 0, 0))
+    im2 = Image.new("RGBA", (1, 2), (255, 0, 0, 0))
+    im.save(out, save_all=True, append_images=[im2])
+
+    with Image.open(out) as reread:
+        assert reread.n_frames == 2
+
+
 def test_palette_save_L(tmp_path):
     # Generate an L mode image with a separate palette
 
diff --git a/src/PIL/GifImagePlugin.py b/src/PIL/GifImagePlugin.py
index 2f92e946751..cf2993e3892 100644
--- a/src/PIL/GifImagePlugin.py
+++ b/src/PIL/GifImagePlugin.py
@@ -569,9 +569,9 @@ def _getbbox(base_im, im_frame):
         delta = ImageChops.subtract_modulo(im_frame, base_im)
     else:
         delta = ImageChops.subtract_modulo(
-            im_frame.convert("RGB"), base_im.convert("RGB")
+            im_frame.convert("RGBA"), base_im.convert("RGBA")
         )
-    return delta.getbbox()
+    return delta.getbbox(alpha_only=False)
 
 
 def _write_multiple_frames(im, fp, palette):
diff --git a/src/PIL/PngImagePlugin.py b/src/PIL/PngImagePlugin.py
index ea3052fbf7c..bfa8cb7ac66 100644
--- a/src/PIL/PngImagePlugin.py
+++ b/src/PIL/PngImagePlugin.py
@@ -1140,7 +1140,7 @@ def _write_multiple_frames(im, fp, chunk, rawmode, default_image, append_images)
                 delta = ImageChops.subtract_modulo(
                     im_frame.convert("RGBA"), base_im.convert("RGBA")
                 )
-                bbox = delta.im.getbbox(False)
+                bbox = delta.getbbox(alpha_only=False)
                 if (
                     not bbox
                     and prev_disposal == encoderinfo.get("disposal")