From 0966293a0df0e45a2d4924542e180e917098c2fc Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Thu, 16 Mar 2023 20:05:56 +1100
Subject: [PATCH] Consider transparency when applying blend mask

---
 Tests/images/blend_transparency.png | Bin 0 -> 211 bytes
 Tests/test_file_apng.py             |   6 ++++++
 src/PIL/PngImagePlugin.py           |  10 +++++++---
 3 files changed, 13 insertions(+), 3 deletions(-)
 create mode 100644 Tests/images/blend_transparency.png

diff --git a/Tests/images/blend_transparency.png b/Tests/images/blend_transparency.png
new file mode 100644
index 0000000000000000000000000000000000000000..cef0a16de1f2db2db74e87d8bed5c8ba32038f28
GIT binary patch
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literal 0
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diff --git a/Tests/test_file_apng.py b/Tests/test_file_apng.py
index b2bec5984..feca72aa6 100644
--- a/Tests/test_file_apng.py
+++ b/Tests/test_file_apng.py
@@ -163,6 +163,12 @@ def test_apng_blend():
         assert im.getpixel((64, 32)) == (0, 255, 0, 255)
 
 
+def test_apng_blend_transparency():
+    with Image.open("Tests/images/blend_transparency.png") as im:
+        im.seek(1)
+        assert im.getpixel((0, 0)) == (255, 0, 0)
+
+
 def test_apng_chunk_order():
     with Image.open("Tests/images/apng/fctl_actl.png") as im:
         im.seek(im.n_frames - 1)
diff --git a/src/PIL/PngImagePlugin.py b/src/PIL/PngImagePlugin.py
index 9078957dc..15a3c8291 100644
--- a/src/PIL/PngImagePlugin.py
+++ b/src/PIL/PngImagePlugin.py
@@ -1003,9 +1003,13 @@ class PngImageFile(ImageFile.ImageFile):
         else:
             if self._prev_im and self.blend_op == Blend.OP_OVER:
                 updated = self._crop(self.im, self.dispose_extent)
-                self._prev_im.paste(
-                    updated, self.dispose_extent, updated.convert("RGBA")
-                )
+                if self.im.mode == "RGB" and "transparency" in self.info:
+                    mask = updated.convert_transparent(
+                        "RGBA", self.info["transparency"]
+                    )
+                else:
+                    mask = updated.convert("RGBA")
+                self._prev_im.paste(updated, self.dispose_extent, mask)
                 self.im = self._prev_im
                 if self.pyaccess:
                     self.pyaccess = None