From 11be1631433f252b816802aef1a3cd109bd308c7 Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Mon, 6 Jun 2022 22:47:58 +1000
Subject: [PATCH 1/2] Added apply_transparency()

---
 Tests/test_image.py | 29 +++++++++++++++++++++++++++++
 src/PIL/Image.py    | 22 ++++++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/Tests/test_image.py b/Tests/test_image.py
index 7922505185b..24289bf517d 100644
--- a/Tests/test_image.py
+++ b/Tests/test_image.py
@@ -840,6 +840,35 @@ def test_zero_tobytes(self, size):
         im = Image.new("RGB", size)
         assert im.tobytes() == b""
 
+    def test_apply_transparency(self):
+        im = Image.new("P", (1, 1))
+        im.putpalette((0, 0, 0, 1, 1, 1))
+        assert im.palette.colors == {(0, 0, 0): 0, (1, 1, 1): 1}
+
+        # Test that no transformation is applied without transparency
+        im.apply_transparency()
+        assert im.palette.colors == {(0, 0, 0): 0, (1, 1, 1): 1}
+
+        # Test that a transparency index is applied
+        im.info["transparency"] = 0
+        im.apply_transparency()
+        assert "transparency" not in im.info
+        assert im.palette.colors == {(0, 0, 0, 0): 0, (1, 1, 1, 255): 1}
+
+        # Test that existing transparency is kept
+        im = Image.new("P", (1, 1))
+        im.putpalette((0, 0, 0, 255, 1, 1, 1, 128), "RGBA")
+        im.info["transparency"] = 0
+        im.apply_transparency()
+        assert im.palette.colors == {(0, 0, 0, 0): 0, (1, 1, 1, 128): 1}
+
+        # Test that transparency bytes are applied
+        with Image.open("Tests/images/pil123p.png") as im:
+            assert isinstance(im.info["transparency"], bytes)
+            assert im.palette.colors[(27, 35, 6)] == 24
+            im.apply_transparency()
+            assert im.palette.colors[(27, 35, 6, 214)] == 24
+
     def test_categories_deprecation(self):
         with pytest.warns(DeprecationWarning):
             assert hopper().category == 0
diff --git a/src/PIL/Image.py b/src/PIL/Image.py
index 0ba2808f851..7edb9a4c293 100644
--- a/src/PIL/Image.py
+++ b/src/PIL/Image.py
@@ -1449,6 +1449,28 @@ def getpalette(self, rawmode="RGB"):
             rawmode = mode
         return list(self.im.getpalette(mode, rawmode))
 
+    def apply_transparency(self):
+        """
+        If a P mode image has a "transparency" key in the info dictionary,
+        remove the key and apply the transparency to the palette instead.
+        """
+        if self.mode != "P" or "transparency" not in self.info:
+            return
+
+        from . import ImagePalette
+
+        palette = self.getpalette("RGBA")
+        transparency = self.info["transparency"]
+        if isinstance(transparency, bytes):
+            for i, alpha in enumerate(transparency):
+                palette[i * 4 + 3] = alpha
+        else:
+            palette[transparency * 4 + 3] = 0
+        self.palette = ImagePalette.ImagePalette("RGBA", bytes(palette))
+        self.palette.dirty = 1
+
+        del self.info["transparency"]
+
     def getpixel(self, xy):
         """
         Returns the pixel value at a given position.

From 70a060e7658469acf33485cb0a6ed198e2d0be2f Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Sat, 11 Jun 2022 22:31:07 +1000
Subject: [PATCH 2/2] Document apply_transparency()

---
 docs/reference/Image.rst    | 1 +
 docs/releasenotes/9.2.0.rst | 9 ++++++---
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/docs/reference/Image.rst b/docs/reference/Image.rst
index 2613b6585b3..ed37521fdae 100644
--- a/docs/reference/Image.rst
+++ b/docs/reference/Image.rst
@@ -123,6 +123,7 @@ methods. Unless otherwise stated, all methods return a new instance of the
 
 
 .. automethod:: PIL.Image.Image.alpha_composite
+.. automethod:: PIL.Image.Image.apply_transparency
 .. automethod:: PIL.Image.Image.convert
 
 The following example converts an RGB image (linearly calibrated according to
diff --git a/docs/releasenotes/9.2.0.rst b/docs/releasenotes/9.2.0.rst
index eddbd07a2d7..424fd487a29 100644
--- a/docs/releasenotes/9.2.0.rst
+++ b/docs/releasenotes/9.2.0.rst
@@ -57,10 +57,13 @@ TODO
 API Additions
 =============
 
-TODO
-^^^^
+Image.apply_transparency
+^^^^^^^^^^^^^^^^^^^^^^^^
 
-TODO
+Added :py:meth:`~PIL.Image.Image.apply_transparency`, a method to take a P mode image
+with "transparency" in ``im.info``, and apply the transparency to the palette instead.
+The image's palette mode will become "RGBA", and "transparency" will be removed from
+``im.info``.
 
 Security
 ========