Added apply_transparency()

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

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.