Preserve transparency when padding and expanding images

Tests/test_imageops.py

@@ -232,6 +232,23 @@ def test_rgba_palette() -> None:
     assert expanded_im.convert("RGBA").getpixel((0, 0)) == translucent_black
 
 
+def test_transparency() -> None:
+    im = Image.new("P", (1, 1))
+    im.info["transparency"] = 0
+
+    expanded_im = ImageOps.expand(im, 1, "blue")
+    assert expanded_im.info["transparency"] == 0
+    expanded_im_rgba = expanded_im.convert("RGBA")
+    assert expanded_im_rgba.getpixel((0, 0)) == (0, 0, 255, 255)
+    assert expanded_im_rgba.getpixel((1, 1)) == (0, 0, 0, 0)
+
+    padded_im = ImageOps.pad(im, (3, 1), color="blue")
+    assert padded_im.info["transparency"] == 0
+    padded_im_rgba = padded_im.convert("RGBA")
+    assert padded_im_rgba.getpixel((0, 0)) == (0, 0, 255, 255)
+    assert padded_im_rgba.getpixel((1, 0)) == (0, 0, 0, 0)
+
+
 def test_pil163() -> None:
     # Division by zero in equalize if < 255 pixels in image (@PIL163)
 

docs/releasenotes/13.0.0.rst

@@ -135,6 +135,14 @@ immediately at the release of 3.15.0 final (2026-10-01, :pep:`790`).
 
 Pillow 13.0.0 now officially supports Python 3.15.
 
+Preserved transparency when padding or expanding images in ImageOps
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+:py:func:`~PIL.ImageOps.pad` and :py:func:`~PIL.ImageOps.expand` now preserve the
+``transparency`` value from the source image's ``info`` dictionary. Transparent
+pixels in L, P and RGB images previously became opaque when a border was added.
+New palette colors are allocated without using reserved transparency entries.
+
 Restore image position if ImageSequence.all_frames() fails
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

src/PIL/ImageOps.py

@@ -349,12 +349,14 @@ def _new_with_fill(
         mode = image.palette.mode
         palette = ImagePalette.ImagePalette(mode, image.getpalette(mode))
         if isinstance(color, tuple) and len(color) in (3, 4):
-            color = palette.getcolor(color)
+            color = palette.getcolor(color, image)
     else:
         palette = None
     out = Image.new(image.mode, size, color)
     if palette:
         out.putpalette(palette.palette, mode)
+    if "transparency" in image.info:
+        out.info["transparency"] = image.info["transparency"]
     return out