fix: keep alpha channel for images with mode P and custom transparency

src/pillow_avif/AvifImagePlugin.py

@@ -233,6 +233,10 @@ def _save(im, fp, filename, save_all=False):
                         "A" in ims.mode
                         or "a" in ims.mode
                         or (ims.mode == "P" and "A" in ims.im.getpalettemode())
+                        or (
+                            ims.mode == "P"
+                            and ims.info.get("transparency", None) is not None
+                        )
                     )
                     rawmode = "RGBA" if alpha else "RGB"
                     frame = ims.convert(rawmode)

tests/test_file_avif.py

@@ -11,7 +11,7 @@
 
 import pytest
 
-from PIL import Image
+from PIL import Image, ImageDraw
 from pillow_avif import AvifImagePlugin
 
 from .helper import (
@@ -565,6 +565,20 @@ def test_decoder_upsampling_invalid(self):
         finally:
             AvifImagePlugin.CHROMA_UPSAMPLING = "auto"
 
+    def test_p_mode_transparency(self):
+        im = Image.new("P", size=(64, 64))
+        draw = ImageDraw.Draw(im)
+        draw.rectangle(xy=[(0, 0), (32, 32)], fill=255)
+        draw.rectangle(xy=[(32, 32), (64, 64)], fill=255)
+
+        buf_png = BytesIO()
+        im.save(buf_png, format="PNG", transparency=0)
+        im_png = Image.open(buf_png)
+        buf_out = BytesIO()
+        im_png.save(buf_out, format="AVIF", quality=100)
+
+        assert_image_similar(im_png.convert("RGBA"), Image.open(buf_out), 1)
+
 
 class TestAvifAnimation:
     @contextmanager