Save as RGB(A) when APNG frames have different palettes

Tests/test_file_apng.py

@@ -299,6 +299,30 @@ def test_apng_mode() -> None:
     assert im_rgba.getpixel((64, 32)) == (0, 0, 255, 128)
 
 
+def test_apng_save_different_palettes(tmp_path: Path) -> None:
+    # APNG does not support multiple palettes
+    test_file = tmp_path / "temp.png"
+    red = Image.new("P", (1, 1), (255, 0, 0))
+    green = Image.new("P", (1, 1), (0, 255, 0))
+    red.save(test_file, save_all=True, append_images=[green])
+
+    with Image.open(test_file) as reloaded:
+        # So the frames must be converted to another mode
+        assert reloaded.mode == "RGB"
+        assert reloaded.getpixel((0, 0)) == (255, 0, 0)
+
+        reloaded.seek(1)
+        assert reloaded.mode == "RGB"
+        assert reloaded.getpixel((0, 0)) == (0, 255, 0)
+
+    # Test that RGBA palettes result in an RGBA image
+    green = Image.new("L", (1, 1))
+    green.putpalette([], "RGBA")
+    red.save(test_file, save_all=True, append_images=[green])
+    with Image.open(test_file) as reloaded:
+        assert reloaded.mode == "RGBA"
+
+
 def test_apng_chunk_errors() -> None:
     with Image.open("Tests/images/apng/chunk_no_actl.png") as im:
         assert isinstance(im, PngImagePlugin.PngImageFile)

src/PIL/PngImagePlugin.py

@@ -1332,25 +1332,44 @@ def _save(
         )
         modes = set()
         sizes = set()
+        palette = None
+        palette_frame_with_alpha = None
+        different_palette = None
         append_images = im.encoderinfo.get("append_images", [])
         for im_seq in itertools.chain([im], append_images):
             for im_frame in ImageSequence.Iterator(im_seq):
                 modes.add(im_frame.mode)
                 sizes.add(im_frame.size)
-        for mode in ("RGBA", "RGB", "P"):
-            if mode in modes:
-                break
+                if im_frame.mode == "P":
+                    if im_frame.palette and im_frame.palette.mode == "RGBA":
+                        palette_frame_with_alpha = im_frame
+                    if different_palette is None:
+                        frame_palette = im_frame.getpalette() or []
+                        if palette is not None:
+                            if frame_palette != palette:
+                                different_palette = True
+                        else:
+                            palette = frame_palette
+                elif im_frame.mode in ("RGBA", "RGBA"):
+                    different_palette = False
+        if different_palette:
+            # APNG can only contain a single PLTE, so use another mode.
+            mode = "RGBA" if palette_frame_with_alpha else "RGB"
         else:
-            mode = modes.pop()
+            for mode in ("RGBA", "RGB", "P"):
+                if mode in modes:
+                    break
+            else:
+                mode = modes.pop()
         size = tuple(max(frame_size[i] for frame_size in sizes) for i in range(2))
     else:
         size = im.size
         mode = im.mode
+        if mode == "P" and im.palette:
+            palette = im.getpalette() or []
+            palette_frame_with_alpha = im if im.palette.mode == "RGBA" else None
 
     outmode = mode
-    palette = []
-    if im.palette:
-        palette = im.getpalette() or []
     if mode == "P":
         #
         # attempt to minimize storage requirements for palette images
@@ -1359,7 +1378,7 @@ def _save(
             colors = min(1 << im.encoderinfo["bits"], 256)
         else:
             # check palette contents
-            if im.palette:
+            if palette:
                 colors = max(min(len(palette) // 3, 256), 1)
             else:
                 colors = 256
@@ -1429,7 +1448,7 @@ def _save(
                 if not after_idat:
                     chunk(fp, cid, data)
 
-    if im.mode == "P":
+    if mode == "P" and palette is not None:
         palette_byte_number = colors * 3
         palette_bytes = bytes(palette[:palette_byte_number])
         while len(palette_bytes) < palette_byte_number:
@@ -1439,7 +1458,7 @@ def _save(
     transparency = im.encoderinfo.get("transparency", im.info.get("transparency"))
 
     if transparency is not None:
-        if im.mode == "P":
+        if mode == "P":
             # limit to actual palette size
             alpha_bytes = colors
             if isinstance(transparency, bytes):
@@ -1473,8 +1492,8 @@ def _save(
             # and it's in the info dict. It's probably just stale.
             msg = "cannot use transparency for this mode"
             raise OSError(msg)
-    elif im.mode == "P" and im.im.getpalettemode() == "RGBA":
-        alpha = im.im.getpalette("RGBA", "A")
+    elif mode == "P" and palette_frame_with_alpha:
+        alpha = palette_frame_with_alpha.im.getpalette("RGBA", "A")
         alpha_bytes = colors
         chunk(fp, b"tRNS", alpha[:alpha_bytes])