Use info disposal and blend when calculating APNG frame deltas

Tests/test_file_apng.py

@@ -686,6 +686,38 @@ def test_apng_save_disposal_previous(tmp_path: Path) -> None:
         assert im.getpixel((64, 32)) == (0, 255, 0, 255)
 
 
+@pytest.mark.parametrize("mode", ("L", "RGB"))
+def test_apng_save_disposal_background_mode(tmp_path: Path, mode: str) -> None:
+    test_file = tmp_path / "temp.png"
+    im = Image.new(mode, (32, 32), 128)
+
+    # the black frame is different from the transparent background
+    im.save(
+        test_file,
+        save_all=True,
+        append_images=[Image.new(mode, (32, 32))],
+        disposal=PngImagePlugin.Disposal.OP_BACKGROUND,
+        duration=100,
+    )
+    with Image.open(test_file) as reloaded:
+        assert isinstance(reloaded, PngImagePlugin.PngImageFile)
+        assert reloaded.n_frames == 2
+
+
+def test_apng_save_info_disposal(tmp_path: Path) -> None:
+    test_file = tmp_path / "temp.png"
+    red = Image.new("RGBA", (32, 32), (255, 0, 0, 255))
+    red_green = red.copy()
+    red_green.paste((0, 255, 0, 255), (0, 0, 10, 10))
+
+    red.info["disposal"] = PngImagePlugin.Disposal.OP_PREVIOUS
+    red.save(test_file, save_all=True, append_images=[red_green])
+    with Image.open(test_file) as im:
+        im.seek(1)
+        assert im.getpixel((0, 0)) == (0, 255, 0, 255)
+        assert im.getpixel((20, 20)) == (255, 0, 0, 255)
+
+
 def test_apng_save_blend(tmp_path: Path) -> None:
     test_file = tmp_path / "temp.png"
     size = (128, 64)

src/PIL/PngImagePlugin.py

@@ -1225,21 +1225,25 @@ def _write_multiple_frames(
                 encoderinfo["duration"] = duration[frame_count]
             elif duration is None and "duration" in im_frame.info:
                 encoderinfo["duration"] = im_frame.info["duration"]
-            if isinstance(disposal, (list, tuple)):
-                encoderinfo["disposal"] = disposal[frame_count]
-            if isinstance(blend, (list, tuple)):
-                encoderinfo["blend"] = blend[frame_count]
+            encoderinfo["disposal"] = (
+                disposal[frame_count]
+                if isinstance(disposal, (list, tuple))
+                else disposal
+            )
+            encoderinfo["blend"] = (
+                blend[frame_count] if isinstance(blend, (list, tuple)) else blend
+            )
             frame_count += 1
 
             if im_frames:
                 previous = im_frames[-1]
-                prev_disposal = previous.encoderinfo.get("disposal")
-                prev_blend = previous.encoderinfo.get("blend")
+                prev_disposal = previous.encoderinfo["disposal"]
+                prev_blend = previous.encoderinfo["blend"]
                 if prev_disposal == Disposal.OP_PREVIOUS and len(im_frames) < 2:
                     prev_disposal = Disposal.OP_BACKGROUND
 
                 if prev_disposal == Disposal.OP_BACKGROUND:
-                    base_im = previous.im.copy()
+                    base_im = previous.im.convert("RGBA")
                     dispose = Image.core.fill("RGBA", im.size, (0, 0, 0, 0))
                     bbox = previous.bbox
                     if bbox:
@@ -1257,8 +1261,8 @@ def _write_multiple_frames(
                 bbox = delta.getbbox(alpha_only=False)
                 if (
                     not bbox
-                    and prev_disposal == encoderinfo.get("disposal")
-                    and prev_blend == encoderinfo.get("blend")
+                    and prev_disposal == encoderinfo["disposal"]
+                    and prev_blend == encoderinfo["blend"]
                     and "duration" in encoderinfo
                 ):
                     previous.encoderinfo["duration"] += encoderinfo["duration"]
@@ -1303,8 +1307,8 @@ def _write_multiple_frames(
         if delay.numerator > 65535:
             msg = "cannot write duration"
             raise ValueError(msg)
-        frame_disposal = encoderinfo.get("disposal", disposal)
-        frame_blend = encoderinfo.get("blend", blend)
+        frame_disposal = encoderinfo["disposal"]
+        frame_blend = encoderinfo["blend"]
         # frame control
         chunk(
             fp,