Save multiple images at different bit depths if provided

Tests/test_file_ico.py

@@ -85,6 +85,39 @@ def test_no_duplicates(tmp_path):
     assert os.path.getsize(temp_file) == os.path.getsize(temp_file2)
 
 
+def test_different_bit_depths(tmp_path):
+    temp_file = str(tmp_path / "temp.ico")
+    temp_file2 = str(tmp_path / "temp2.ico")
+
+    im = hopper()
+    im.save(temp_file, "ico", bitmap_format="bmp", sizes=[(128, 128)])
+
+    hopper("1").save(
+        temp_file2,
+        "ico",
+        bitmap_format="bmp",
+        sizes=[(128, 128)],
+        append_images=[im],
+    )
+
+    assert os.path.getsize(temp_file) != os.path.getsize(temp_file2)
+
+    # Test that only matching sizes of different bit depths are saved
+    temp_file3 = str(tmp_path / "temp3.ico")
+    temp_file4 = str(tmp_path / "temp4.ico")
+
+    im.save(temp_file3, "ico", bitmap_format="bmp", sizes=[(128, 128)])
+    im.save(
+        temp_file4,
+        "ico",
+        bitmap_format="bmp",
+        sizes=[(128, 128)],
+        append_images=[Image.new("P", (64, 64))],
+    )
+
+    assert os.path.getsize(temp_file3) == os.path.getsize(temp_file4)
+
+
 @pytest.mark.parametrize("mode", ("1", "L", "P", "RGB", "RGBA"))
 def test_save_to_bytes_bmp(mode):
     output = io.BytesIO()

src/PIL/IcoImagePlugin.py

@@ -41,52 +41,65 @@ _MAGIC = b"\0\0\1\0"
 
 def _save(im, fp, filename):
     fp.write(_MAGIC)  # (2+2)
+    bmp = im.encoderinfo.get("bitmap_format") == "bmp"
     sizes = im.encoderinfo.get(
         "sizes",
         [(16, 16), (24, 24), (32, 32), (48, 48), (64, 64), (128, 128), (256, 256)],
     )
+    frames = []
+    provided_ims = [im] + im.encoderinfo.get("append_images", [])
     width, height = im.size
-    sizes = filter(
-        lambda x: False
-        if (x[0] > width or x[1] > height or x[0] > 256 or x[1] > 256)
-        else True,
-        sizes,
-    )
-    sizes = set(sizes)
-    fp.write(o16(len(sizes)))  # idCount(2)
-    offset = fp.tell() + len(sizes) * 16
-    bmp = im.encoderinfo.get("bitmap_format") == "bmp"
-    provided_images = {
-        im.size: im for im in [im] + im.encoderinfo.get("append_images", [])
-    }
-    for size in sizes:
-        width, height = size
+    for size in sorted(set(sizes)):
+        if size[0] > width or size[1] > height or size[0] > 256 or size[1] > 256:
+            continue
+
+        for provided_im in provided_ims:
+            if provided_im.size != size:
+                continue
+            frames.append(provided_im)
+            if bmp:
+                bits = BmpImagePlugin.SAVE[provided_im.mode][1]
+                bits_used = [bits]
+                for other_im in provided_ims:
+                    if other_im.size != size:
+                        continue
+                    bits = BmpImagePlugin.SAVE[other_im.mode][1]
+                    if bits not in bits_used:
+                        # Another image has been supplied for this size
+                        # with a different bit depth
+                        frames.append(other_im)
+                        bits_used.append(bits)
+            break
+        else:
+            # TODO: invent a more convenient method for proportional scalings
+            frame = provided_im.copy()
+            frame.thumbnail(size, Image.Resampling.LANCZOS, reducing_gap=None)
+            frames.append(frame)
+    fp.write(o16(len(frames)))  # idCount(2)
+    offset = fp.tell() + len(frames) * 16
+    for frame in frames:
+        width, height = frame.size
         # 0 means 256
         fp.write(o8(width if width < 256 else 0))  # bWidth(1)
         fp.write(o8(height if height < 256 else 0))  # bHeight(1)
-        fp.write(b"\0")  # bColorCount(1)
+
+        bits, colors = BmpImagePlugin.SAVE[frame.mode][1:] if bmp else (32, 0)
+        fp.write(o8(colors))  # bColorCount(1)
         fp.write(b"\0")  # bReserved(1)
         fp.write(b"\0\0")  # wPlanes(2)
-
-        tmp = provided_images.get(size)
-        if not tmp:
-            # TODO: invent a more convenient method for proportional scalings
-            tmp = im.copy()
-            tmp.thumbnail(size, Image.Resampling.LANCZOS, reducing_gap=None)
-        bits = BmpImagePlugin.SAVE[tmp.mode][1] if bmp else 32
         fp.write(o16(bits))  # wBitCount(2)
 
         image_io = BytesIO()
         if bmp:
-            tmp.save(image_io, "dib")
+            frame.save(image_io, "dib")
 
             if bits != 32:
-                and_mask = Image.new("1", tmp.size)
+                and_mask = Image.new("1", size)
                 ImageFile._save(
-                    and_mask, image_io, [("raw", (0, 0) + tmp.size, 0, ("1", 0, -1))]
+                    and_mask, image_io, [("raw", (0, 0) + size, 0, ("1", 0, -1))]
                 )
         else:
-            tmp.save(image_io, "png")
+            frame.save(image_io, "png")
         image_io.seek(0)
         image_bytes = image_io.read()
         if bmp: