Deprecate saving I mode images as PNG (#9023)

Tests/test_file_png.py

@@ -100,11 +100,11 @@ class TestFilePng:
             assert im.format == "PNG"
             assert im.get_format_mimetype() == "image/png"
 
-        for mode in ["1", "L", "P", "RGB", "I", "I;16", "I;16B"]:
+        for mode in ["1", "L", "P", "RGB", "I;16", "I;16B"]:
             im = hopper(mode)
             im.save(test_file)
             with Image.open(test_file) as reloaded:
-                if mode in ("I", "I;16B"):
+                if mode == "I;16B":
                     reloaded = reloaded.convert(mode)
                 assert_image_equal(reloaded, im)
 
@@ -801,6 +801,16 @@ class TestFilePng:
         with Image.open("Tests/images/truncated_end_chunk.png") as im:
             assert_image_equal_tofile(im, "Tests/images/hopper.png")
 
+    def test_deprecation(self, tmp_path: Path) -> None:
+        test_file = tmp_path / "out.png"
+
+        im = hopper("I")
+        with pytest.warns(DeprecationWarning):
+            im.save(test_file)
+
+        with Image.open(test_file) as reloaded:
+            assert_image_equal(im, reloaded.convert("I"))
+
 
 @pytest.mark.skipif(is_win32(), reason="Requires Unix or macOS")
 @skip_unless_feature("zlib")

docs/deprecations.rst

@@ -193,6 +193,20 @@ Image.Image.get_child_images()
 method uses an image's file pointer, and so child images could only be retrieved from
 an :py:class:`PIL.ImageFile.ImageFile` instance.
 
+Saving I mode images as PNG
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. deprecated:: 11.3.0
+
+In order to fit the 32 bits of I mode images into PNG, when PNG images can only contain
+at most 16 bits for a channel, Pillow has been clipping the values. Rather than quietly
+changing the data, this is now deprecated. Instead, the image can be converted to
+another mode before saving::
+
+    from PIL import Image
+    im = Image.new("I", (1, 1))
+    im.convert("I;16").save("out.png")
+
 Removed features
 ----------------
 

docs/releasenotes/11.3.0.rst

@@ -23,10 +23,17 @@ TODO
 Deprecations
 ============
 
-TODO
+Saving I mode images as PNG
-^^^^
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-TODO
+In order to fit the 32 bits of I mode images into PNG, when PNG images can only contain
+at most 16 bits for a channel, Pillow has been clipping the values. Rather than quietly
+changing the data, this is now deprecated. Instead, the image can be converted to
+another mode before saving::
+
+    from PIL import Image
+    im = Image.new("I", (1, 1))
+    im.convert("I;16").save("out.png")
 
 API changes
 ===========

src/PIL/PngImagePlugin.py

@@ -48,6 +48,7 @@ from ._binary import i32be as i32
 from ._binary import o8
 from ._binary import o16be as o16
 from ._binary import o32be as o32
+from ._deprecate import deprecate
 from ._util import DeferredError
 
 TYPE_CHECKING = False
@@ -1368,6 +1369,8 @@ def _save(
     except KeyError as e:
         msg = f"cannot write mode {mode} as PNG"
         raise OSError(msg) from e
+    if outmode == "I":
+        deprecate("Saving I mode images as PNG", 13, stacklevel=4)
 
     #
     # write minimal PNG file

src/PIL/_deprecate.py

@@ -12,6 +12,7 @@ def deprecate(
     *,
     action: str | None = None,
     plural: bool = False,
+    stacklevel: int = 3,
 ) -> None:
     """
     Deprecations helper.
@@ -67,5 +68,5 @@ def deprecate(
     warnings.warn(
         f"{deprecated} {is_} deprecated and will be removed in {removed}{action}",
         DeprecationWarning,
-        stacklevel=3,
+        stacklevel=stacklevel,
     )