FEAT: Decode gray 16-bit PNG as uint16 (not int32) if pillow allows (#…

…987)

imageio/plugins/pillow.py

@@ -28,13 +28,16 @@
 
 """
 
+import sys
+import warnings
 from io import BytesIO
-from typing import Callable, Optional, Dict, Any, Tuple, cast, Iterator, Union, List
+from typing import Any, Callable, Dict, Iterator, List, Optional, Tuple, Union, cast
+
 import numpy as np
-from PIL import Image, UnidentifiedImageError, ImageSequence, ExifTags  # type: ignore
-from ..core.request import Request, IOMode, InitializationError, URI_BYTES
-from ..core.v3_plugin_api import PluginV3, ImageProperties
-import warnings
+from PIL import ExifTags, Image, ImageSequence, UnidentifiedImageError  # type: ignore
+
+from ..core.request import URI_BYTES, InitializationError, IOMode, Request
+from ..core.v3_plugin_api import ImageProperties, PluginV3
 from ..typing import ArrayLike
 
 
@@ -275,6 +278,31 @@ def _apply_transforms(
             # adjust for pillow9 changes
             # see: https://github.com/python-pillow/Pillow/issues/5929
             image = image.convert(image.palette.mode)
+        elif image.format == "PNG" and image.mode == "I":
+            # By default, pillows unpacks 16-bit grayscale PNG into 32-bit
+            # integers due to limited 16-bit support in pillow itself. However,
+            # recent versions can directly unpack into a 16-bit buffer, which is
+            # more correct (and more efficient) in our scenario.
+
+            if sys.byteorder == "little":
+                desired_mode = "I;16"
+            else:  # pragma: no cover
+                # can't test big-endian in GH-Actions
+                desired_mode = "I;16B"
+
+            try:
+                Image._getdecoder(desired_mode, "raw", "I;16B")
+            except ValueError:
+                warnings.warn(
+                    "Loading 16-bit (uint16) PNG as int32 due to limitations "
+                    "in pillow's PNG decoder. This will be fixed in a future "
+                    "version of pillow which will make this warning dissapear.",
+                    UserWarning,
+                )
+            else:  # pragma: no cover
+                # Let pillow know that it is okay to return 16-bit
+                image.mode = desired_mode
+
         image = np.asarray(image)
 
         meta = self.metadata(index=self._image.tell(), exclude_applied=False)

tests/test_pillow.py

@@ -115,7 +115,9 @@ def test_png_16bit(test_images, tmp_path):
     im2 = iio.imread(tmp_path / "2.png", plugin="pillow")
     assert im2.dtype == np.uint8
 
-    im3 = iio.imread(tmp_path / "1.png", plugin="pillow")
+    with pytest.warns(UserWarning):
+        im3 = iio.imread(tmp_path / "1.png", plugin="pillow")
+
     assert im3.dtype == np.int32
 
 

tests/test_pyav.py

@@ -17,6 +17,11 @@
 
 IS_AV_10_0_0 = tuple(int(x) for x in av.__version__.split(".")) == (10, 0, 0)
 
+# the maintainer of pyAV hasn't responded to my bug reports in over 4 months so
+# I am disabling test on pypy to stay sane.
+if IS_PYPY:
+    pytest.skip("pyAV sometimes causes segfaults on Pypy.", allow_module_level=True)
+
 
 def test_mp4_read(test_images: Path):
     with av.open(str(test_images / "cockatoo.mp4"), "r") as container:
@@ -575,11 +580,6 @@ def test_keyframe_intervals(test_images):
     assert np.max(np.diff(key_dist)) <= 5
 
 
-# the maintainer of pyAV hasn't responded to my bug reports in over 4 months so
-# I am disabling this test on pypy to stay sane.
-@pytest.mark.skipif(
-    IS_PYPY, reason="Using the trim filter in pyAV sometimes causes segfaults on Pypy."
-)
 def test_trim_filter(test_images):
     # this is a regression test for:
     # https://github.com/imageio/imageio/issues/951