Avoid eager C-order copy in NibabelReader (fixes #8107)

monai/data/image_reader.py

@@ -1217,7 +1217,7 @@ def _get_array_data(self, img, filename):
             data_offset = img.dataobj.offset
             data_dtype = img.dataobj.dtype
             return image[data_offset:].view(data_dtype).reshape(data_shape, order="F")
-        return np.asanyarray(img.dataobj, order="C")
+        return np.asanyarray(img.dataobj)
 
 
 class NumpyReader(ImageReader):

tests/data/test_init_reader.py

@@ -11,8 +11,12 @@
 
 from __future__ import annotations
 
+import os
+import tempfile
 import unittest
 
+import numpy as np
+
 from monai.data import ITKReader, NibabelReader, NrrdReader, NumpyReader, PILReader, PydicomReader
 from monai.transforms import LoadImage, LoadImaged
 from tests.test_utils import SkipIfNoModule
@@ -76,6 +80,26 @@ def test_readers_to_gpu(self):
             inst = NibabelReader(to_gpu=to_gpu)
             self.assertIsInstance(inst, NibabelReader)
 
+    @SkipIfNoModule("nibabel")
+    def test_nibabel_reader_avoids_eager_c_order_copy(self):
+        import nibabel as nib
+
+        test_image = np.arange(2 * 3 * 4, dtype=np.int16).reshape(2, 3, 4)
+        with tempfile.TemporaryDirectory() as tempdir:
+            for suffix in (".nii", ".nii.gz"):
+                with self.subTest(suffix=suffix):
+                    filename = os.path.join(tempdir, f"test_image{suffix}")
+                    nib.save(nib.Nifti1Image(test_image, np.eye(4)), filename)
+
+                    reader = NibabelReader(mmap=False)
+                    img = reader.read(filename)
+                    data, _ = reader.get_data(img)
+
+                    np.testing.assert_array_equal(data, test_image)
+                    # The reader must not force an eager C-order copy; the native
+                    # (F-order) layout from nibabel should be preserved here.
+                    self.assertFalse(data.flags.c_contiguous)
+
 
 if __name__ == "__main__":
     unittest.main()