fix: set x-forms on resampled images

Resolves: #175.

Author: oesteban

nitransforms/base.py

@@ -11,6 +11,7 @@
 import numpy as np
 import h5py
 import warnings
+from collections import namedtuple
 from nibabel.loadsave import load as _nbload
 from nibabel import funcs as _nbfuncs
 from nibabel.nifti1 import intent_codes as INTENT_CODES
@@ -92,7 +93,7 @@ def shape(self):
 class ImageGrid(SampledSpatialData):
     """Class to represent spaces of gridded data (images)."""
 
-    __slots__ = ["_affine", "_inverse", "_ndindex"]
+    __slots__ = ["_affine", "_inverse", "_ndindex", "_header"]
 
     def __init__(self, image):
         """Create a gridded sampling reference."""
@@ -101,6 +102,7 @@ def __init__(self, image):
 
         self._affine = image.affine
         self._shape = image.shape
+        self._header = getattr(image, "header", None)
 
         self._ndim = getattr(image, "ndim", len(image.shape))
         if self._ndim >= 4:
@@ -117,6 +119,11 @@ def affine(self):
         """Access the indexes-to-RAS affine."""
         return self._affine
 
+    @property
+    def header(self):
+        """Access the original reference's header."""
+        return self._header
+
     @property
     def inverse(self):
         """Access the RAS-to-indexes affine."""
@@ -293,12 +300,15 @@ def apply(
         )
 
         if isinstance(_ref, ImageGrid):  # If reference is grid, reshape
+            hdr = None
+            if _ref.header is not None:
+                hdr = _ref.header.copy()
+                hdr.set_data_dtype(output_dtype)
             moved = spatialimage.__class__(
                 resampled.reshape(_ref.shape).astype(output_dtype),
                 _ref.affine,
-                spatialimage.header
+                hdr,
             )
-            moved.set_data_dtype(output_dtype)
             return moved
 
         return resampled