fix: pass tests, serialization implemented

nitransforms/resampling.py

@@ -102,15 +102,7 @@ def apply(
         xfm_nvols = len(transform)
     else:
         xfm_nvols = transform.ndim
-    """
-    if data_nvols == 1 and xfm_nvols > 1:
-        data = data[..., np.newaxis]
-    elif data_nvols != xfm_nvols:
-        raise ValueError(
-            "The fourth dimension of the data does not match the transform's shape."
-        )
-    RESAMPLING FAILS. SUGGEST:
-    """
+
     if data.ndim < transform.ndim:
         data = data[..., np.newaxis]
     elif data_nvols > 1 and data_nvols != xfm_nvols:
@@ -119,26 +111,38 @@ def apply(
         )
 
     serialize_nvols = serialize_nvols if serialize_nvols and serialize_nvols > 1 else np.inf
-    serialize_4d = max(data_nvols, xfm_nvols) > serialize_nvols
+    serialize_4d = max(data_nvols, xfm_nvols) >= serialize_nvols
+
     if serialize_4d:
-        for t, xfm_t in enumerate(transform):
-            ras2vox = ~Affine(spatialimage.affine)
-            input_dtype = get_obj_dtype(spatialimage.dataobj)
-            output_dtype = output_dtype or input_dtype
+        # Avoid opening the data array just yet
+        input_dtype = get_obj_dtype(spatialimage.dataobj)
+        output_dtype = output_dtype or input_dtype
+
+        # Prepare physical coordinates of input (grid, points)
+        xcoords = _ref.ndcoords.astype("f4").T
+
+        # Invert target's (moving) affine once
+        ras2vox = ~Affine(spatialimage.affine)
+        dataobj = (
+            np.asanyarray(spatialimage.dataobj, dtype=input_dtype)
+            if spatialimage.ndim in (2, 3)
+            else None
+        )
 
+        # Order F ensures individual volumes are contiguous in memory
+        # Also matches NIfTI, making final save more efficient
+        resampled = np.zeros(
+            (xcoords.shape[0], len(transform)), dtype=output_dtype, order="F"
+        )
+
+        for t, xfm_t in enumerate(transform):
             # Map the input coordinates on to timepoint t of the target (moving)
-            xcoords = _ref.ndcoords.astype("f4").T
             ycoords = xfm_t.map(xcoords)[..., : _ref.ndim]
 
             # Calculate corresponding voxel coordinates
             yvoxels = ras2vox.map(ycoords)[..., : _ref.ndim]
 
             # Interpolate
-            dataobj = (
-                np.asanyarray(spatialimage.dataobj, dtype=input_dtype)
-                if spatialimage.ndim in (2, 3)
-                else None
-            )
             resampled[..., t] = ndi.map_coordinates(
                 (
                     dataobj