test: add xfail for 3d transform on 4d data

Author: oesteban

nitransforms/tests/test_resampling.py

@@ -330,6 +330,30 @@ def test_apply_transformchain(tmp_path, testdata_path):
     assert (np.abs(diff) > 1e-3).sum() / diff.size < RMSE_TOL_LINEAR
 
 
+@pytest.mark.xfail(reason="gh-281: applying a single 3D transform to 4D data")
+def test_apply_single_3d_on_4d():
+    """Apply one 3D transform across all timepoints of a 4D dataset."""
+    nvols = 5
+    data = np.zeros((10, 5, 5, nvols), dtype=np.float32)
+    for i in range(nvols):
+        data[i + 1, 2, 2, i] = i + 1
+
+    img = nb.Nifti1Image(data, np.eye(4))
+
+    mat = np.eye(4)
+    mat[0, 3] = -1.0
+    ref = nb.Nifti1Image(np.zeros((10, 5, 5), dtype=np.uint8), np.eye(4))
+    xfm = nitl.Affine(mat, reference=ref)
+
+    moved = apply(xfm, img, order=0)
+    moved_data = np.asanyarray(moved.dataobj)
+
+    assert moved_data.shape == data.shape
+    for i in range(nvols):
+        assert moved_data[i + 2, 2, 2, i] == i + 1
+        assert moved_data[i + 1, 2, 2, i] == 0
+
+
 @pytest.mark.parametrize("serialize_4d", [True, False])
 def test_LinearTransformsMapping_apply(
     tmp_path, data_path, testdata_path, serialize_4d