enh: add one test comparing to TOPUP's interpolator

Author: oesteban

sdcflows/interfaces/bspline.py

@@ -130,12 +130,11 @@ def _run_interface(self, runtime):
             bs_levels.append(level)
             ncoeff.append(level.dataobj.size)
 
-            if regressors is None:
-                regressors = bspline_weights(fmap_points, level)
-            else:
-                regressors = sparse_vstack(
-                    (regressors, bspline_weights(fmap_points, level))
-                )
+            regressors = (
+                bspline_weights(fmap_points, level)
+                if regressors is None
+                else sparse_vstack((regressors, bspline_weights(fmap_points, level)))
+            )
 
         # Fit the model
         model = lm.Ridge(alpha=self.inputs.ridge_alpha, fit_intercept=False)
@@ -563,10 +562,11 @@ def bspline_weights(points, ctrl_nii, blocksize=None, mem_percent=None):
 
         weights[weights < 1e-6] = 0.0
 
-        if wmatrix is None:
-            wmatrix = csc_matrix(weights)
-        else:
-            wmatrix = hstack((wmatrix, csc_matrix(weights)))
+        wmatrix = (
+            csc_matrix(weights)
+            if wmatrix is None
+            else hstack((wmatrix, csc_matrix(weights)))
+        )
         idx = end
     return wmatrix.tocsr()
 

sdcflows/interfaces/tests/test_bspline.py

@@ -84,3 +84,27 @@ def test_topup_coeffs(tmpdir, testdata_dir):
     # Test automatic output file name generation, just for coverage
     with pytest.raises(ValueError):
         _fix_topup_fieldcoeff("failing.nii.gz", str(testdata_dir / "epi.nii.gz"))
+
+
+@pytest.mark.skipif(os.getenv("GITHUB_ACTIONS") == "true", reason="this is GH Actions")
+def test_topup_coeffs_interpolation(tmpdir, testdata_dir):
+    """Check that our interpolation is not far away from TOPUP's."""
+    tmpdir.chdir()
+    result = Coefficients2Warp(
+        in_target=str(testdata_dir / "epi.nii.gz"),
+        in_coeff=str(testdata_dir / "topup-coeff-fixed.nii.gz"),
+        pe_dir="j-",
+        ro_time=1.0,
+    ).run()
+    assert (
+        np.sqrt(
+            np.mean(
+                (
+                    nb.load(result.outputs.out_field).get_fdata()
+                    - nb.load(testdata_dir / "topup-field.nii.gz").get_fdata()
+                )
+                ** 2
+            )
+        )
+        < 3
+    )