Support writing of hex elements to VTKHDF format.

docs/source/usersguide/processing.rst

@@ -97,7 +97,15 @@ VTK Mesh File Generation
 ------------------------
 
 VTK files of OpenMC meshes can be created using the
-:meth:`openmc.Mesh.write_data_to_vtk`  method. Data can be applied to the
+:meth:`openmc.Mesh.write_data_to_vtk` method. This method supports several VTK
+formats depending on the mesh type. Structured meshes
+(:class:`~openmc.RegularMesh`, :class:`~openmc.RectilinearMesh`,
+:class:`~openmc.CylindricalMesh`, and :class:`~openmc.SphericalMesh`) can be
+exported to legacy VTK format (``.vtk``). The :class:`~openmc.UnstructuredMesh`
+class supports VTK unstructured grid formats (``.vtu``) as well as an HDF5-based
+format (``.vtkhdf``) that does not require the ``vtk`` module to write.
+
+Data can be applied to the
 elements of the resulting mesh from mesh filter objects. This data can be
 provided either as a flat array or, in the case of structured meshes
 (:class:`~openmc.RegularMesh`, :class:`~openmc.RectilinearMesh`,

openmc/mesh.py

@@ -2737,7 +2737,8 @@ class UnstructuredMesh(MeshBase):
     _UNSUPPORTED_ELEM = -1
     _LINEAR_TET = 0
     _LINEAR_HEX = 1
-    _VTK_TETRA = 10
+    _VTK_TET = 10
+    _VTK_HEX = 12
 
     def __init__(self, filename: PathLike, library: str, mesh_id: int | None = None,
                  name: str = '', length_multiplier: float = 1.0,
@@ -3048,7 +3049,9 @@ def _write_data_to_vtk_ascii_format(
                 n_skipped += 1
                 continue
             else:
-                raise RuntimeError(f"Invalid element type {elem_type} found")
+                raise RuntimeError(
+                    f"Invalid element type {elem_type} found in mesh {self.id}"
+                )
 
             for i, c in enumerate(conn):
                 if c == -1:
@@ -3105,35 +3108,42 @@ def _write_data_to_vtk_hdf5_format(
         datasets: dict | None = None,
         volume_normalization: bool = True,
     ):
-        def append_dataset(dset, array):
-            """Convenience function to append data to an HDF5 dataset"""
-            origLen = dset.shape[0]
-            dset.resize(origLen + array.shape[0], axis=0)
-            dset[origLen:] = array
-
-        if self.library != "moab":
-            raise NotImplementedError("VTKHDF output is only supported for MOAB meshes")
-
-        # the self.connectivity contains arrays of length 8 to support hex
-        # elements as well, in the case of tetrahedra mesh elements, the
-        # last 4 values are -1 and are removed
-        trimmed_connectivity = []
-        for cell in self.connectivity:
-            # Find the index of the first -1 value, if any
-            first_negative_index = np.where(cell == -1)[0]
-            if first_negative_index.size > 0:
-                # Slice the array up to the first -1 value
-                trimmed_connectivity.append(cell[: first_negative_index[0]])
+        # This writer supports linear tetrahedra and linear hexahedra elements
+        conn_list = []           # flattened connectivity ids
+        cell_sizes = []          # number of points per cell
+        vtk_types = []           # VTK cell types per cell (uint8)
+        n_skipped = 0
+
+        for conn, etype in zip(self.connectivity, self.element_types):
+            if etype == self._LINEAR_TET:
+                ids = conn[:4]
+                vtk_types.append(self._VTK_TET)
+            elif etype == self._LINEAR_HEX:
+                ids = conn[:8]
+                vtk_types.append(self._VTK_HEX)
+            elif etype == self._UNSUPPORTED_ELEM:
+                n_skipped += 1
+                continue
             else:
-                # No -1 values, append the whole cell
-                trimmed_connectivity.append(cell)
-        trimmed_connectivity = np.array(trimmed_connectivity, dtype="int32").flatten()
+                raise RuntimeError(
+                    f"Invalid element type {etype} found in mesh {self.id}"
+                )
+            conn_list.extend(ids.tolist())
+            cell_sizes.append(len(ids))
 
-        # MOAB meshes supports tet elements only so we know it has 4 points per cell
-        points_per_cell = 4
+        if n_skipped > 0:
+            warnings.warn(
+                f"{n_skipped} elements were not written because "
+                "they are not of type linear tet/hex"
+            )
 
-        # offsets are the indices of the first point of each cell in the array of points
-        offsets = np.arange(0, self.n_elements * points_per_cell + 1, points_per_cell)
+        connectivity = np.asarray(conn_list, dtype=np.int64)
+
+        # Offsets must be length (numCells + 1) with a leading 0 and
+        # cumulative end-indices thereafter, per VTK's layout
+        cell_sizes_arr = np.asarray(cell_sizes, dtype=np.int64)
+        offsets = np.zeros(cell_sizes_arr.size + 1, dtype=np.int64)
+        np.cumsum(cell_sizes_arr, out=offsets[1:])
 
         for name, data in datasets.items():
             if data.shape != self.dimension:
@@ -3155,42 +3165,27 @@ def append_dataset(dset, array):
                 dtype=h5py.string_dtype("ascii", len(ascii_type)),
             )
 
-            # create hdf5 file structure
-            root.create_dataset("NumberOfPoints", (0,), maxshape=(None,), dtype="i8")
-            root.create_dataset("Types", (0,), maxshape=(None,), dtype="uint8")
-            root.create_dataset("Points", (0, 3), maxshape=(None, 3), dtype="f")
-            root.create_dataset(
-                "NumberOfConnectivityIds", (0,), maxshape=(None,), dtype="i8"
-            )
-            root.create_dataset("NumberOfCells", (0,), maxshape=(None,), dtype="i8")
-            root.create_dataset("Offsets", (0,), maxshape=(None,), dtype="i8")
-            root.create_dataset("Connectivity", (0,), maxshape=(None,), dtype="i8")
-
-            append_dataset(root["NumberOfPoints"], np.array([len(self.vertices)]))
-            append_dataset(root["Points"], self.vertices)
-            append_dataset(
-                root["NumberOfConnectivityIds"],
-                np.array([len(trimmed_connectivity)]),
-            )
-            append_dataset(root["Connectivity"], trimmed_connectivity)
-            append_dataset(root["NumberOfCells"], np.array([self.n_elements]))
-            append_dataset(root["Offsets"], offsets)
+            # Create HDF5 file structure compliant with VTKHDF UnstructuredGrid
+            n_points = int(len(self.vertices))
+            n_cells = int(len(cell_sizes))
+            n_conn_ids = int(len(connectivity))
 
-            append_dataset(
-                root["Types"], np.full(self.n_elements, self._VTK_TETRA, dtype="uint8")
-            )
+            root.create_dataset("NumberOfPoints", data=(n_points,), dtype="i8")
+            root.create_dataset("NumberOfCells", data=(n_cells,), dtype="i8")
+            root.create_dataset("NumberOfConnectivityIds", data=(n_conn_ids,), dtype="i8")
+            root.create_dataset("Points", data=self.vertices.astype(np.float64, copy=False), dtype="f8")
+            root.create_dataset("Types", data=np.asarray(vtk_types, dtype=np.uint8), dtype="uint8")
+            root.create_dataset("Offsets", data=offsets.astype("i8"), dtype="i8")
+            root.create_dataset("Connectivity", data=connectivity.astype("i8"), dtype="i8")
 
             cell_data_group = root.create_group("CellData")
 
             for name, data in datasets.items():
-
-                cell_data_group.create_dataset(
-                    name, (0,), maxshape=(None,), dtype="float64", chunks=True
-                )
-
                 if volume_normalization:
                     data /= self.volumes
-                append_dataset(cell_data_group[name], data)
+                cell_data_group.create_dataset(
+                    name, data=data, dtype="float64", chunks=True
+                )
 
     @classmethod
     def from_hdf5(cls, group: h5py.Group, mesh_id: int, name: str):

tests/unit_tests/test_mesh.py

@@ -490,22 +490,38 @@ def test_umesh(run_in_tmpdir, simple_umesh, export_type):
         simple_umesh.write_data_to_vtk(datasets={'mean': ref_data[:-2]}, filename=filename)
 
 
+vtkhdf_tests = [
+    (
+        Path("test_mesh_dagmc_tets.vtk"),
+        "moab"
+    ),
+    (
+        Path("test_mesh_hexes.exo"),
+        "libmesh"
+    )
+]
 @pytest.mark.skipif(not openmc.lib._dagmc_enabled(), reason="DAGMC not enabled.")
-def test_write_vtkhdf(request, run_in_tmpdir):
+@pytest.mark.parametrize('mesh_file, mesh_library', vtkhdf_tests)
+def test_write_vtkhdf(mesh_file, mesh_library, request, run_in_tmpdir):
     """Performs a minimal UnstructuredMesh simulation, reads in the resulting
     statepoint file and writes the mesh data to vtk and vtkhdf files. It is
     necessary to read in the unstructured mesh from a statepoint file to ensure
     it has all the required attributes
     """
+    if mesh_library == 'moab' and not openmc.lib._dagmc_enabled():
+        pytest.skip("DAGMC not enabled.")
+    if mesh_library == 'libmesh' and not openmc.lib._libmesh_enabled():
+        pytest.skip("LibMesh not enabled.")
+
     model = openmc.Model()
 
     surf1 = openmc.Sphere(r=1000.0, boundary_type="vacuum")
     cell1 = openmc.Cell(region=-surf1)
     model.geometry = openmc.Geometry([cell1])
 
     umesh = openmc.UnstructuredMesh(
-        request.path.parent / "test_mesh_dagmc_tets.vtk",
-        "moab",
+        request.path.parent / mesh_file,
+        mesh_library,
         mesh_id = 1
     )
     mesh_filter = openmc.MeshFilter(umesh)
@@ -514,6 +530,8 @@ def test_write_vtkhdf(request, run_in_tmpdir):
     mesh_tally = openmc.Tally(name="test_tally")
     mesh_tally.filters = [mesh_filter]
     mesh_tally.scores = ["flux"]
+    if mesh_library == "libmesh":
+        mesh_tally.estimator = "collision"
 
     model.tallies = [mesh_tally]
 
@@ -556,6 +574,26 @@ def test_write_vtkhdf(request, run_in_tmpdir):
     with h5py.File("test_mesh.vtkhdf", "r"):
         ...
 
+    import vtk
+    reader = vtk.vtkHDFReader()
+    reader.SetFileName("test_mesh.vtkhdf")
+    reader.Update()
+
+    # Get mean from file and make sure it matches original data
+    num_elements = reader.GetOutput().GetNumberOfCells()
+    assert num_elements == umesh_from_sp.n_elements
+
+    num_vertices = reader.GetOutput().GetNumberOfPoints()
+    assert num_vertices == umesh_from_sp.vertices.shape[0]
+
+    arr = reader.GetOutput().GetCellData().GetArray("mean")
+    mean = np.array([arr.GetTuple1(i) for i in range(my_tally.mean.size)])
+    np.testing.assert_almost_equal(mean, my_tally.mean.flatten()/umesh_from_sp.volumes)
+
+    arr = reader.GetOutput().GetCellData().GetArray("std_dev")
+    std_dev = np.array([arr.GetTuple1(i) for i in range(my_tally.std_dev.size)])
+    np.testing.assert_almost_equal(std_dev, my_tally.std_dev.flatten()/umesh_from_sp.volumes)
+
 
 def test_mesh_get_homogenized_materials():
     """Test the get_homogenized_materials method"""