writeFenicsXML.py

# ***************************************************************************
# *                                                                         *
# *   Copyright (c) 2017 - Johannes Hartung <j.hartung@gmx.net>             *
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from __future__ import print_function

__title__ = "FreeCAD Fenics XML mesh writer"
__author__ = "Johannes Hartung"
__url__ = "http://www.freecadweb.org"

## @package exportFenicsXML
#  \ingroup FEM
#  \brief FreeCAD Fenics Mesh XML writer for FEM workbench


from importToolsFem import get_FemMeshObjectDimension, get_FemMeshObjectElementTypes, get_MaxDimElementFromList
from xml.etree import ElementTree as ET  # parsing xml files and exporting


def write_fenics_mesh_xml(fem_mesh_obj, outputfile):
    """
        For the export, we only have to use the highest dimensional entities and their
        vertices to be exported. (For second order elements, we have to delete the mid element nodes.)
    """

    # TODO: check for second order elements (what to do? deny export or reduce element order?)

    FreeCAD_to_Fenics_dict = {
        "Triangle": "triangle",
        "Tetra": "tetrahedron",
        "Hexa": "hexahedron",
        "Edge": "interval",
        "Node": "point",
        "Quadrangle": "quadrilateral",

        "Polygon": "unknown", "Polyhedron": "unknown",
        "Prism": "unknown", "Pyramid": "unknown",
    }

    print("Converting " + fem_mesh_obj.Label + " to fenics XML File")
    print("Dimension of mesh: %d" % (get_FemMeshObjectDimension(fem_mesh_obj),))

    elements_in_mesh = get_FemMeshObjectElementTypes(fem_mesh_obj)
    print("Elements appearing in mesh: %s" % (str(elements_in_mesh),))
    celltype_in_mesh = get_MaxDimElementFromList(elements_in_mesh)
    (num_cells, cellname_fc, dim_cell) = celltype_in_mesh
    cellname_fenics = FreeCAD_to_Fenics_dict[cellname_fc]
    print("Celltype in mesh -> %s and its Fenics name: %s" % (str(celltype_in_mesh), cellname_fenics))

    root = ET.Element("dolfin", dolfin="http://fenicsproject.org")
    meshchild = ET.SubElement(root, "mesh", celltype=cellname_fenics, dim=str(dim_cell))
    vertices = ET.SubElement(meshchild, "vertices", size=str(fem_mesh_obj.FemMesh.NodeCount))

    for (nodeind, fc_vec) in fem_mesh_obj.FemMesh.Nodes.iteritems():  # python2
        ET.SubElement(
            vertices, "vertex", index=str(nodeind - 1),
            # FC starts from 1, fenics starts from 0 to size-1
            x=str(fc_vec[0]), y=str(fc_vec[1]), z=str(fc_vec[2]))

    cells = ET.SubElement(meshchild, "cells", size=str(num_cells))
    if dim_cell == 3:
        fc_cells = fem_mesh_obj.FemMesh.Volumes
    elif dim_cell == 2:
        fc_cells = fem_mesh_obj.FemMesh.Faces
    elif dim_cell == 1:
        fc_cells = fem_mesh_obj.FemMesh.Edges
    else:
        fc_cells = ()

    for (fen_ind, fc_volume_ind) in enumerate(fc_cells):
        # FC starts after all other entities, fenics start from 0 to size-1
        nodeindices = fem_mesh_obj.FemMesh.getElementNodes(fc_volume_ind)

        cell_args = {}
        for (vi, ni) in enumerate(nodeindices):
            cell_args["v" + str(vi)] = str(ni - 1)
        # generate as many v entries in dict as nodes are listed in cell (works only for first order elements)

        ET.SubElement(cells, cellname_fenics, index=str(fen_ind), **cell_args)

    ET.SubElement(meshchild, "data")

    fp = open(outputfile, "w")  
    fp.write(ET.tostring(root))
    # xml core functionality does not support pretty printing
    # so the output file looks quite ugly
    fp.close()