OCC entities for arbitrary 3D geometry

While meshwell provides convenient polygon-based entities (PolySurface, PolyPrism), you can also wrap arbitrary OpenCASCADE shapes via OCC_entity. This gives you full access to the OCP geometric modeler while still benefiting from meshwell’s fragment/tagging/meshing workflow.

from functools import partial

from OCP.BRepPrimAPI import BRepPrimAPI_MakeBox
from OCP.gp import gp_Pnt

from meshwell.cad_occ import cad_occ
from meshwell.occ_entity import OCC_entity
from meshwell.occ_xao_writer import write_xao

/home/runner/work/meshwell/meshwell/.venv/lib/python3.13/site-packages/tqdm/auto.py:21: TqdmWarning: IProgress not found. Please update jupyter and ipywidgets. See https://ipywidgets.readthedocs.io/en/stable/user_install.html
  from .autonotebook import tqdm as notebook_tqdm

def _make_box():
    return BRepPrimAPI_MakeBox(gp_Pnt(0, 0, 0), 1, 1, 1).Shape()


box_entity = OCC_entity(
    occ_function=_make_box,
    physical_name="box",
    mesh_order=1,
    dimension=3,
)

write_xao(cad_occ([box_entity]), "box.xao")

You can wrap any zero-argument callable that returns a TopoDS_Shape: primitives, boolean results, imported STEP bodies, etc. For parameterized shapes, use functools.partial or a closure.

def cylinder(radius, height):
    from OCP.BRepPrimAPI import BRepPrimAPI_MakeCylinder

    return BRepPrimAPI_MakeCylinder(radius, height).Shape()


custom_entity = OCC_entity(
    occ_function=partial(cylinder, radius=0.5, height=2.0),
    physical_name="cyl",
    mesh_order=1,
    dimension=3,
)

write_xao(cad_occ([custom_entity]), "cyl.xao")

Use OCC_entity when you need:

• Primitive shapes beyond polygon extrusion (spheres, cones, tori)

• Shapes from external STEP / BREP files

• Any OCP boolean composition you want processed as a single labeled entity

Naming individual points (0D entities)

meshwell meshes are keyed by physical name. Regions (PolySurface), curves (PolyLine), and auto-generated interfaces/boundaries all get names – and so can an individual point, by wrapping a vertex in an OCC_entity with dimension=0 and a physical_name. Build the vertex with BRepBuilderAPI_MakeVertex(gp_Pnt(x, y, 0.0)).Vertex(). The named vertex survives CAD fragmentation and becomes a named mesh node (a vertex cell block) within meshwell’s point-tolerance (~1e-5) of the requested coordinate. Works inside a region, on an interface, or on a corner.

This is the building block for features that reference a specific vertex by name (e.g. boundary-layer fan points at a convex corner – see notebook 25).

import shapely  # noqa: E402
from OCP.BRepBuilderAPI import BRepBuilderAPI_MakeVertex  # noqa: E402

from meshwell.orchestrator import generate_mesh  # noqa: E402
from meshwell.polysurface import PolySurface  # noqa: E402


def named_point(x, y, name):
    return OCC_entity(
        occ_function=lambda: BRepBuilderAPI_MakeVertex(gp_Pnt(x, y, 0.0)).Vertex(),
        physical_name=name,
        dimension=0,
    )


sheet = PolySurface(
    polygons=shapely.box(0, 0, 4, 2), physical_name="sheet", mesh_order=1
)
point_mesh = generate_mesh(
    entities=[sheet, named_point(2.0, 1.0, "probe")],
    dim=2,
    output_mesh="named_point.msh",
    default_characteristic_length=0.5,
)
print(
    "named groups:",
    sorted(k for k in point_mesh.cell_sets if not k.startswith("gmsh:")),
)

Info    : Clearing all models and views...
Info    : Done clearing all models and views
Info    : Reading '/tmp/tmpmk7ayg6q/cad.xao'...
Info    : Done reading '/tmp/tmpmk7ayg6q/cad.xao'

named groups: ['probe', 'sheet', 'sheet___None']