GMSH Backend

GMSH Backend#

The GMSH backend (default) provides full control over mesh parameters and supports both surface and volume meshing.

Basic Usage#

import cadquery as cq
from cad_to_dagmc import CadToDagmc

sphere = cq.Workplane().sphere(5)
small_sphere = cq.Workplane().moveTo(10, 0).sphere(2)

assembly = cq.Assembly()
assembly.add(sphere)
assembly.add(small_sphere)

model = CadToDagmc()
model.add_cadquery_object(cadquery_object=assembly, material_tags=["mat1", "mat2"])

model.export_dagmc_h5m_file(
    filename="dagmc.h5m",
    meshing_backend="gmsh",  # Default
    min_mesh_size=0.5,
    max_mesh_size=1.0e6,
)

Mesh Size Parameters#

Control overall mesh density:

model.export_dagmc_h5m_file(
    filename="dagmc.h5m",
    min_mesh_size=0.5,   # Minimum element size
    max_mesh_size=10.0,  # Maximum element size
)

Smaller values = finer mesh = more triangles = slower transport but more accurate geometry.

Mesh Algorithms#

GMSH provides multiple meshing algorithms:

model.export_dagmc_h5m_file(
    filename="dagmc.h5m",
    mesh_algorithm=6,  # Frontal-Delaunay
)

Available algorithms:

Algorithm	Name	Notes
1	MeshAdapt	Default, adaptive
2	Automatic	GMSH chooses
5	Delaunay	Classic Delaunay
6	Frontal-Delaunay	Good for most cases
7	BAMG	Anisotropic
8	Frontal-Delaunay for Quads	Quad elements
9	Packing of Parallelograms	Structured regions

See GMSH mesh options for details.

Per-Volume Mesh Sizing#

Control mesh density per volume:

import cadquery as cq
from cad_to_dagmc import CadToDagmc

coarse_box = cq.Workplane().box(1, 1, 2)
fine_box = cq.Workplane().moveTo(1, 0.5).box(1, 1, 1.5)
default_box = cq.Workplane().moveTo(2, 1).box(1, 1, 1)

assembly = cq.Assembly()
assembly.add(coarse_box, name="coarse")
assembly.add(fine_box, name="fine")
assembly.add(default_box, name="global")

model = CadToDagmc()
model.add_cadquery_object(assembly, material_tags="assembly_names")

model.export_dagmc_h5m_file(
    filename="different_resolution_meshes.h5m",
    min_mesh_size=0.01,
    max_mesh_size=10,
    set_size={
        "coarse": 0.9,  # By material tag name
        "fine": 0.1,
        # "global" uses min/max only
    },
)

See Mesh Sizing for more details.

CAD Transfer Method#

Control how geometry is transferred to GMSH:

# File method (default) - more compatible
model.export_dagmc_h5m_file(
    filename="dagmc.h5m",
    method="file",  # Write temp BREP file
)

# In-memory method - faster for large geometries
model.export_dagmc_h5m_file(
    filename="dagmc.h5m",
    method="inMemory",  # Direct transfer
)

Method	Pros	Cons
`"file"`	More compatible, works with pip install	Slower (file I/O)
`"inMemory"`	Faster for large geometries	Requires matching OCC versions

Advanced GMSH Options#

For fine-grained control, access GMSH directly:

import gmsh
import cad_to_dagmc

# Initialize GMSH
gmsh_obj = cad_to_dagmc.init_gmsh()

# ... add geometry and configure mesh ...

# Set advanced GMSH options
gmsh.option.setNumber("Mesh.OptimizeNetgen", 1)
gmsh.option.setNumber("Mesh.Smoothing", 5)
gmsh.option.setNumber("Mesh.Algorithm", 6)

# Generate mesh
gmsh.model.mesh.generate(2)

# Export
cad_to_dagmc.export_gmsh_object_to_dagmc_h5m_file(filename="dagmc.h5m")

gmsh.finalize()

Volume Meshing#

Only GMSH backend supports volume meshes:

import cadquery as cq
from cad_to_dagmc import CadToDagmc

assembly = cq.Assembly()
assembly.add(cq.Workplane("XY").sphere(10))

model = CadToDagmc()
model.add_cadquery_object(assembly, material_tags=["mat1"])

# Volume mesh for unstructured mesh tallies
model.export_unstructured_mesh_file(
    filename="umesh.vtk",
    min_mesh_size=1.0,
    max_mesh_size=5.0,
)

Parameters Reference#

Parameter	Type	Default	Description
`min_mesh_size`	float	None	Minimum mesh element size
`max_mesh_size`	float	None	Maximum mesh element size
`mesh_algorithm`	int	1	GMSH meshing algorithm
`set_size`	dict	None	Per-volume mesh sizes
`method`	str	“file”	CAD transfer method