mohm-dhi/dmsh

Simple mesh generator inspired by distmesh.

The worst mesh generator you'll ever use.

Inspired by distmesh, dmsh can be slow, requires a lot of memory, and isn't terribly robust either.

On the plus side,

• it's got a user-friendly interface,
• is pure Python (and hence easily installable on any system), and
• it produces pretty high-quality meshes.

Combined with optimesh, dmsh produces the highest-quality 2D meshes in the west.

Examples

Primitives

import dmsh
import meshio
import optimesh

geo = dmsh.Circle([0.0, 0.0], 1.0)
X, cells = dmsh.generate(geo, 0.1)

# optionally optimize the mesh
X, cells = optimesh.optimize_points_cells(X, cells, "CVT (full)", 1.0e-10, 100)

# visualize the mesh
dmsh.helpers.show(X, cells, geo)

# and write it to a file
meshio.Mesh(X, {"triangle": cells}).write("circle.vtk")

import dmsh

geo = dmsh.Rectangle(-1.0, +2.0, -1.0, +1.0)
X, cells = dmsh.generate(geo, 0.1)

import dmsh

geo = dmsh.Polygon(
    [
        [0.0, 0.0],
        [1.1, 0.0],
        [1.2, 0.5],
        [0.7, 0.6],
        [2.0, 1.0],
        [1.0, 2.0],
        [0.5, 1.5],
    ]
)
X, cells = dmsh.generate(geo, 0.1)

Combinations

Difference

import dmsh

geo = dmsh.Circle([-0.5, 0.0], 1.0) - dmsh.Circle([+0.5, 0.0], 1.0)
X, cells = dmsh.generate(geo, 0.1)

import dmsh

geo = dmsh.Circle([0.0, 0.0], 1.0) - dmsh.Polygon([[0.0, 0.0], [1.5, 0.4], [1.5, -0.4]])
X, cells = dmsh.generate(geo, 0.1, tol=1.0e-10)

The following example uses a nonconstant edge length; it depends on the distance to the circle c.

import dmsh
import numpy as np

r = dmsh.Rectangle(-1.0, +1.0, -1.0, +1.0)
c = dmsh.Circle([0.0, 0.0], 0.3)
geo = r - c

X, cells = dmsh.generate(geo, lambda pts: np.abs(c.dist(pts)) / 5 + 0.05, tol=1.0e-10)

Union

import dmsh

geo = dmsh.Circle([-0.5, 0.0], 1.0) + dmsh.Circle([+0.5, 0.0], 1.0)
X, cells = dmsh.generate(geo, 0.15)

import dmsh

geo = dmsh.Rectangle(-1.0, +0.5, -1.0, +0.5) + dmsh.Rectangle(-0.5, +1.0, -0.5, +1.0)
X, cells = dmsh.generate(geo, 0.15)

import dmsh
import numpy as np

angles = np.pi * np.array([3.0 / 6.0, 7.0 / 6.0, 11.0 / 6.0])
geo = dmsh.Union(
    [
        dmsh.Circle([np.cos(angles[0]), np.sin(angles[0])], 1.0),
        dmsh.Circle([np.cos(angles[1]), np.sin(angles[1])], 1.0),
        dmsh.Circle([np.cos(angles[2]), np.sin(angles[2])], 1.0),
    ]
)
X, cells = dmsh.generate(geo, 0.15)

Intersection

import dmsh

geo = dmsh.Circle([0.0, -0.5], 1.0) & dmsh.Circle([0.0, +0.5], 1.0)
X, cells = dmsh.generate(geo, 0.1, tol=1.0e-10)

import dmsh
import numpy as np

angles = np.pi * np.array([3.0 / 6.0, 7.0 / 6.0, 11.0 / 6.0])
geo = dmsh.Intersection(
    [
        dmsh.Circle([np.cos(angles[0]), np.sin(angles[0])], 1.5),
        dmsh.Circle([np.cos(angles[1]), np.sin(angles[1])], 1.5),
        dmsh.Circle([np.cos(angles[2]), np.sin(angles[2])], 1.5),
    ]
)
X, cells = dmsh.generate(geo, 0.1, tol=1.0e-10)

The following uses the HalfSpace primtive for cutting off a circle.

import dmsh

geo = dmsh.HalfSpace([1.0, 1.0]) & dmsh.Circle([0.0, 0.0], 1.0)
X, cells = dmsh.generate(geo, 0.1)

Rotation, translation, scaling

import dmsh
import numpy as np

geo = dmsh.Rotation(dmsh.Rectangle(-1.0, +2.0, -1.0, +1.0), 0.1 * np.pi)
X, cells = dmsh.generate(geo, 0.1, tol=1.0e-10)

import dmsh

geo = dmsh.Rectangle(-1.0, +2.0, -1.0, +1.0) + [1.0, 1.0]
X, cells = dmsh.generate(geo, 0.1)

import dmsh

geo = dmsh.Rectangle(-1.0, +2.0, -1.0, +1.0) * 2.0
X, cells = dmsh.generate(geo, 0.1, tol=1.0e-5)

Local refinement

All objects can be used to refine the mesh according to the distance to the object; e.g. a Path:

import dmsh

geo = dmsh.Rectangle(0.0, 1.0, 0.0, 1.0)

p1 = dmsh.Path([[0.4, 0.6], [0.6, 0.4]])


def edge_size(x):
    return 0.03 + 0.1 * p1.dist(x)


X, cells = dmsh.generate(geo, edge_size, tol=1.0e-10)

Custom shapes

It is also possible to define your own geometry. Simply create a class derived from dmsh.Geometry that contains a dist method and a method to project points onto the boundary.

import dmsh
import numpy as np


class MyDisk(dmsh.Geometry):
    def __init__(self):
        self.r = 1.0
        self.x0 = [0.0, 0.0]
        bounding_box = [-1.0, 1.0, -1.0, 1.0]
        feature_points = np.array([[], []]).T
        super().__init__(bounding_box, feature_points)

    def dist(self, x):
        assert x.shape[0] == 2
        y = (x.T - self.x0).T
        return np.sqrt(np.einsum("i...,i...->...", y, y)) - self.r

    def boundary_step(self, x):
        # project onto the circle
        y = (x.T - self.x0).T
        r = np.sqrt(np.einsum("ij,ij->j", y, y))
        return ((y / r * self.r).T + self.x0).T


geo = MyDisk()
X, cells = dmsh.generate(geo, 0.1)

Debugging

dmsh is rather fragile, but sometimes the break-downs are due to an incorrectly defined geometry. Use

geo.show()

to inspect the level set function of your domain. (It must be negative inside the domain and positive outside. The 0-level set forms the domain boundary.)

Installation

dmsh is available from the Python Package Index, so simply type

pip install dmsh

to install.

Testing

To run the dmsh unit tests, check out this repository and type

tox

License

This software is published under the MIT license.