This tutorial is part of viskex.

Plot subdomains mesh tags of a 2D dolfinx mesh

import dolfinx.mesh
import mpi4py.MPI

import viskex
import viskex.utils

import common_dolfinx as common  # isort: skip

Generate meshes of the unit square by dividing each edge of the square in 6 segments, using either a triangular or quadrangular mesh.

square_tria = dolfinx.mesh.create_unit_square(mpi4py.MPI.COMM_WORLD, 6, 6, dolfinx.mesh.CellType.triangle)
square_quad = dolfinx.mesh.create_unit_square(mpi4py.MPI.COMM_WORLD, 6, 6, dolfinx.mesh.CellType.quadrilateral)

Mark subdomains according to the $(x, y)$ position of the vertices of the mesh:

• if $x \leq \frac{1}{3}$, assign subdomain mesh tag equal to $1$;
• if $x \geq \frac{2}{3}$, assign subdomain mesh tag equal to $2$;
• otherwise, leave the subdomain mesh tag unassigned.

square_tria_subdomains = common.mark_subdomains(square_tria)
square_quad_subdomains = common.mark_subdomains(square_quad)

We plot the resulting subdomains.

viskex.dolfinx.plot_mesh_tags(square_tria, square_tria_subdomains, "subdomains")

error: XDG_RUNTIME_DIR is invalid or not set in the environment.
MESA: error: ZINK: failed to choose pdev
glx: failed to create drisw screen

viskex.dolfinx.plot_mesh_tags(square_quad, square_quad_subdomains, "subdomains")

error: XDG_RUNTIME_DIR is invalid or not set in the environment.
MESA: error: ZINK: failed to choose pdev
glx: failed to create drisw screen

We can also plot all subdomains with tag equal to $2$, which will be displayed with colors. The rest of the mesh cells are still included in the plot, but are colored in gray.

viskex.dolfinx.plot_mesh_tags(
    square_tria, square_tria_subdomains, "subdomains_2", viskex.utils.values_in([2]))

error: XDG_RUNTIME_DIR is invalid or not set in the environment.
MESA: error: ZINK: failed to choose pdev
glx: failed to create drisw screen

viskex.dolfinx.plot_mesh_tags(
    square_quad, square_quad_subdomains, "subdomains_2", viskex.utils.values_in([2]))

error: XDG_RUNTIME_DIR is invalid or not set in the environment.
MESA: error: ZINK: failed to choose pdev
glx: failed to create drisw screen

Alternatively, we can clip the plot so that it only shows the subset of the mesh with subdomain tag equal to $2$.

viskex.dolfinx.plot_mesh_tags(
    square_tria, square_tria_subdomains, "subdomains_2",
    lambda grid: grid.threshold(2))  # type: ignore[no-untyped-call]

error: XDG_RUNTIME_DIR is invalid or not set in the environment.
MESA: error: ZINK: failed to choose pdev
glx: failed to create drisw screen

viskex.dolfinx.plot_mesh_tags(
    square_quad, square_quad_subdomains, "subdomains_2",
    lambda grid: grid.threshold(2))  # type: ignore[no-untyped-call]

error: XDG_RUNTIME_DIR is invalid or not set in the environment.
MESA: error: ZINK: failed to choose pdev
glx: failed to create drisw screen

What's next?

• next step: plot boundaries
• repeat this step with firedrake