Hello everyone,

I want to create a submesh utilizing the binary image data. The following figure represents what I am trying to do. Let’s assume a 2*2 image data with 0s and 1s as depicted in the left figure. Each pixel should be represented by a quadrilateral element.

All the cells/elements with the zero values should be deleted from the submesh, and remaining submesh should look like the figure on the right.

I tried the following code. I represented the input data with the help of DG0 elements. This could could have worked if the marker function in mesh.locate_entities would take x as the co-ordinates of dofs instead of vertices. Please let me know how I should proceed.

```
import numpy as np
import ufl
from dolfinx import fem, io, mesh, plot
from ufl import ds, dx, grad, inner, nabla_grad
from mpi4py import MPI
from petsc4py.PETSc import ScalarType
## input data (typical to 2D binary image)
data = np.zeros((2,2))
data[0,0] = 1.0
data[1,1] = 1.0
msh = mesh.create_rectangle(comm=MPI.COMM_WORLD,
points=((0.0, 0.0), (1.0, 1.0 )), n=(2, 2),
cell_type=mesh.CellType.quadrilateral,)
V = fem.FunctionSpace(msh, ('DG', 0))
dof_coo = V.tabulate_dof_coordinates()
def submesh_marker(x,data):
## x is co-ordinates of all the vertices
# this code could have worked, if x would have been the coordinates of dofs.
ix = np.round((x[0])/0.5).astype(int)
iy = np.round((x[1])/0.5).astype(int)
tol=1e-14
return abs(data[ix,iy]-1) < tol
submesh_entities = mesh.locate_entities(msh, msh.topology.dim, marker = lambda x: submesh_marker(x,data))
submesh_omega_tilde = mesh.create_submesh(msh, msh.topology.dim, submesh_entities)[0]
dx_subdomain = ufl.Measure("dx", domain = submesh_omega_tilde)
V_sub = fem.FunctionSpace(submesh_omega_tilde, ('DG', 0))
```