Can I extract surface mesh from a solid mesh?

[srhouyu]

Hello, I have some FE mesh consisting tet4 elements. Their cells_dict only have tetra key and no 'triangle' key. Then can I extract its surface or boundary using meshio?

[logari81]

This is deepseek's solution to this problem, seems to work.

import meshio
import numpy as np

# Load the volume mesh
volume_mesh = meshio.read("volume_mesh.vtk")  # Replace with your file

# Extract tetrahedron cells
tetra_cells = volume_mesh.get_cells_type("tetra")

# Get the faces of all tetrahedrons
# Each tetrahedron has 4 triangular faces
faces = np.vstack([
    tetra_cells[:, [0, 1, 2]],  # Face 1
    tetra_cells[:, [0, 1, 3]],  # Face 2
    tetra_cells[:, [0, 2, 3]],  # Face 3
    tetra_cells[:, [1, 2, 3]],  # Face 4
])

# Sort each face to ensure consistent ordering
faces = np.sort(faces, axis=1)

# Find unique faces (boundary faces)
# A face is on the surface if it appears only once
unique_faces, face_counts = np.unique(faces, axis=0, return_counts=True)
surface_faces = unique_faces[face_counts == 1]

# Create the surface mesh
surface_mesh = meshio.Mesh(
    points=volume_mesh.points,  # Use the same points as the volume mesh
    cells=[("triangle", surface_faces)]  # Use the boundary faces
)

# Save the surface mesh
meshio.write("surface_mesh.stl", surface_mesh)  # Save as STL or another surface format