Python script to extract a submesh give an input mesh vertices and faces array and a list of vertices or faces to keep.

submesh.py

""" Python script to extract a submesh give an input mesh vertices and faces array and a list of vertices or faces to keep."""
def get_submesh(verts, faces, verts_retained=None, faces_retained=None, min_vert_in_face=2):
    '''
        Given a mesh, create a (smaller) submesh
        indicate faces or verts to retain as indices or boolean

        verts : Input mesh vertices (Nx3)
        faces : Input mesh faces (Fx3)
        verts_retained : list of the vertices indices to keep

        @return new_verts: the new array of 3D vertices
                new_faces: the new array of faces
                bool_faces: the faces indices wrt the input mesh
                vetex_ids: the vertex_ids wrt the input mesh
        '''

    if verts_retained is not None:
        # Transform indices into bool array
        if verts_retained.dtype != 'bool':
            vert_mask = np.zeros(len(verts), dtype=bool)
            vert_mask[verts_retained] = True
        else:
            vert_mask = verts_retained

        # Faces with at least min_vert_in_face vertices
        bool_faces = np.sum(vert_mask[faces.ravel()].reshape(-1, 3), axis=1) > min_vert_in_face

    elif faces_retained is not None:
        # Transform indices into bool array
        if faces_retained.dtype != 'bool':
            bool_faces = np.zeros(len(faces_retained), dtype=bool)
        else:
            bool_faces = faces_retained

    new_faces = faces[bool_faces]
    # just in case additional vertices are added
    vertex_ids = np.where(vert_mask)[0]

    oldtonew = -1 * np.ones([len(verts)])
    oldtonew[vertex_ids] = range(0, len(vertex_ids))

    new_verts = verts[vertex_ids]
    new_faces = oldtonew[new_faces].astype('int32')

    return (new_verts, new_faces, bool_faces, vertex_ids)

#Input
input_mesh = None #Load a trimesh mesh here 
submesh_verts_indices = None # List of vertex indices to keep
  
new_verts, new_faces, bool_faces, vertex_ids = get_submesh(input_mesh.vertices, input_mesh.faces, verts_retained=submesh_verts_indices)

# Output
submesh = trimesh.Trimesh(vertices=new_verts, faces=new_faces, process=False, maintain_order=True)
    
# For each face of the submesh, gives its index in the original mesh (can be usefull)
submesh_global_faces_index = np.where(bool_faces)[0]