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extract_surface フィルタを使用して， pyvista 内のほぼすべてのオブジェクトのサーフェスを抽出できます．

import numpy as np

import pyvista as pv
from pyvista import CellType

2次セルを作成し，そのサーフェスを抽出します

ここでは，1つの2次6面体セルを作成し，そのサーフェスを抽出して，UnstructuredGridのサーフェスを抽出する方法を示します．

lin_pts = np.array(
    [
        [-1, -1, -1],  # point 0
        [1, -1, -1],  # point 1
        [1, 1, -1],  # point 2
        [-1, 1, -1],  # point 3
        [-1, -1, 1],  # point 4
        [1, -1, 1],  # point 5
        [1, 1, 1],  # point 6
        [-1, 1, 1],  # point 7
    ],
    np.double,
)

# these are the "midside" points of a quad cell.  See the definition of a
# vtkQuadraticHexahedron at:
# https://vtk.org/doc/nightly/html/classvtkQuadraticHexahedron.html
quad_pts = np.array(
    [
        (lin_pts[1] + lin_pts[0]) / 2,  # between point 0 and 1
        (lin_pts[1] + lin_pts[2]) / 2,  # between point 1 and 2
        (lin_pts[2] + lin_pts[3]) / 2,  # and so on...
        (lin_pts[3] + lin_pts[0]) / 2,
        (lin_pts[4] + lin_pts[5]) / 2,
        (lin_pts[5] + lin_pts[6]) / 2,
        (lin_pts[6] + lin_pts[7]) / 2,
        (lin_pts[7] + lin_pts[4]) / 2,
        (lin_pts[0] + lin_pts[4]) / 2,
        (lin_pts[1] + lin_pts[5]) / 2,
        (lin_pts[2] + lin_pts[6]) / 2,
        (lin_pts[3] + lin_pts[7]) / 2,
    ]
)

# introduce a minor variation to the location of the mid-side points
quad_pts += np.random.random(quad_pts.shape) * 0.3
pts = np.vstack((lin_pts, quad_pts))

# create the grid
cells = np.hstack((20, np.arange(20))).astype(np.int64, copy=False)
celltypes = np.array([CellType.QUADRATIC_HEXAHEDRON])
grid = pv.UnstructuredGrid(cells, celltypes, pts)

# finally, extract the surface and plot it
surf = grid.extract_surface()
surf.plot(show_scalar_bar=False)

非線形サーフェスサブディビジョン

UnstructuredGridに2次セルが含まれている場合， "中間" 節点の位置に基づいて滑らかなサーフェスを生成できます．これにより，曲率を含むセルをプロットできます．詳細については， https://prod.sandia.gov/techlib-noauth/access-control.cgi/2004/041617.pdf を参照してください．

surf_subdivided = grid.extract_surface(nonlinear_subdivision=5)
surf_subdivided.plot(show_scalar_bar=False)