code: def test_vertex_outer_normals():
      # compute normals for a flat horizontal mesh, they should all be (0, 0, 1)
      mesh = flat_mesh(5, 7)
      computed_normals = surface._vertex_outer_normals(mesh)
      true_normals = np.zeros((len(mesh[0]), 3))
      true_normals[:, 2] = 1
      assert_array_almost_equal(computed_normals, true_normals)
  
imports: import os
  import tempfile
  import warnings
  import nibabel
  import numpy
  import pytest
  from nibabel import gifti
  from numpy.testing import assert_array_almost_equal, assert_array_equal
  from scipy.spatial import Delaunay
  from scipy.stats import pearsonr
  from nilearn import datasets, image
  from nilearn._utils import data_gen
  from nilearn.image import resampling
  from nilearn.surface import Mesh, Surface, load_surf_data, load_surf_mesh, surface
  from nilearn.surface.surface import _gifti_img_to_mesh, _load_surf_files_gifti_gzip
  from nilearn.surface.tests._testing import flat_mesh, generate_surf, z_const_img
label_desc: Use image node_id 19 for calling a global function with example usage: test_vertex_outer_normals() without return types