code: def test_preprocess():
      def preprocess(x):
          x[:] = 1
          return x[:, :, 0]
  
      with random_images(4, (2, 3, 4)) as globstring:
          im = da_imread(globstring, preprocess=preprocess)
          assert (im.compute() == np.ones((4, 2, 3), dtype="u1")).all()
  
file_path: dask/dask/array/tests/test_image.py
imports: from __future__ import annotations
  import os
  from contextlib import contextmanager
  import pytest
  import numpy
  from skimage.io import imsave
  from dask.array.image import imread
  from dask.utils import tmpdir
label_desc: Use image node_id 4 for calling a global function with example usage: test_preprocess() and returns: x