code: def test_linear_gradient():
      # linear light source centered at top left corner
      x, y = np.meshgrid(range(100), range(100))
      img = y * 20 + x * 20
  
      expected_img = 19 * np.ones_like(img)
      expected_img[0, 0] = 0
  
      background = rolling_ball(img, radius=1)
      assert np.allclose(img - background, expected_img)
  
file_path: scikit-image/skimage/restoration/tests/test_rolling_ball.py
imports: import numpy
  import pytest
  from skimage import data
  from skimage.restoration._rolling_ball import rolling_ball
  from skimage.restoration._rolling_ball import ellipsoid_kernel
label_desc: Use image node_id 5 for calling a global function with example usage: test_linear_gradient() without return types