code: def volume_mapper():
      vol = pv.ImageData(dimensions=(10, 10, 10))
      vol["scalars"] = 255 - vol.z * 25
      pl = pv.Plotter()
      actor = pl.add_volume(vol)
      return actor.mapper
  
imports: import pytest
  import vtk
  import pyvista
label_desc: Use image node_id 1 for calling a global function with example usage: volume_mapper() and returns: actor