docstr: This function calculates the ratio of difference in volume between
  the reference and segmentation images.
  
  :return: vol_diff
docstr_tok: ["This","function","calculates","the","ratio","of","difference","in","volume","between","the","reference","and","segmentation","images",".",":","return",":","vol_diff"]
code: def vol_diff(self):
      """
      This function calculates the ratio of difference in volume between
      the reference and segmentation images.
  
      :return: vol_diff
      """
      return (
          np.abs(self.n_pos_ref() - self.n_pos_seg()) / self.n_pos_ref()
      )
  
imports: from __future__ import absolute_import, print_function
  import numpy
  from scipy import ndimage
  from niftynet.utilities.util_common import MorphologyOps, CacheFunctionOutput
label_desc: Use image node_id 34 for calling the PairwiseMeasures obj's underlying member method code with example usage: obj.vol_diff() and returns: unknown