docstr: This function calculates the false negative map
  
  :return: FN map
docstr_tok: ["This","function","calculates","the","false","negative","map",":","return",":","FN","map"]
code: def __FNmap(self):
      """
      This function calculates the false negative map
  
      :return: FN map
      """
      self.check_binary()
      return np.asarray((self.ref - self.seg) > 0.0, dtype=np.float32)
  
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 4 for calling the PairwiseMeasures obj's underlying member method code with example usage: obj.__FNmap() and returns: np