docstr: This function calculates the true negative map
  
  :return: TN map
docstr_tok: ["This","function","calculates","the","true","negative","map",":","return",":","TN","map"]
code: def __TNmap(self):
      """
      This function calculates the true negative map
  
      :return: TN map
      """
      self.check_binary()
      return np.logical_and(self.ref < 0.5, self.seg < 0.5).astype(
          float
      )
  
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 6 for calling the PairwiseMeasures obj's underlying member method code with example usage: obj.__TNmap() and returns: np