code: def preprocess(self, data_group):
      if self.mask_mode == "lower":
          data_group.preprocessed_case[
              data_group.preprocessed_case < self.mask_threshold
          ] = self.mask_value
      elif self.mask_mode == "higher":
          data_group.preprocessed_case[
              data_group.preprocessed_case > self.mask_threshold
          ] = self.mask_value
      elif self.mask_mode == "equal":
          data_group.preprocessed_case[
              data_group.preprocessed_case == self.mask_threshold
          ] = self.mask_value
      else:
          raise NotImplementedError
  
      self.output_data = data_group.preprocessed_case
  
imports: import subprocess
  import os
  import numpy
  from deepneuro.preprocessing.preprocessor import Preprocessor
  from deepneuro.utilities.conversion import read_image_files
  from deepneuro.utilities.util import add_parameter, quotes
label_desc: Use image node_id 2 for calling the MaskValues obj's underlying member method code with example usage: obj.preprocess(data_group) without return types