code: def whitening(image):
      """Whitening. Normalises image to zero mean and unit variance."""
  
      image = image.astype(np.float32)
  
      mean = np.mean(image)
      std = np.std(image)
  
      if std > 0:
          ret = (image - mean) / std
      else:
          ret = image * 0.0
      return ret
  
file_path: DLTK/dltk/io/preprocessing.py
imports: from __future__ import unicode_literals
  from __future__ import print_function
  from __future__ import division
  from __future__ import absolute_import
  import numpy
label_desc: Use image node_id 1 for calling a global function with example usage: whitening(image) and returns: ret