docstr: The alignment energy is defined as the log-ratio between the
  average temporal variance in the sequence and the global
  spatio-temporal variance.
docstr_tok: ["The","alignment","energy","is","defined","as","the","log-ratio","between","the","average","temporal","variance","in","the","sequence","and","the","global","spatio-temporal","variance","."]
code: def _energy(self):
      """
      The alignment energy is defined as the log-ratio between the
      average temporal variance in the sequence and the global
      spatio-temporal variance.
      """
      return np.log(self._V / self._V0)
  
imports: import os
  import warnings
  import numpy
  from nibabel import io_orientation
  from nibabel.affines import apply_affine
  from ...core.image.image_spaces import as_xyz_image, make_xyz_image, xyz_affine
  from ...io.nibcompat import get_header
  from ..slicetiming import timefuncs
  from ._registration import _cspline_sample3d, _cspline_sample4d, _cspline_transform
  from .affine import Affine, Rigid
  from .optimizer import configure_optimizer, use_derivatives
  from .type_check import check_type, check_type_and_shape
label_desc: Use image node_id 8 for calling the Realign4dAlgorithm obj's underlying member method code with example usage: obj._energy() and returns: np