docstr: Shear variance.
  
  Returns
  -------
  v_shear : numpy.ndarray
  
  Notes
  -----
  Shear variance is calculated as
  
      .. math::
  
          V_      ext{shear} = \mathbb{C} : \mathbb{E}_   ext{shear}
docstr_tok: ["Shear","variance",".","Returns","--","--","--","-","v_shear",":","numpy.ndarray","Notes","--","--","-","Shear","variance","is","calculated","as","..","math",":",":","V_","ext","{","shear","}","=","\\mathbb","{","C","}",":","\\mathbb","{","E","}","_","ext","{","shear","}"]
code: def v_shear(self):
      """Shear variance.
  
      Returns
      -------
      v_shear : numpy.ndarray
  
      Notes
      -----
      Shear variance is calculated as
  
          .. math::
  
              V_\text{shear} = \mathbb{C} : \mathbb{E}_\text{shear}
      """
      v_shear = np.matmul(
          self.params[..., np.newaxis, 7::], from_6x6_to_21x1(E_shear)
      )[..., 0, 0]
      return v_shear
  
imports: from warnings import warn
  import numpy
  from dipy.reconst.base import ReconstModel
  from dipy.reconst.dti import auto_attr
  from dipy.utils.optpkg import optional_package
label_desc: Use image node_id 6 for calling the QtiFit obj's underlying member method code with example usage: obj.v_shear() and returns: v_shear