docstr: The design matrix for a SFM.
  
  Returns
  -------
  ndarray
      The design matrix, where each column is a rotated version of the
      response function.
docstr_tok: ["The","design","matrix","for","a","SFM",".","Returns","--","--","--","-","ndarray","The","design","matrix",",","where","each","column","is","a","rotated","version","of","the","response","function","."]
code: def design_matrix(self):
      """
      The design matrix for a SFM.
  
      Returns
      -------
      ndarray
          The design matrix, where each column is a rotated version of the
          response function.
      """
      return sfm_design_matrix(
          self.gtab, self.sphere, self.response, "signal"
      )
  
imports: import warnings
  import numpy
  import gc
  from collections import OrderedDict
  from dipy.utils.optpkg import optional_package
  from dipy.utils.multiproc import determine_num_processes
  import dipy.core.gradients
  import dipy.core.optimize
  import dipy.sims.voxel
  import dipy.data
  from dipy.reconst.base import ReconstModel, ReconstFit
  from dipy.reconst.cache import Cache
  from dipy.core.onetime import auto_attr
label_desc: Use image node_id 2 for calling the SparseFascicleModel obj's underlying member method code with example usage: obj.design_matrix() and returns: sfm_design_matrix