code: def ssim(test, ref, mask=None):
      
  
      if not import_skimage:  # pragma: no cover
          raise ImportError('Scikit-Image package not found')
  
      test, ref, mask = _preprocess_input(test, ref, mask)
      assim, ssim = compare_ssim(test, ref, full=True)
  
      if mask is None:
          return assim
  
      else:
          return (mask * ssim).sum() / mask.sum()
docs: Structural Similarity (SSIM)
  
      Calculate the SSIM between a test image and a reference image.
  
      Parameters
      ----------
      ref : np.ndarray
          the reference image
      test : np.ndarray
          the tested image
      mask : np.ndarray, optional
          the mask for the ROI
  
      Notes
      -----
      Compute the metric only on magnetude.
  
      Returns
      -------
      ssim: float, the snr