code: def _bss_image_crit(s_true, e_spat, e_interf, e_artif):
      
      # energy ratios
      sdr = _safe_db(np.sum(s_true**2), np.sum((e_spat+e_interf+e_artif)**2))
      isr = _safe_db(np.sum(s_true**2), np.sum(e_spat**2))
      sir = _safe_db(np.sum((s_true+e_spat)**2), np.sum(e_interf**2))
      sar = _safe_db(np.sum((s_true+e_spat+e_interf)**2), np.sum(e_artif**2))
      return (sdr, isr, sir, sar)
docs: Measurement of the separation quality for a given image in terms of
      filtered true source, spatial error, interference and artifacts.