docs: def calc_fwhm(img, region, fexpand=3, axis=0):
      """Compute the FWHM in the direction given by axis"""
  
      # We compute know the FWHM of the slit
      # Given the computed position of the slit
      # Expand 'fexpand' pixels around
      # and cut an slice in the median filtered image
  
      xpregion = expand_region(region, fexpand, fexpand)
      cslit = img[xpregion]
  
      # Collapse it
      pslit = cslit.mean(axis=axis)
  
      # Estimate the background as a flat line
      # starting in pslit[0] and ending in pslit[-1]
      x2 = len(pslit)
      y1, y2 = pslit[0], pslit[-1]
      mslope = (y2-y1) / x2
      # background estimation
      backstim = mslope*numpy.arange(x2) + y1
  
      # We subtract background
      qslit = pslit-backstim
      # and find the pixel of the maximum
      pidx = numpy.argmax(qslit)
      peak, fwhm = fmod.compute_fwhm_1d_simple(qslit, pidx)
      return fwhm