code: def undo_filter_paeth(filter_unit, scanline, previous, result):
      
  
      # Also used for ci.
      ai = -filter_unit
      for i in range(len(result)):
          x = scanline[i]
          if ai < 0:
              a = c = 0
          else:
              a = result[ai]
              c = previous[ai]
          b = previous[i]
          p = a + b - c
          pa = abs(p - a)
          pb = abs(p - b)
          pc = abs(p - c)
          if pa <= pb and pa <= pc:
              pr = a
          elif pb <= pc:
              pr = b
          else:
              pr = c
          result[i] = (x + pr) & 0xff
          ai += 1
docs: Undo Paeth filter.