code: def from_stream(cls, stream, marker_code, offset):
          
          # field                 off  len  type   notes
          # --------------------  ---  ---  -----  ----------------------------
          # segment length         0    2   short
          # Exif identifier        2    6   6 chr  'Exif\x00\x00'
          # TIFF byte order        8    2   2 chr  'II'=little 'MM'=big endian
          # meaning of universe   10    2   2 chr  '*\x00' or '\x00*' depending
          # IFD0 off fr/II or MM  10   16   long   relative to ...?
          # --------------------  ---  ---  -----  ----------------------------
          segment_length = stream.read_short(offset)
          if cls._is_non_Exif_APP1_segment(stream, offset):
              return cls(marker_code, offset, segment_length, 72, 72)
          tiff = cls._tiff_from_exif_segment(stream, offset, segment_length)
          return cls(
              marker_code, offset, segment_length, tiff.horz_dpi, tiff.vert_dpi
          )
docs: Extract the horizontal and vertical dots-per-inch value from the APP1
          header at *offset* in *stream*.