code: def from_stream(cls, stream, marker_code, offset):
          
          # field                 off  type   notes
          # ------------------  ---  -----  ----------------------------
          # segment length       0   short
          # Data precision       2   byte
          # Vertical lines       3   short  px_height
          # Horizontal lines     5   short  px_width
          # ------------------  ---  -----  ----------------------------
          segment_length = stream.read_short(offset)
          px_height = stream.read_short(offset, 3)
          px_width = stream.read_short(offset, 5)
          return cls(marker_code, offset, segment_length, px_width, px_height)
docs: Return an |_SofMarker| instance for the SOFn marker at *offset* in
          stream.