docs: def from_offset(cls, chunk_type, stream_rdr, offset):
          """
          Return a _pHYsChunk instance containing the image resolution
          extracted from the pHYs chunk in *stream* at *offset*.
          """
          horz_px_per_unit = stream_rdr.read_long(offset)
          vert_px_per_unit = stream_rdr.read_long(offset, 4)
          units_specifier = stream_rdr.read_byte(offset, 8)
          return cls(
              chunk_type, horz_px_per_unit, vert_px_per_unit, units_specifier
          )