'Return a _pHYsChunk instance containing the image resolution
extracted from the pHYs chunk in *stream* at *offset*.'

@classmethod
def from_offset(cls, chunk_type, stream_rdr, 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)
