def from_offset(cls, chunk_type, stream_rdr, offset):
        """
        Return an _IHDRChunk instance containing the image dimensions
        extracted from the IHDR chunk in *stream* at *offset*.
        """
        px_width = stream_rdr.read_long(offset)
        px_height = stream_rdr.read_long(offset, 4)
        return cls(chunk_type, px_width, px_height)