Integer dots per inch for the width of this image. Defaults to 72
        when not present in the file, as is often the case.

def horz_dpi(self):
        """
        Integer dots per inch for the width of this image. Defaults to 72
        when not present in the file, as is often the case.
        """
        pHYs = self._chunks.pHYs
        if pHYs is None:
            return 72
        return self._dpi(pHYs.units_specifier, pHYs.horz_px_per_unit)