Reason is that hinting distorts
        the glyph shape for optimal rasterization, and this process
        sometimes creates outlines which have considerably different
        metrics.  The TrueType format provides the (optional)
        &lsquo;hdmx&rsquo; table which contains device horizontal metrics
        for selected pixel sizes, but even here the vertical metrics are
        missing.</p>

        <p>It is probably best to use both a glyph and a metrics cache to
        avoid recomputation.</p>


   