[HarfBuzz] how to detect missing glyphs e.g. for font substitition

[Khaled Hosny]

On Mon, May 11, 2015 at 08:49:48PM +0000, Louis Semprini wrote:
> 
> 
> > Date: Mon, 11 May 2015 21:35:49 +0200
> > From: khaledhosny at eglug.org
> > To: lsemprini at hotmail.com
> > CC: harfbuzz at lists.freedesktop.org
> > Subject: Re: [HarfBuzz] how to detect missing glyphs e.g. for font substitition
> > 
> > On Mon, May 11, 2015 at 07:56:19AM +0000, Louis Semprini wrote:
> > > Or, must Harfbuzz callers first do a complete, separate pass where
> > > they run all code points of the input through some kind of mapping
> > > routine that uses the fonts' 'cmap' and other tables?  The latter
> > > would be a shame because it would require the Harfbuzz caller to
> > > duplicate a vast amount of the complexity that is nicely hidden in
> > > Harfbuzz in their own code.  It's also a shame because in most cases,
> > > no font substitution would be needed and so it would be inefficient in
> > > the average case.
> > 
> > Some HarfBuzz users do that i.e. check the font’s cmap table to see what
> > characters it supports and selects fallback fonts for what it doesn’t
> > before even calling HarfBuzz. Others rely on HarfBuzz, for example in
> > LibreOffice the run is first shaped with the user selected font, then
> > any contiguous runs of missing glyphs are reshaped with fallback fonts,
> > this have also the advantage of letting HarfBuzz do its normalisation
> > which can result in the font supporting more characters than it declares
> > in its cmap table.
> 
> That's good to know, but for the second group of users, how do they
> detect the missing glyphs?  By looking for glyph index 0?

Yes (but as Konstantin said, it depends on what your font functions
return for missing glyphs).

Regards,
Khaled