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

[Louis Semprini]

> 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?

Thanks.

 		 	   		  
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