[HarfBuzz] Arabic Presentation Forms-A in fallback shaping

Khaled Hosny khaledhosny at eglug.org
Tue Mar 12 18:21:24 PDT 2013

On Tue, Mar 12, 2013 at 09:08:41PM -0400, Behdad Esfahbod wrote:
> On 13-03-12 03:10 PM, Khaled Hosny wrote:
> > Hi Behdad,
> > 
> > While comparing the fallback shaping ICU-based XeTeX with the
> > HarfBuzz-based one, I noticed that ICU uses the Arabic Presentation
> > Forms-A ligatures if the font have them unlike HarfBuzz. It is not a big
> > issue (I don’t even have such font, I striped OT tables from fonts to
> > test), but it would be nice to have.
> I'm not a huge fan of those, mostly because in Persian Naskh style they are
> typically not desired.  That said, changing the code to do it is trivial.  I
> already have it for all two-character ligatures.  Extending it to
> three-character ones takes another twenty minutes.
> But I'm not sure whether we want this.  The size increment in the binary is
> about 5k, which is not a big deal, but then I wonder whether I should start
> adding build options to disable fallbacks.  Or perhaps we build the best
> engine first and wait till someone complains about size before we figure out
> how to build smaller subsets.

I have no preference either way, and I don’t think many such old fonts
have those ligatures either (If it weren’t for the fact that Amiri has
the presentation forms glyphs for the sake of completeness I would have
never found this out).


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