[HarfBuzz] Arabic Presentation Forms-A in fallback shaping

[Jonathan Kew]

On 13/3/13 01:08, 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'll go ahead and fix this.

FWIW, my vote would be to -not- "fix" this. We should not do anything 
that might appear to encourage font developers to think those lists of 
"ligatures" have any real meaning in terms of Arabic-script type design.

I think it was a mistake for Unicode (or indeed anyone) to attempt to 
define such a repertoire of ligatures for Arabic script, as the decision 
as to what's included and what is excluded - and indeed where to split 
longer sequences into 3-character "ligatures" in the first place - is 
often rather arbitrary, and heavily dependent on the style involved.

My preference would be for the Arabic engine to ignore them entirely.

JK