[HarfBuzz] Arabic Presentation Forms-A in fallback shaping

[Behdad Esfahbod]

On 13-03-13 04:14 AM, Jonathan Kew wrote:
> 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.

Been thinking about it more.  It's code that no one will ever going to see in
action in reality.  As such I changed my mind and leaving it out.

behdad