[HarfBuzz] Interaction between default and non default features

[Behdad Esfahbod]

On 08/10/2012 10:48 AM, Khaled Hosny wrote:
> While trying to added a stylistic set to an Arabic font, I found that
> they are applied after all default features no matter how the lookups
> are ordered in the font.
> 
> I sort of understand why it is done this way,

It's actually just to match Uniscribe.  Otherwise I would have loved not to do
it this way.

> but it strikes me as it
> renders stylistic sets near useless when any complex substitution is
> needed. In my case I want to convert all regular yeh letters to yeh
> barree form, but it have to be done before any other substitution to
> allow for contextual substitution to take place, doing it after
> contextual substitution will require much more complex rules.
> 
> Here is the font for testing:
> http://khaledhosny.org/files/tmp/hussaini-nastaleeq_ss01.ttf
> 
> (with something like “ى ىى لى فى”)
> 
> So I’m asking if the current behaviour is “final” and no intentions to
> change it, and if anyone knows how other implementations deal with this
> (the Uniscribe backed seems to ignore the stylistic set completely, so I
> don’t know how to test this).

Yeah, the Uniscribe backend doesn't have features wired up.  You just gave me
perfect reason to hack that in today.

Your best bet is 'locl' I'm afraid.  If Uniscribe compatibility is not an
issue, I'm open to providing HarfBuzz-specific behavior.  For example, to let
Arabic fonts opt in to have all features applied together.

Cheers,
behdad

> Regards,
>  Khaled