[HarfBuzz] Fwd: ZWNJ is being ignored

[Jonathan Kew]

On 30/4/13 21:49, Behdad Esfahbod wrote:
> On 13-04-28 05:27 PM, Georg Duffner wrote:
>> Hi,
>>
>> I posted this on the XeTeX list. Khaled meant it could have something to do
>> with
>> http://cgit.freedesktop.org/harfbuzz/commit/?id=607feb7cff0e50f8738d2e49ca463fc9d7d494de
>>
>>
>> Recently I’ve changed the behaviour of liga. The f-ligatures are now formed by
>> component glyphs which get substituted by means of contextual chaining
>> lookups. Thus, instead of
>>
>> sub f f by f_f; the lookup is now sub f' f by f._f;
>
> What you should be doing is:
>
> sub f' f' by f._f f;
>
> Not sure if the format supports that.  However, I understand that this would
> cause a problem in that every other f won't be substituted.
>
> We really do want to ignore ZWNJ in backtrack/lookahead.  The problem is that
> you are abusing those contexts to do a ligation using single substitutions.
>
> Jonathan, do you have any suggestions?
>

Presumably, the way to handle this as a font developer is to explicitly 
handle the ZWNJ, by adding something like

   sub f' zwnj f by f;

ahead of the existing rule. That should in effect block the "ligature" 
when ZWNJ is present, if I'm understanding correctly.

JK