[HarfBuzz] 'calt' in Indic shaper

[John Hudson]

Dear Behdad,

>> So apparently Uniscribe isn't applying 'calt' by default in Gujarati. That
>> kinda seems reasonable, given that the Indic specs refer to it as
>> "Discretionary presentation forms", although it differs from Adobe's
>> registration of the feature (targeted at cursive Latin fonts), which says it
>> should be active by default.

I don't think that is what what 'discretionary' usually means in the 
contexts of Microsoft's Indic font specs. If 'calt' is not being applied 
in Gujurati, I think that might be a bug and something you should query 
with Andrew Glass (currently responsible for Uniscribe, and cc'd).

>> In the interests of consistency with Uniscribe (and therefore to minimize
>> surprises for font developers), I think we should probably remove it from the
>> Indic shaper.

> But it sounds like an interesting feature for font developers.  How about
> other features that we apply that Uniscribe may not be applying?  clig, curs,
> rclt?  But then again I see that the MS spec singles calt out.  I'll push this
> out, but like to hear from John Hudson and other Indic font developers.

 From a font developer perspective, the preference is that 'calt' is 
always (for all scripts) on by default but possible to turn off. If that 
is not possible because software does not provide a UI to toggle the 
feature, then I think the fallback preference would definitely be for 
the 'calt' feature to be always on. Because there has not been until 
very recently a required contextual alternates feature, font developers 
have often used 'calt' for substitutions that are really intrinsic to 
particular styles of type, and should not be disabled.

With regard to 'consistency with Uniscribe', I am wary of that as a 
criterion for layout behaviour. Sometimes what Uniscribe does is 
difficult to correlate with the font specs for a given script 
(Malayalam, for instance), and as I documented there are serious issues 
for Indic typography that result from failure to apply lookups across 
cluster boundaries:
http://www.tiro.com/John/Problems_for_Indic_Typography.pdf


Regards, John




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