[HarfBuzz] patch for apply_lookup (applying contextual lookups)

[Behdad Esfahbod]

On 13-09-28 12:09 PM, Jonathan Kew wrote:
> Hi Behdad,
> 
> I've been experimenting a bit with Noto Devanagari, and come up with the
> attached change to apply_lookup(). This fixes the problem where we were
> advancing too far during processing the contextual lookups for the first
> syllable, and hence missing the application of the same lookup at the first
> glyph of the next syllable.
> 
> This greatly improves the results for the Noto Devanagari font (e.g. Hindi
> failures down from 2.57% to 0.408%), as well as for Gujarati (2.57% down to
> 1.08%) and Kannada (27.2% down to 0.095%!) which have similar issues. The
> other Noto fonts are unaffected by the change; I haven't tested against other
> font collections yet.
> 
> Anyhow, please have a look and see if this makes sense at all. I'm far from
> confident that I actually understand what's going among in all these subtables
> and recursive lookup calls.....

Thanks Jonathan.  I've been thinking about this problem in the context of
other issues we've been seeing with Noto Bengali.  I concluded that that piece
of code is too delicate for me to try to touch alone, and as such I added it
to the agenda for the hackfest.  The details are plenty and to get them right
would require at least an hour of active discussion :).  So lets get back to
it in two weeks in Paris!


-- 
behdad
http://behdad.org/