[HarfBuzz] Detecting mandatory ligatures

[Behdad Esfahbod]

On Mon, Apr 4, 2016 at 12:45 PM, Khaled Hosny <khaledhosny at eglug.org> wrote:

> On Tue, Mar 22, 2016 at 10:52:43PM +0000, Jamie Dale wrote:
> > Hey all,
> >
> > I've spent today fixing some issues in our editable text controls, mostly
> > relating to issues caused by the difference between characters and
> grapheme
> > clusters.
> >
> > I've sorted most of my issues now, but I'm still having an issue the لا
> > ligature in Arabic.
> >
> > My current code that performs picking on text, or applies formatting to
> > text that spans a ligature, assumes that any ligature can be split into
> its
> > component grapheme clusters, however this assumption does not hold true
> for
> > that ligature as it cannot be split.
> >
> > Does HarfBuzz have a way to identify these mandatory ligatures, or
> failing
> > that, how do people generally deal with this sort of thing? I have ICU
> > available if it has anything that can help?
>
> You just don’t try to identify mandatory ligatures. What we are doing in
> Scribus (that bit of code is not published yet) is to treat all
> ligatures as unbreakable. You simply find how many characters in a
>

s/characters/Unicode graphemes/


> ligature, distribute the width on them and find the width of the
> selected part, draw the selection rectangle then render the ligature
> twice once with the highlight color clipped to the width of the selected
> area and once with the regular color clipped to the rest of the glyph
> width. You can try to use hb_ot_layout_get_ligature_carets() to get
> better positions than simply distributing the width over the number of
> components, but very few fonts support it and you will need a fallback
> code anyway.
>
> I believe this is essentially what Firefox does as well.
>
> Regards,
> Khaled
> >
> > Thanks,
> > Jamie.
>
> > _______________________________________________
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> > HarfBuzz at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/harfbuzz
>
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-- 
behdad
http://behdad.org/
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