[HarfBuzz] Hebrew composition to presentation forms

[Khaled Hosny]

On Sun, Feb 02, 2014 at 09:45:16AM +0000, Jonathan Kew wrote:
> On 2/2/14 01:13, Khaled Hosny wrote:
> >Hi,
> >
> >Someone reported an issue with the hireq placement under the yodh with
> >Ezra SIL font[1]. When I checked this, it seemed to be because HarfBuzz
> >is composing U+05D9 + U+05B4 to U+FB1D and the font has a glyph for
> >U+FB1D that has a not so good placement for hireq.
> >
> >I thought this composition is result of Unicode normalisation, so
> >HarfBuzz is doing the right thing, but the comment in
> >hb-ot-shape-complex-hebrew.cc:75 indicates otherwise. I’m no very sure,
> >but I feel this kind of composition should fits more into fallback
> >shaping like done with Arabic and not something to be done
> >unconditionally, WDYT?
> 
> This is a difficult call. Note that U+FB1D does have a canonical
> decomposition to <U+05D9, U+05B4>; the comment in
> hb-ot-shape-complex-hebrew.cc relates only to the fact that these Hebrew
> presentation forms are excluded from the composition rules for NFC; thus,
> both NFC and NFD representations use the decomposed sequence.
> 
> Nevertheless, the two representations *are* canonically equivalent, and
> therefore it's appropriate that they should be rendered the same.

I was not sure about this, given that it is decomposition only, but I’ll
take your word on this.

> IMO, this is a font bug in Ezra SIL; if a font has positioning rules for yod
> + hireq, and also has a precomposed yod-hireq glyph, the two should look
> identical. A font that gives the impression that entering U+FB1D will result
> in one appearance, while <U+05D9, U+05B4> will result in something
> different, is misleading its users.

Indeed.

> As for what harfbuzz should do: currently, it deliberately uses the
> precomposed Hebrew presentation-form glyphs, because there are many
> (generally older) fonts out there that lack good (or any) mark positioning
> rules, and so decomposed sequences look terrible. Using the presentation
> forms gives a much better result.
> 
> However, perhaps we should try to be more sophisticated, and do something
> like "compose to the presentation forms if the font doesn't have GPOS mark
> positioning; otherwise prefer decomposed sequences".

Yes, this what I had in mind, similar to what HarfBuzz currently does
for Arabic presentation forms shaping.

Regards,
Khaled