[HarfBuzz] Better handling variation selectors

[Khaled Hosny]

On Mon, Apr 22, 2013 at 09:06:53PM -0400, Behdad Esfahbod wrote:
> On 13-04-22 08:20 PM, Khaled Hosny wrote:
> > On Sat, Apr 20, 2013 at 10:49:26AM +0100, Jonathan Kew wrote:
> >> On 19/4/13 22:13, Behdad Esfahbod wrote:
> >>
> >>>   - What should HB_BUFFER_FLAG_PRESERVE_DEFAULT_IGNORABLES do to variation
> >>> selectors?  Right now they do not become visible no matter what.  Is it
> >>> relevant whether the variation selector was actually used during glyph lookup
> >>> or not?
> >>
> >> Yes, I think it's relevant. A variation selector that was used
> >> during glyph lookup has (in a sense) become "visible", manifested as
> >> a particular choice of glyph. I don't think it should -also- appear
> >> as a separate glyph, even in a "show invisibles" mode. In effect,
> >> the <base, VS> pair has been ligated - it's just a "ligation" that's
> >> handled by the cmap instead of GSUB.
> > 
> > But this can be said about many other control characters, ZWJ or ZWNJ
> > for example, they have a visual effect.
> > 
> > IMO PRESERVE_DEFAULT_IGNORABLES should simply show any default ignorable
> > character that is being removed from the output not as a result of
> > OpenType glyph substitution. One use of such feature (the only use?) is
> > to make visible all characters that has mysterious effects on the output
> > so they can be easily checked/edited, and I can see variation selectors
> > benefiting from this just like any other default ignorable.
> 
> 
> Note that if the font has a lookup ligating with ZWJ / ZWNJ (or any other
> default-ignorable) those won't be display even with
> PRESERVE_DEFAULT_IGNORABLES either.

Which is fine since it can happen to any other usually visible glyph and
one can switch fonts to make them visible.

Regards,
Khaled