[HarfBuzz] Better handling variation selectors

[Khaled Hosny]

On Tue, Apr 23, 2013 at 10:00:26AM +0900, suzuki toshiya wrote:
> Dear Khaled,
> 
> 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.
> 
> I think the requirement to make visible all characters is reasonable,
> but I'm not sure PRESERVE_DEFAULT_IGNORABLES is the appropriate place
> to serve the request. For example, if ZJW or ZWNJ should be displayed
> visibly, various spaces (e.g. U+2000 - U+200B) are not expected to be
> displayed with clearly different glyphs?

That is certainly valid point, but HarfBuzz already has
PRESERVE_DEFAULT_IGNORABLES, so applying it to variation selectors IMHO
makes it consistent.

Regards,
Khaled