[HarfBuzz] Better handling variation selectors

[suzuki toshiya]

Dear Behdad,

> Though, we still have to assess what
> effect it would have on fonts that implement variation-selectors via GSUB
> instead of cmap.  Does anyone know any fonts like that?

Maybe some Mongolian fonts have VS to be handled via GSUB,
instead of cmap format 14 (because Mongolian VS were standardized
before cmap format 14). Please let me know what I should
check in detail.

Regards,
mpsuzuki

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.  We have to go out of our way to make that
> happen (may be very hard for GSUB).  It just happens that for
> variation-selectors it may be easier.  Though, we still have to assess what
> effect it would have on fonts that implement variation-selectors via GSUB
> instead of cmap.  Does anyone know any fonts like that?
> 
> behdad
> 
>> Regards,
>> Khaled
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>