[HarfBuzz] Better handling variation selectors

[Behdad Esfahbod]

Some progress, but there's more to do.  From the commit I just pushed out:

commit 79d1007a501fd63c0ba4d51038c513e6b8b94740
Author: Behdad Esfahbod <behdad at behdad.org>
Date:   Thu Jun 13 19:01:07 2013 -0400

    If variation selector is not consumed by cmap, pass it on to GSUB

    This changes the semantics of get_glyph() callback and expect that
    callbacks return false if the requested variant is not available, and
    then we will call them back with variation_selector=0 and will retain
    the glyph for the selector in the glyph stream.

    Apparently most Mongolian fonts implement the Mongolian Variation
    Selectors using GSUB, not cmap.

    https://bugs.freedesktop.org/show_bug.cgi?id=65258

    Note that this doesn't fix the Mongolian shaping yet, because the way
    that's implemented is that the, say, 'init' feature ligates the letter
    and the variation-selector.  However, since currently the variation
    selector doesn't have the 'init' mask on, it will not be matched...


I'm not quite sure how to address this.  Previously we only used to match the
feature mask on the first glyph in a ligature / sequence.  We changed that two
hackfests ago.

behdad



On 13-06-06 06:00 PM, Behdad Esfahbod wrote:
> On 13-04-20 05:49 AM, Jonathan Kew wrote:
>>
>> The remaining question is what to do about a VS that was part of a valid
>> sequence, but the font being used didn't handle it.
> 
> So, apparently Mongolian fonts in the wild implement the Mongolian Variation
> Selectors using GSUB, not cmap. [1]
> 
> So, we either need to keep the "unused" variation selectors in the glyph
> stream, or remove the Mongolian ones from the cmap processing.
> 
> Fixing this the way I like would require a change to the get_glyph() callback
> API.  Right now we have:
> 
> typedef hb_bool_t
> (*hb_font_get_glyph_func_t) (hb_font_t *font, void *font_data,
>                              hb_codepoint_t unicode,
>                              hb_codepoint_t variation_selector,
>                              hb_codepoint_t *glyph,
>                              void *user_data);
> 
> Now ideally I would have wanted to split that in two:
> 
>   bool get_glyph (unicode)
> 
> and
> 
>   bool get_glyph_variant (unicode, selector)
> 
> but I'm afraid it's too late to make such a change.  Instead, I propose that
> we just change the semantics of the existing call and expect that callbacks
> return false if the requested variant is not available, and then we will call
> them back with variation_selector=0 and will retain the glyph for the selector
> in the glyph stream.
> 
> Makes sense?
> 
> 
> [1] https://bugs.freedesktop.org/show_bug.cgi?id=65258
> 

-- 
behdad
http://behdad.org/