[HarfBuzz] Contextual shaping of Malayalam post(pre)/below base forms

[Behdad Esfahbod]

On 13-06-26 07:59 PM, Richard Wordingham wrote:
> On Wed, 26 Jun 2013 19:09:54 -0400
> Behdad Esfahbod <behdad at behdad.org> wrote:
> 
>> Humm.  Ok, now I know.  In the particular sequence y1 is the base,
>> right?  If yes, then it is NOT marked for the pref feature.  <pref>
>> is applied to all characters after the base.
> 
> Glyph y1 was the base in my test case, but the problem still applies
> if I expand the input to the unlikely sequence <PA, VIRAMA, YA,
> VIRAMA, RA> (as in the string പ്യ്രക്രഖ്ര ).  The <VIRAMA, RA> still forms the
> prebase ra.
> 
>>  I believe that's why your original design wasn't working.
> 
> The logic to mark for the <pref> feature in 0.9.18 is 
> 
>     for (unsigned int i = base + 1; i + 1 < end; i++) {
>       hb_codepoint_t glyphs[2] = {info[i].codepoint, info[i +
> 1].codepoint};
>       if (indic_plan->pref.would_substitute (glyphs, ARRAY_LENGTH
>       (glyphs), true, face))
>       {
>         info[i++].mask |= indic_plan->mask_array[PREF];
>         info[i++].mask |= indic_plan->mask_array[PREF];
>         ...
> 
> in function initial_reordering_consonant_syllable() in
> hb-ot-shape-complex-indic.cc.
> 
> I surmise that the marking is more specific for <pref> because pre-base
> rearrangement looks for an isolated glyph marked for <pref>; this
> should work, because Uniscribe forbids Malayalam from having two
> pre-base ra's in the same syllable.

Right.  That's why, indeed.


>> Still I'm
>> interested to double check that Uniscribe does the same.
> 
> Test font is being sent off-list.
> 
>>> I haven't yet worked out how HarfBuzz distinguishes a pre-base
>>> <VIRAMA,
>>> RA> ligature formed by the <pref> feature from a post-base <VIRAMA,
>>> RA> RA>
>>> ligature formed by the <pstf> feature,
> 
>> It doesn't.
> 
> One fix would be to remove <pref> marking from consecutive pairs of
> glyphs immediately after the <pref> feature has been applied.  I can't
> quantify the practical benefit.

Yeah, lets wait until someone needs that and then try to figure out what
Uniscribe does.

-- 
behdad
http://behdad.org/