[HarfBuzz] OpenType "mark" Ligatures ... ?

[Behdad Esfahbod]

On 13-04-23 05:29 PM, Khaled Hosny wrote:
> On Tue, Apr 23, 2013 at 03:59:32PM -0400, Ed Trager wrote:
>> Hi, Behdad and OpenType Experts-at-Large!
>>
>> I'm trying to find a good OpenType solution to a problem in the Tai Tham
>> font that I am developing.
>>
>> The problem, as illustrated in (1) in the attached image, is that the
>> subjoined form of consonant u1A36 TAI THAM LETTER NA or subjoined form of
>> consonant u1A3E TAI THAM LETTER MA may commonly be followed by a subjoined
>> dependent vowel u1A69 TAI THAM VOWEL SIGN U or u1A6A TAI THAM VOWEL SIGN UU
>> and we don't want these to overlap.
>>
>> The solution that I thought would work quite well is to create ligature
>> glyphs with the correct spacing, as shown in (2) in the attached figure.
>> Note that these "ligature" glyphs are "mark" glyphs
>>
>> The lookup table, as one would expect it to be, is shown in (3).
>>
>> However, this apparently does not work!
> 
> Ligatures should work for mark glyphs (I had used them few times, though
> I generally prefer positioning with ‘mkmk’ anchors), so you have to
> elaborate on how you are making your ligatures.

Correct.

> Another possibility is
> that ‘liga’ features or not activated for Thai shaping by default (I
> don’t know), so may be using another tag would help.

Correct.  For Tai *Tham* (ie. in the SEA shaper) we force disable 'liga'.
This is to allow developers to use liga to get the font working on systems
without a dedicated Tai Tham shaper.  You can use, eg. the 'blws' feature to
do what you want to do.  It can even reference the same lookups that 'liga' does.

behdad

> Regards,
> Khaled
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-- 
behdad
http://behdad.org/