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

[Khaled Hosny]

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. 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.

Regards,
Khaled