[HarfBuzz] Tai Tham NGA, SAKOT is not Kinzi

[Richard Wordingham]

On Thu, 25 Apr 2013 19:22:17 +0700
Theppitak Karoonboonyanan <thep at linux.thai.net> wrote:

> On Thu, Apr 25, 2013 at 8:55 AM, Richard Wordingham
> <richard.wordingham at ntlworld.com> wrote:
> > On Sun, 21 Apr 2013 22:07:53 +0700
> > Theppitak Karoonboonyanan <thep at linux.thai.net> wrote:
> >
> >> This could be the solution we're seeking. But how should the font
> >> do the signalling?
> >
> > The method used before was the presence or absence of a certain
> > substitution within some feature.
> >
> > [in another mail]:
> >
> > According to
> > http://lists.freedesktop.org/archives/harfbuzz/2013-January/002842.html ,
> > the need for MEDIAL RA to be reordered is signalled by having a
> > lookup for it in the 'pref' feature.  (I am not sure of the point of
> > implementation of this.)  The same could be done for MAI KANG LAI in
> > the 'abvf' feature.


> In fact, the signal for that case is actually the presence of 'lana'
> script tag. Harfbuzz will ignore all tables for 'DFLT' script tag if
> 'lana' tag is present, to avoid the conflict with fallback rules.

I'm not sure it has been implemented as described - perhaps this
intended change has been abandoned.

> So, what should be the contents of 'abvf' for signalling? Full
> reordering rules or just emptiness? The latter is quite awkward
> design, IMHO. For the former, I think it could be a single 'ccmp'
> fallback with both 'DFLT' and 'lana' script tags instead, and
> Harfbuzz doesn't need to handle Mai Kang Lai at all, *except*
> allowing GSUB rules to be applied across text clusters.

Having slept on the idea, I think 'pref' would be a more appropriate.
As this is for items *before* the base consonant, it makes sense to
exclude adjustments to MAI KANG LAI from that feature unless it is in
some sense coming from before the base consonant.  Also, 'pref' is the
first feature applied after rearrangement.

The necessary condition for re-ordering would be the presence of the MAI
KANG LAI glyph (as defined by the cmap table) in the initial character
coverage for the 'pref' feature in GSUB.  I'm not sure if it is
sufficient, or whether the lookup might have to be a single
substitution.  A bigger design change would be to require it in the
'rphf' feature, used for kinzi in the Myanmar script.  While this would
be even better, at present the SEA shaper ignores the 'rphf' feature.

> > I assume you're using the ligature substitution (look-up type 4)
> > followed by the mulitiple substitution (look-up type 2).  One
> > solution is to use different glyphs for swapped and unswapped MAI
> > KANG LAI.
> 
> OK. I've tried it and it works. The problem is that it only works in
> Fontforge metrics window, not on Harfbuzz. It looks like Harfbuzz
> does not allow GSUB rules to be applied across text cluster boundary
> currently (while old Pango does).

Which feature did you put it in, and which HarfBuzz shaper did you use?
In HarfBuzz, the features 'pref', 'abvf', 'blwf' and 'pstf' do not work
across syllables in the SEA shaper.  I don't think 'locl' and 'ccmp'
work across syllable boundaries in the SEA shaper either.

Without the ability to disable the rearrangement, editing the Khuen
spelling ᩋᩘᨩᩮᩥ᩠ᨬ to the Maefahluang spelling ᩋᩢ᩠ᨦᨩᩮᩥ᩠ᨬ could be hard
work. At the very least, <MAI KANG LAI, LOW CA, E, I> will be a single
cluster!

Richard.