[HarfBuzz] Tai Tham NGA, SAKOT is not Kinzi

[Richard Wordingham]

On Wed, 1 May 2013 15:50:54 +0700
Theppitak Karoonboonyanan <thep at linux.thai.net> wrote:

> On Wed, May 1, 2013 at 2:39 PM, Richard Wordingham
> <richard.wordingham at ntlworld.com> wrote:

> > (b) That word is not a problem for a Tai Khuen font.  How should a
> > Lao Tham font handle it?

> No need to handle it, because Mai Kang Lai is not that heavily used
> in Lao Tham. It should rather be spelt as ᩋᩢ᩠ᨦᨠᩕᩥ᩠ᩈ instead.

That depends on whether one expects to have to choose the font
according to the spelling habits of the font.  I would hope a font
could handle all the languages, though of cause it need not be beautiful
for all languages.

> >  How would it handle the Pali word _sankilesa_
> > spelt with MAI KANG LAI - I would expect text written for a Tai
> > Khuen font to use MAI KANG LAI.
> 
> The conflict like this is usually avoided in Lao Tham manuscripts by
> using alternative spelling, probably ᩈᨦ᩠ᨠᩥᩃᩮᩈ. (I haven't found a real
> sample of this yet,
> just imply it from the tendency of other cases.)

That is what I would expect - but see my comment above.  Possibly a Lao
Tham font could protect itself by adding GSUB rules such as

<KA, MAI KANG LAI> becomes <NGA, <SAKOT KA>> before SIGN I, though
the rearrangement is not nice.  Perhaps the solution for a Lao Tham font
is a special <KA, MAI KANG LAI> ligature that looks very like <NGA,
SAKOT,
KA>.   

> > (i) ᩁᩘᩈᩦ <RA, MAI KANG LAI, SA, SIGN II> 'ray of light'
> > (ii) ᩈᩘᨠᩕᩣᨶ᩠ᨲᩴ <HIGH SA, MAI KANG LAI, HIGH KA, MEDIAL RA, AA, NA,
> > SAKOT, HIGH TA, RA HAAM> 'songkran'

> So, the question is for Lanna. Do such spellings exist, and how are
> they handled? If they exist, yes, the rendering engine 'must' handle
> it.

The problem with handling the differences when reordering is that Lao
Tham reorders <MAI KANG LAI, HIGH KA, MEDIAL RA>, but the MFL style
does not.  This gives us at least three different reordering behaviours.

As to whether these spellings exist, I wish I had manuscript evidence.

> >  I'm still some way
> > from being ready to test the ability of GPOS to undo shaping.  The
> > sort of GPOS rule I have in mind is:

> > Context (lookup type 7): consonant consonant <MAI KANG LAI>
> > Lookup for context: At position 0, mark to base (lookup type 4),
> > skipping bases and other marks, to position MAI KANG LAI on the
> > consonant.

> > In example (i) above, rearrangement would deliver <g(RA), g(SA),
> > g(MAI KANG LAI), g(SIGN II)>, and we need to be able to attach MAI
> > KANG LAI to RA.  I don't know if this sort of rule works.
> 
> I think having the rendering engine do it should be simpler.

The problem is that the reordering logic is not very flexible.  We may
need GPOS rules to handle the exceptions that vary from style to style.

Richard.