[HarfBuzz] Tai Tham Shaper in HarfBuzz

[Jonathan Kew]

Hi Ed,

I put together a New Tai Lue shaper for hb-ng a while ago, as a simple example of a script needing minor reordering. The patch was forwarded to the hb mailing list on March 3, 2011 (see message "Shape for New Tai Lue"), if you'd like to have a look; that might provide you with a starting-point.

JK

On 22 Apr 2011, at 17:31, Ed wrote:

> Hi, Behdad and other HB contributors,
> 
> As some of you know, I'm working on creating a Unicode font for the
> Tai Tham script which is one of the many Brāhmī-derived abugidas of
> Southeast Asia (our project URL: http://hariphunchai.unifont.org/ ).
> 
> Like other Indic scripts, Tai Tham shaping is complex and requires
> reordering of certain glyphs.  Currently, no operating system provides
> support for Tai Tham re-ordering.  However Tai Tham's re-ordering
> requirements are much simpler than (for example) those Devanagari.
> 
> As it seems that HarfBuzz is now reaching a more mature state, I am
> hoping that now is a good time to bring up the idea of having HB
> support Tai Tham re-ordering.
> 
> Here, in general, is what is required:
> 
> (1)  U+1A55 TAI THAM CONSONANT SIGN MEDIAL RA needs to be reordered to
> appear *before* a consonant cluster (CC):
> 
>                                    LOGICAL BACKSTORE => VISUAL LAYOUT
>                                    <CC> + <U+1A55>     =>     <U+1A55><CC>
> 
> (2) The following five Prefixed Dependent Vowel (PDV) signs, U+1A6E,
> U+1A6F, U+1A70, U+1A71, U+1A72, need to be reordered to appear
> *before* a consonant cluster (CC) which may include MEDIAL RA (w/MR):
> 
>                                             <CC> + <PDV>  => <PDV><CC>
>                                   <CCw/MR> + <PDV>  =>  <PDV><CCw/MR>
> 
> That's about it!
> 
> There are other features of Tai Tham, such as subjoined consonants,
> but I believe only the 2 features above require specialized shaper
> code while everything else can be achieved using standard OpenType
> features such as GSUB without resorting to specialized shaper code.
> 
> Note that if Tai Tham is implemented, then it will be simple to
> implement other 'Tai' scripts like Tai Viet and New Tai Lue that have
> very similar features.
> 
> Anyone interested in helping sort this out, please contact me.  Thanks!  - Ed
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