[HarfBuzz] Tai Tham / Lanna (iso15924="lana") shaping question

[Behdad Esfahbod]

On 05/18/2012 04:02 PM, Ed Trager wrote:

> The first area of investigation is prefixed dependent vowel symbols.
> In Tai Tham, these are the 5 letters U+1A6E → U+1A72
> ( specifically: U+1A6E TAI THAM VOWEL SIGN E, U+1A6F TAI THAM VOWEL
> SIGN AE, U+1A70 TAI THAM VOWEL SIGN OO, U+1A71 TAI THAM VOWEL SIGN AI,
> and U+1A72 TAI THAM VOWEL SIGN THAM AI ).
> 
> My initial look at this today strongly suggests that the latest
> HarfBuzz-ng  will only "shift" these dependent vowel signs over to the
> left by a *SINGLE* base consonant:  HarfBuzz appears to have no
> knowledge about *consonant clusters* in Tai Tham script.
> 
> Here is an example:  For the purpose of this email, let's suppose:
> 
> * "E" represents U+1A6E VOWEL SIGN E
> * "P" represents U+1A38 CONSONANT HIGH PA
> * "L" represents U+1A56  CONSONANT SIGN MEDIAL LA
> 
> In the logical backstore of the string, we type "PLE" (sounds
> something similat to the English word "play").
> 
> In Tai Tham, U+1A6E VOWEL SIGN E needs to be shifted all the way to
> the left so that the final visual appearance would be:

Are you sure?  Without U+1A60 TAI THAM SIGN SAKOT before the subjoined
consonant?  Reading Unicode suggests that you need that sign betwee PA and LA.
 In which case, HarfBuzz will recognize the entire thing as one syllable and
you get the vowel sign correctly shifted all the way to the left.

behdad


> "EPL"
> 
> (in this email we will ignore the fact that in reality the "L" needs
> to be subjoined and hang below the "P")
> 
> But what I get from HarfBuzz is only this:
> 
> "PEL"
> 
> ... which is of course wrong.
> 
> Can someone please confirm that, based on the current code, the
> expected behavior of HarfBuzz at this point in time cannot do anything
> other than what I have just described?
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