[HarfBuzz] A problem in thai shaper

[Martin Hosken]

Dear Behdad,

> >> I don't know about Thai, but the handling of "invalid" Arabic combining
> >> marks in Uniscribe is completely brain dead and a real PITA and I'd
> >> really like not to see HarfBuzz going there, a shaping engine is not a
> >> spell checker and should not enforce any input pattern.
> >>
> >> http://www.microsoft.com/typography/OpenType%20Dev/arabic/shaping.mspx#invalid
> > 
> > Incidentally, I came across this Typophile post, which is one example of
> > why this "invalid" mark handling is not really a good idea:
> > http://typophile.com/node/92130
> 
> Interesting.  I'm undecided about this as of now.

I agree that shaping should not be used to constrain what might be valid sequences. But I think a shaping engine can be used to mark (think dotted circle) sequences that are structurally invalid. By these I mean sequences that would not otherwise show any visual difference from a valid sequence. For example diacritics in the wrong order (not covered by normalization) that show no visual difference (e.g. upper diacritic preceding lower when both have 0 combining order). Such validity will be script specific but not language specific. The aim here is not to limit spellings but to ensure matchable sequencess.

In addition, a shaping engine is not designed to ensure that the lowest common denominator font for a script can handle anything thrown at it.

Yours,
Martin