[HarfBuzz] script segmentation

[Martin Hosken]

Dear Richard,

I would reply to my own message, but that never comes back to me. Thank you (NOT) Google. So here goes.

I've started a discussion document here: https://github.com/OpenType/opentype-layout/blob/master/docs/script_segmentation.md. Please feel free to interact with it. If you are one of the implementers of the segmentation algorithms summarised, please feel free to correct me. I'm certain I've made lots of mistakes in describing this.

Yours,
Martin

> > 1. Do we have a standard algorithm for this?  
> Well, the obvious fix is a per-block default script, just as some
> unassigned characters have a default property of AL or R.  The problem
> comes with Indic scripts, though a default of consonant will often work.
> 
> > 2. Do we want one?  
> I suspect you're the expert.  How well does MultiScribe work on
> Windows?  On Apple systems, the answer for ordinary users is to use
> AAT, and I suspect that will soon extend to Linux applications courtesy
> of HarfBuzz.  I don't know if that would work on ChromeOS.
> 
> On the other hand, in the free world it would be nice to test out
> OpenType fonts.  Several applications already use a Linux sharable
> object for HarfBuzz, and one could in principle replace them with a
> version that already included the new characters.  LibreOffice is one
> such application.
> 
> > 3. How can we make it more future resilient?  
> 
> A mechanism that ascribes properties to PUA points could be extended to
> unassigned characters in general.
> 
> In principal, the USE grammar policeman is a problem.  Combining marks
> can usually be identified by an OpenType glyph category of 'mark', but
> unassigned combining marks are unlikely to get a security clearance, so
> the obvious relaxation will not work.
> 
> Richard.
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