Private Use areas

[WORDINGHAM RICHARD via Unicode]

> On 21 August 2018 at 01:04 "Mark E. Shoulson via Unicode" <unicode at unicode.org> wrote:
> 
>     It is kind of a bummer, though, that you can't experiment (easily?  or at all?) in the PUA with scripts that have complex behavior, or even not-so-complex behavior like accents & combining marks, or RTL direction (here, also, am I speaking true?  Is there a block of RTL PUA also?  I guess there's always RLO, but meh.)  Still, maybe it doesn't really matter much: your special-purpose font can treat any codepoint any way it likes, right?
> 
>     ~mark
> 
> 
Back in 2006, I was typing the Tai Tham script (then being proposed as the Lanna script) using the PUA and exploring the issue of selecting between what are now <MEDIAL RA> and <SAKOT, RA> based on the preceding character and between what are now <SIGN AA> and <SIGN TALL AA> based on the preceding base character and its subscripts.  I was also looking at using variation selectors to override the rules.  I was using SIL Graphite fonts when they was getting intermittent support in OpenOffice and Firefox - my main display engine was WorldPad.  Nowadays, SIL Graphite seems to be securely supported in LibreOffice and Firefox.  Now, back then, Graphite was at least attempting to support RTL; I would expect the RTL support to work well by now.

On the other hand, experimenting with OpenType is much harder.  The best I've found is transcoding to a Latin range and using an ssxx feature to convert the Latin glyphs back to those for the complex script.  I do that to render Tai Tham in Internet Explorer 11 on Windows 7; this complex scheme is a fallback for when the rendering engine fails.

Richard.
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