Private Use areas - Vertical Text

[WORDINGHAM RICHARD via Unicode]

> 
>     On 27 August 2018 at 15:22 Peter Constable via Unicode <unicode at unicode.org> wrote:
> 
>     Layout engines that support CJK vertical layout do not rely on the 'vert' feature to rotate glyphs for CJK ideographs, but rather rotate the glyph 90° and switch to using vertical glyph metrics. The 'vert' feature is used to substitute vertical alternate glyphs as needed, such as for punctuation that isn't automatically rotated (and would probably need a differently-positioned alternate in any case).
> 
>     Cf. UAX 50.
> 

There have been some pretty confused statements. I believe the observed problem is that PUA characters for Zhuang CJK ideographs get rotated when displayed vertically rather than left-to-right.

Unicode is doing what it can in this matter:

(a) Zhuang PUA characters are being made individually obsolete.

(b) By default, PUA characters have the value of Vertical_orientation=upright as do CJK ideographs.

For CJK ideographs, it is not clear to me when the vert feature (if present) would be applied.  Is it only for some codepoints (vo=tu), or is it for all that the engine expects to be displayed ‘upright’ in vertical text?  The vrtr feature (if present) would be applied when glyphs are to be rotated.  Is it for all such glyphs, or only those for which rotation is expected to be inadequate (vo=tr)?  It seems that feature vrt2 is to be applied to all glyphs; perhaps rotation is the default behaviour when there is no look-up value for a glyph that the engine expects to be rotated.  The truly difficult case would be when there is no attempt to apply a look-up – possibly vrtr would not apply to /p{vo=r}.

I would expect that defining the lookup vrt2 or vrtr to map Zhuang glyphs to themselves (or something prerotated) would cure the problem.  This would not work for sequences of Zhuang ideographs treated as RTL text - but that is unlikely to happen.

Richard.
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