>
> On 27 August 2018 at 15:22 Peter Constable via Unicode
> <[email protected]> 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.
