[HarfBuzz] vertical text for RTL scripts?

[Richard Wordingham]

On Mon, 13 Jul 2020 11:24:23 -0400
Phil M Perry <philperry at hvc.rr.com> wrote:

> (my first digest copy arrived moments after posting my previous
> update)
> 
> Richard,
> 
> "Top To Bottom" means that the characters (glyphs) are upright (top 
> uppermost), as they would be written horizontally, but stacked in a 
> column one-at-a-time and reading from the top to the bottom. BTT
> (Bottom To Top) would be the same, except reading from bottom to top.
> 
> I noted that with a cursive script such as Arabic, when written 
> horizontally, connects letters (also there are a lot of ligatures).
> When written vertically, the individual "standalone" forms of letters
> are used, so no connections. I would presume that Mongolian and
> Phags-pa would do something similar. Latin text (and CJK) are
> normally non-cursive (unconnected), as you note.

Jonathan Kew's post spelled out the complexity - one needs to specify
the orientation of the glyphs and the ordering of the lines.  Mongolian
is a TTB cursive script.  When it is written horizontally, the glyphs
are normally rotated.  Apparently users learn it as a CV syllabary, so
separating consonants and vowels is quite unnatural.  Phags-pa appears
to be treated similarly.

Now, Ogham is reported to be a BTT script.  I wonder if we should get
bidi in mixed script contexts?  When Ogham is written horizontally, it
appears as left-to-right and 'cursive'.

I don't think the issue has been solved for spacing marks in Indic
scripts.  I think vowels stay next to their consonants in Thai despite
being letters, but examples of vertical Thai with upright text are
vanishingly rare.

Richard.