[HarfBuzz] vertical text for RTL scripts?

[Richard Wordingham]

On Sun, 12 Jul 2020 10:15:31 -0400
Phil M Perry <philperry at hvc.rr.com> wrote:

> (A week ago I sent an earlier version of this query, as an
> unregistered user, and it doesn't seem to have shown up. I apologize
> if this is a duplicate. I'm registered to receive the digest now.)
> 
> I have been using the Perl library HarfBuzz::Shaper to provide 
> HarfBuzz-based complex script support for PDF::Builder (PDF creation 
> library). LTR Latin script (English) and CJK script (Chinese) work as 
> expected when rendered vertically (TTB or BTT).

What do you mean by top-to-bottom?  Are the tops of letters
then uppermost or rightmost?  In some scripts, such as Mongolian and
Phags-pa, changing the direction of the text like that changes the
orientation of the characters, so the parts that were neighbours
before remain neighbours.  The difference between those two and CJK is
that the first two are cursive.

Richard.