Cc: harfbuzz@lists.freedesktop.org
From: Phil M Perry <philpe...@hvc.rr.com>
Date: Mon, 13 Jul 2020 11:11:51 -0400
Eli, I realize that (except for Chinese, Japanese, and possibly Korean),
text is normally written horizontally (LTR or RTL). Vertical text is for
special uses such as signage and advertising.
Yes, I understand that, and was replying with that in mind.
Anyway, I'm still not sure what the convention is for writing vertical
text in RTL languages. There's not much discussion of this online,
except for "I want to get a Hebrew tattoo down my spine saying 'daughter
of Jehovah' -- which way will read correctly?" The convention for LTR
scripts is to start at the top and grow downwards, which is like taking
the original LTR coordinate system and rotating it 90 degrees clockwise
(with individual letters rotated back). The next line (column) is to the
LEFT.
Yes, agreed.
For RTL, my sources suggest that the last letter input (first one
read)
This is fundamentally incorrect: both input from keyboard and reading
are done in the same order, even for RTL languages. The only order
which is reversed for RTL languages is the left-to-right order on
display: the first RTL letter read is generally the rightmost, unlike
with LTR scripts.
I think the above observation is important, because I'm guessing it is
the basis of your confusion regarding the vertical layout. In the
vertical layout, the left vs right issue no longer exists (at least as
long as we are talking about a single column), so the distinction
between LTR and RTL scripts also disappears.
Therefore:
should be at the TOP of the text column, which means rotating the
original horizontal coordinate system 90 degrees COUNTERclockwise. For
TTB of a RTL script, it is like a clockwise rotation, with the first
input letter at the top, but reading from the bottom/original right.
No, the first input letter is at the top, and the first one you read
is also at the top.
Embedded LTR text is read TTB. For BTT, it is like a COUNTERclockwise
rotation, with the first input letter at the bottom, reading from
top/original right. Unfortunately, this leaves embedded LTR text
backwards from what would be expected
No. Embedded LTR text will also be laid out TTB, i.e. without
reordering it.
In short, in vertical layout there's no bidi reordering at all: both
LTR and RTL characters are displayed in the logical order, top to
bottom.
