[HarfBuzz] vertical text for RTL scripts?

[Jonathan Kew]

On 13/07/2020 17:45, Eli Zaretskii wrote:
>> Cc: harfbuzz at lists.freedesktop.org
>> From: Phil M Perry <philperry at 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.

Just to confirm the expectation here from the CSS layout world: in terms 
of CSS properties, this would correspond to the use of 
writing-mode:vertical-rl or vertical-lr (depending which direction you 
want the columns of text to progress), together with 
text-orientation:upright to request characters from naturally-horizontal 
scripts to remain upright, stacked above each other (rather than each 
entire line of horizontally-laid-out text being rotated 90° to be 
written "sideways").

In this case, the CSS writing modes spec[1] is clear that characters 
from both horizontal-LTR and horizontal-RTL are to progress downwards; 
all characters are treated (for inline bidi-reordering purposes in the 
vertical line) as though they had LTR directionality.

JK

[1] 
https://drafts.csswg.org/css-writing-modes-3/#valdef-text-orientation-upright