Tex Texin replied to Marco Cimarosti:
>> Right-to-left vs. left-to-right are attributes of arbitrary *spans* of
text,
>> which can easily be mixed within the same paragraph.
>>
>> On the other hand, horizontal vs. vertical are attributes that can be only
>> be applied to a whole paragraph or section.
>
> Marco, is that true? I thought that sometimes numbers for example "123."
> might be written horizontally in the middle of a vertical run.
Marco responded:
> But that would a limited case for horizontal text embedded in vertical text:
> I cannot imagine a real-world situation for a vertical text embedded in
> horizontal text.
And Sampo Syreeni weighed in:
> I think this is something better handled by special-casing in rendering
> software -- the numbers (and whatnot) could be rendered as rotated or
> straight top-to-bottom as well. Considering this, it seems like a
> stylistic variation better controlled by an upper level protocol, if at
> all.
Tex's example may or may not be realistic -- I have no way of knowing -- but
in suggesting a top-to-bottom directional override, I had hoped it would be
possible to represent a run of text such as Tex describes without resorting
to the infamous "higher protocol."
TUS 3.0 states (p. 24): "In contrast to the bidirectional case, the choice to
lay out text either vertically or horizontally is treated as a formatting
style. Therefore, the Unicode Standard does not provide directionality
controls to specify that choice." This may seem arbitrary to some; why
should overrides of default horizontal directionality be a plain-text issue
but overrides of default vertical directionality be a higher-level
"formatting style" issue? I hope this discussion can shed some light on this
question, and possibly help me see what I may be missing.
Actually, there is a more serious problem involved with vertical directional
overrides: They would force the Unicode plain-text mechanism to become aware
of both vertical directionality and directional priority. This sounds
obvious, but in fact there are not two, but THREE issues involved with text
directionality:
1. Horizontal, that is, left-to-right (LTR) versus right-to-left (RTL).
2. Vertical, that is, top-to-bottom (TTB) versus bottom-to-top (BTT).
3. Priority of direction (e.g. (LTR, TTB) versus (TTB, LTR)).
If you think about it, all text of non-trivial length has both horizontal and
vertical directionality, and also a priority to the directionality.
Horizontal and vertical directionalities are not opposites, they are
complements. The Latin script is written (LTR, TTB) which means not only
that there is a horizontal directionality of left-to-right and a vertical
directionality of top-to-bottom, but also that the horizontal directionality
takes precedence over the vertical. That is, we complete a horizontal (LTR)
line before moving down the page (TTB) to start another line.
According to TUS 3.0,
Latin and most other European scripts are (LTR, TTB).
Arabic and most other Middle Eastern scripts are (RTL, TTB).
Ogham is either (LTR, TTB) or (BTT, ???).
Han is traditionally written (TTB, RTL) and more recently (LTR, TTB).
Mongolian is written (TTB, LTR).
Unicode characters have a default directionality, but both this and the
override mechanism cover only the horizontal aspect, not the vertical aspect
or the priority of one over the other. Thus, Mongolian characters are
assigned the same directionality code as Latin ("L") even though the TTB
directionality takes precedence over the LTR, the opposite of Latin. And
there is no plain-text way to indicate the alternative directionality of
Ogham or Han.
An elaboration of the directional override mechanism to handle vertical
directionality would have to take priority into account as well. Instead of
two directionalities, LTR and RTL, the Unicode Standard would have to
consider eight. The Bidirectional Algorithm might have to become
Octodirectional, with a commensurate increase in complexity. Perhaps this is
the problem that is avoided by declaring vertical directionality to be a
higher-level "formatting style" issue. But it still seems arbitrary.
-Doug Ewell
Fullerton, California
