[XeTeX] TeX--XeT and OpenType fonts

[Jonathan Kew]

On 01/03/2017 11:59, Joseph Wright wrote:
> Hello all,
>
> With example
>
>     \font\OTtenrm="[lmroman10-regular.otf]/OT"
>     \OTtenrm
>     \TeXXeTstate=1
>     \beginR
>     abc
>     \endR
>     \bye
>
> the output is LTR with TL'16. Is this a known issue?

Yes, this is expected behavior. The TeX--XeT direction controls (\beginR 
etc) control the ordering of words within a line, etc. (slightly more 
accurately, the direction in which nodes in an hlist progress), but do 
not override the inherent directionality of Unicode characters, so "abc" 
is still a sequence of three strong-LTR letters and they stay in their 
left-to-right order.

(However, if you try

   \beginR
   abc def
   \endR

I'd expect you to get output that reads "def abc" because the two words
are ordered RTL, even though each of them remains LTR internally.)

This is why it is possible -- for better or worse -- to do something like

     ...english text {\arabfont العربي} more english

in a xetex document and have the isolated Arabic word appear with 
correct (internal) RTL directionality, without having to explicitly 
surround it with \beginR...\endR (although for a multi-word Arabic 
phrase that would be necessary); the RTL-ness of the characters controls 
their behavior within the word, despite the TeX direction remaining LTR.

Currently, there isn't an option to make the TeX-level direction 
override the Unicode character directionality (comparable to the CSS 
property "unicode-bidi:bidi-override;"). Perhaps that would occasionally 
be useful, though people haven't exactly been clamouring for it AFAIK.

JK