[XeTeX] XeTeX bugs in bidirectional typesetting

David Carlisle d.p.carlisle at gmail.com
Sat Nov 19 22:17:46 CET 2016

Vafa Khalighi wrote

>  A) Drop the TeX--XeT model entirely and switch to LuaTeX (Omega)
> bidi model
> Unfortunately Omega's bidi model has its own set of bugs so even if
> XeTeX goes this route, we need to fix quite some issues in its bidi
> model.

Long term I think that this would be a good development for xetex.
Having tex engines with two completely different models for directionality
causes real issues for formats and packages that need to build on the
base provided by the engines. It is clear that of the currently implemented
possibilities the omega/luatex model is more functional, having more
support for right to left text, but also supporting vertical text as well.

If you have a list of issues it would be good to raise them (on the luatex
initially would presumably be best).

> B) Switch to the original TeX-XeT model by Knuth
> I would prefer this approach (considering my over 10 years of
> experience with both TeX--XeT and Omega bidi model) as it is more
> stable than Omega's bidi model. If I recall correctly, Khaled Hosny
> did this in 2013 and when I was testing most of the problems were
> fixed. The only issue was that the original TeX-XeT model adds \beginL
> ... \endL and that would cause some problem in math typesetting
> (sorry, I can not remember what exactly it was about but I remember it
> was reported by David Carlisle). For this problem, one can look at
> TeX--XeT and see how display math are done there and we could apply
> the same approach to TeX-XeT model.

I really could not recommend this, the extra nodes added by tex-xet
interfere with typesetting in ways that are very hard to work around.
Particularly but not limited to math mode.
The only way shown so far to avoid those issues is not to add the nodes,
which essentially leads to tex--xet  rather than tex-xet.

Again, if you have an explicit list of issues with tex--xet that are
not shared with tex-xet it would be good if you could post them here.

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