[XeTeX] XeTeX bugs in bidirectional typesetting

Vafa Khalighi vk489 at uowmail.edu.au
Sat Nov 19 16:23:04 CET 2016


XeTeX uses TeX--XeT algorithm and has some serious problems in typesetting right to left text. About few years ago, I wrote to Peter Bre­it­en­lohner and we discussed together what needs to be fixed. Peter promised that he would fix the issues but unfortunately he passed away. I believe he at least fixed one of the bugs I reported to him.

Bidirectional typesetting is a very fundamental area which unfortunately has been ignored so far. I can think of two ways to improve the situation:

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.

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.

Once this is done, we can also look at ParsiTeX source[1] which is also based on TeX--XeT and add few primitives which are essential for bidi such as \Rhalign (making RTL tables).

Conclusion: I am not going to sit and watch like before! if XeTeX bidi model is not improved until TeXLive 2017, I am going to fork XeTeX, rename it to something else and make the changes I need. This is indeed last time I try to talk about it with the XeTeX team. I have done it numerous times and each time the bidi problems were ignored.

[1] http://ctan.org/pkg/parsitex

CTAN: Package parsitex<http://ctan.org/pkg/parsitex>
Sugges­tions. Maybe you are in­ter­ested in the fol­low­ing pack­ages as well. nts: New Type­set­ting Sys­tem; vtex-free: T e X sys­tem and PDF sup­port ...

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