[XeTeX] XeTeX bugs in bidirectional typesetting

[Vafa Khalighi]

Hi


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>
ctan.org
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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