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I have been seen terms like multidirectional typesetting, bidirectional typesetting/work, bidirectional editor ...

For example, in this brilliant answer to The differences between TeX engines, there is:

... LuaTeX also incorporates some ideas from Omega and Aleph like multidirectional typesetting ...

... XeTeX having been around longer and being probably the more widely-used for bidirectional work at present ...

Can someone help me to understand what these terms mean, and how they work? Thanks!

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How much detail do you want/need on 'how they work': could get quite technical. –  Joseph Wright Jul 19 at 8:55
    
@JosephWright Thank you very much for the answer, I now understand what bidirectional/multidirectional typesetting means in Tex. As for 'How they work', I think a few XeLaTex and/or LuaLaTex examples are good enough to me. –  YaOzI Jul 19 at 12:59
    
@YaOzl At the low level, setting up bidi/multi-directional stuff is hard (particularly as there are serious technical issues with the TeX--XeT model used in XeTeX). I would strongly suggest looking for 'higher level' interfaces that answer the question of what you want to do: perhaps post a question with your actual use case issue rather than asking about the general case (unless you really want to learn about those low level things). –  Joseph Wright Jul 19 at 13:06

1 Answer 1

up vote 10 down vote accepted

In the Western European tradition, text runs from left-to-right (LTR) across the page, with lines stacked from top-to-bottom (TTB) down the page

1 abcdefgh
2 abcdefgh
3 abcdefgh
....

Arabic, Persian, Hebrew and related middle Eastern systems have a reversal of the line direction, with text running from right-to-left (RTL) across the page. The lines still read from the top of the page down:

hgfedcba 1
hgfedcba 2
hgfedcba 3
....

These two systems reverse the line direction but nothing else: handling both is therefore referred to as 'bidirection' work (LTR and RTL).

In writing systems from the far East, 'lines' of text run from the top of the page to the bottom. Lines can then be stacked next to each other either left-to-right

123
aaa
bbb
ccc
ddd
eee
fff
ggg

(Chinese, Japanese, Korean, ...) or right-to-left

321
aaa
bbb
ccc
ddd
eee
fff
ggg

(Mongolian). As these break out of the model used by European and middle Eastern traditions, these add a multiple' nature to the direction of text.

The TeX--XeT model currently implemented in pdfTeX and XeTeX is currently limited to handling LTR/RTL only, while the Omega-derived model in LuaTeX can also handle the far Eastern top-to-bottom cases. However, even that model is not perfect. John Plaice talked about improving handling for all of these cases in a simple way at TUG 2013: his article in TUGboat is available currently only to TUG members.

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