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I am trying to find out how to have Kashida justification in Luatex (in TexLive) for bidi text.

Background:

  • I can see by searching that support might appear in babel here but nothing implemented yet.
  • In Xetex (bidi or xeperisan ?), Kashida justification is implemented by \XetexInterCharToks, using \hrule but not the glyph ( which was visually problematic in letter joints).
  • Using (stretching) the Kashida glyph directly or applying the new variable fonts feature are the two alternatives.
  • I also noticed that \XetexInterCharToks can be implemented in Luatex in someway here.
  • Previously, I asked about finding the width of glue space between chars here.
  • No idea, if harfbuzz implementation (any support for Kashida ?) can be related. Harfbuzz can be used through luahbtex here

Edit (About Kashida):

Kashida (from keshideh which means "stretched" in Persian) is a dash-like justification that can be added between (actually to the end of the first letter) some letters in Arabic/Persian/Urdu scripts (and probably Hebrew see here). Its Unicode character is (U+0640), also known as Arabic Tatweel.

It has two functionalities:

  1. aesthetically: more beautiful typesetting and a replacement for white-space justification
  2. It can be used in stressing some words, as a replacement for capital letters or small-caps which do not exist in those scripts.

One can add manually the default character ((U+0640)) as many time as it seems appropriate:

کشیده
vs
کشیـــــــده

However, (1) any use of Kashida letter might create problem when searching for a word, and (2), the right amount of width of Keshida is not a proper multiple of the default width of it in a font. Therefore, a right solution might be to use variable fonts technology. This also can fix another problem that Kashida is not always a stretchable dash but it can be/should be a stretchable curve part of a glyph.

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    You can't implement \XetexInterCharToks in LuaTeX like this anymore because the token_filter callback is gone. – Henri Menke Dec 11 '19 at 6:35
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    in luatex rather than using interchar tokens and two runs to see the stretched glue you could probably do it in one pass by adjusting a glue node in the post linebreak callback – David Carlisle Dec 11 '19 at 7:50
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    for harfbuzz you probably want to watch this issue github.com/harfbuzz/harfbuzz/issues/586 – David Carlisle Dec 11 '19 at 7:53
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    What is Kashida? I can guess from your question that it has something to do with spaces but the rules are unclear. – Ulrike Fischer Dec 11 '19 at 7:55
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    @UlrikeFischer stretchy line-y character thing that can stretch to avoid stretching space. en.wikipedia.org/wiki/Kashida – David Carlisle Dec 11 '19 at 7:57

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