[luatex] \predisplaydirection from etex in luatex has no effect

Khaled Hosny khaledhosny at eglug.org
Thu Apr 8 13:57:44 CEST 2010

On Thu, Apr 08, 2010 at 11:07:52AM +0200, Taco Hoekwater wrote:
> Vafa Khalighi wrote:
> > 
> > Hi Taco
> > 
> > Thanks a lot for this fix.
> > 
> > The good news is that the bug that exist with pdftex and xetex, does not
> > exist with luatex.
> > 
> > The bad news is that, this fix introduces two new bugs shown in the
> > attached.
> This stuff is trickier then one would expect, because of interactions
> between \mathdir, \pardir, and \leqno / \eqno. So let's forget about
> the ability to set \predisplaydirection manually for a moment, and
> focus on the core functionality first.
> The big question is this: should \eqno and \leqno adhere to \pardir,
> to \mathdir, or simply be physical right and left without regard to
> direction specifiers at all? I don't know the answer to that, so
> somebody has to enlighten me.

May preference is to treat left and right as "start of the line" and
"end of the line", so that existing macro code will give the logically
expected result when one switches direction (one setting equation number
to the right is actually expecting it at the start of the line). If one
wants right and left to be always physical right and left, then this has
to be done at macro not at engine level. In an ideal world, we would use
a start/end nomenclature (lets not forget about vertical typesetting,
even if it isn't working properly right now), but I'm not going to
demand this.


 Khaled Hosny
 Arabic localiser and member of Arabeyes.org team
 Free font developer

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