[XeTeX] Problems with some "exotic" RTL scripts

François Charette firmicus at ankabut.net
Wed May 9 18:16:40 CEST 2007


Jonathan Kew a écrit :
> I'd think that it is impossible to judge "properly implemented"  
> without a shaping engine specification.
>   
You're right. But since N'Ko contextual shaping is a more or less 
identical to Arabic, I naïvely assumed that it would be taken care of in 
a generic manner by ICU or Pango. Thanks for clarifying this.

> I'm not surprised by this; the ICU layout library has no shaping  
> engine for N'Ko, as far as I know. Presumably Pango doesn't either.  
> Nor do I see a specification for OpenType features for N'Ko on the  
> Microsoft typography site; do you know if there is any information  
> available? It would presumably be a fairly simple extension to the  
> Arabic engine, but without a font spec, any implementation is going  
> to be an arbitrary, hit-or-miss affair.
> While I'd love to have complete support for everything :-) the  
> reality is that N'Ko is not at the top of my personal priorities for  
> now. But if someone cares to offer a patch, either for XeTeX or  
> (better) directly to the ICU project, I'd be happy for it to be  
> included in due course.
>   
By all means, of course do work according to your priorities! My idea 
was simply to test bidirectionality with various not-so-common scripts. 
I do not think I will use N'Ko ever again in my life ...

> Old Italic is encoded in Unicode with left-to-right directionality;  
> see The Unicode Standard 5.0, p 474. Right-to-left rendering will  
> require directional overrides (and currently, you'll have to apply  
> these to each word in XeTeX; this is a shortcoming of the  
> implementation).
>   
I have checked the Unicode Standard 4.0 online (since I don't have the 
5.0 book at hand). I understand why LTR has been given preference, 
despite the fact that the vast majority of historical inscriptions are 
written RTL (*). Yet in the two fonts I mentioned, the glyphs are facing 
left, which makes them adequate for RTL rendering. For LTR rendering 
they should be mirrored.

(*) Note: This is also the case with the current proposal for 
Hieroglyphic Egyptian. (See 
http://www.evertype.com/standards/iso10646/pdf/towards-egyptian.pdf).

Anyway, if there is demand for supporting right-to-left Old Italic with 
XeLaTeX I could perhaps write a little package to automate the 
directional overrides, and write a font-mapping that provides a 
convenient input encoding ... But the likelihood that an Etruscanist 
reads this list cannot be very high... ;-)


Cheers
F



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