[XeTeX] Arabic ligatures

[Jonathan Kew]

On 5 Jul 2004, at 9:12 pm, Musa Furber wrote:

> I started playing around with Arabic in XeLaTeX.
>
> Geeza Pro works well, though it is not close enough to nuskhi (the 
> model for printed books) or thuluth (the model for Qur'an) for my 
> tastes.
>
> Al Bayan Plain is closer. Unfortunately, something odd is happening 
> with the ligatures.
>
>
> Compare this sample using Geeza Pro:
> <pastedGraphic4.tiff>
> with the same using Al Bayan Plain:
> <pastedGraphic5.tiff>
> Notice that in the Geeza sample, there is an alif. It's the vertical 
> line to the left of the blank space between the two words. That alif 
> is missing in the Al Bayan sample.
>
> I checked the sample in TextEdit, which takes advantage of typography 
> gobbledygook. The same thing happened here as with XeLaTeX.
>
> I then checked the sample in Mellel, which does not seem to use 
> typography gobbledygook. There, Al Bayan did not lose its alif.
>
> So, the problem is not XeLaTeX.
>
> Can anyone suggest a solution around this?
>

Well, it's clearly a bug in the AAT tables in Al Bayan. It's got a 
ligature rule to access a special glyph for "Allah", but the ligature 
glyph doesn't actually include the initial alef. I guess maybe Mellel 
is using some other way of rendering Arabic, not through ATSUI; perhaps 
it's using the Presentation Forms codepoints. (If so, I'll be very 
disappointed. But that's off-topic here.) Or perhaps it's just 
disabling optional ligatures.

I'll file a bug report with Apple re Al Bayan, but obviously it'll take 
some time before a fixed version appears.

Meanwhile, I suppose you could insert a zero-width non-joiner between 
the alef and lam, to prevent the ligature formation; but it will 
prevent getting the special ligature for "Allah". And you don't really 
want to clutter your text like that. Better to use a different font. 
Does Baghdad look any better than Geeza to you? Or how about DecoType 
Naskh, if you have it; I'm not sure which systems that shipped with.

Jonathan