[XeTeX] Ligatures and searching in PDFs

Gareth Hughes garzohugo at gmail.com
Thu Jun 10 19:26:12 CEST 2010

David J. Perry wrote:
> I am curious; are you using standard Unicode Syriac fonts?  In such
> fonts, there is no need for, nor should there be, PUA assignments for
> the joined shapes.  (And any font whose maker puts joined shapes
> "somewhere that's going to spare" needs to go back to Unicode 101 and
> learn some good practices.  There is no such place in Unicode and
> putting one's private characters in codepoints marked reserved or used
> for other scripts is really bad.)  I just looked at the Estrangelo
> Edessa font and it (correctly) has no PUA assignments for other than the
> isolated shapes.  (If you are using older fonts, created before Syriac
> was supported in Unicode, of course there will be all sorts of
> nonstandard things.  But we can't use those to judge whether XeTeX is
> doing the right thing.)
> Another fundamental question is whether Adobe even claims that rtl or
> mixed directional text can be searched or copied correctly from a PDF. 
> I did some googling on RTL support in PDFs and didn't really find an
> answer.  But the overall support for RTL in PDF seems pretty spotty,
> which is perhaps not surprising given Adobe's track record with RTL in
> other products such as InDesign.  So the non-searchable PDFs may not be
> the fault of XeTeX.  If you or anyone else knows the answer, please let
> us know--I agree with you completely that it is an important issue.
> David

Thanks, David.

In Estrangelo Edessa the joined glyphs are 'unmapped' (don't have
Unicode code points). So, is it that they are unmapped that makes them
unsearchable or that PDFs baulk at RTL scripts? Some Latin ligatures
have Unicode code points at U+FB00-6. The Unicode blocks Arabic
Presentation Forms-A and -B provide the joined forms for Arabic-based
scripts, so it can be searched and copied from a PDF (although when
pasted I get the isolated forms separated by spaces, which is still
better than copying the raw joining glyphs). This would suggest that a
similar thing would be possible for Syriac too, but, like a 'Th'
ligature, would we need to have explicit Unicode code points for
these to work properly? For starters, how much is possible within XeTeX,
and how much would require mass lobbying of Adobe or the Unicode
Consortium to make it work?


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