[XeTeX] Handling of combining and variant selector characters in math

[David J. Perry]

I know nothing whatsoever about math, so perhaps I shouldn't even join 
this discussion, but I am curious.  I do have considerable experience in 
font development and supporting things like the use of combining marks 
and variation selectors outside of math contexts.

I looked in a font editor at the font (XITS) that was used to produce 
the sample PDF.  As far as I can see, it has no support for combining 
marks or variation selectors of the sort that I would expect, based on 
my non-math experience.  (For instance, fonts that support combining 
marks usually have a Mark to Base lookup.)   Is there an expectation 
that in TeX the math typesetting can properly position combining 
characters and handle variation selectors without support from the font?

David

On 2/9/2016 8:54 AM, David Carlisle wrote:
> Sending this separately to xetex  luatex lists...
>
> The current Unicode math alphabets in U+1D400 ...include a "script"
> alphabet this was intended to cover both "caligraphic/chancery" and
> "script/roundhand" as font variants, in the hope that no document
> would need both.
>
> Unicode is considering adding separate markup for the two forms, see
> Murray's blog here
>
> http://blogs.msdn.com/b/murrays/archive/2016/02/05/unicode-math-calligraphic-alphabet.aspx
>
> As explained in the article at least two possible suggestions are
> being considered:  adding the new alphabet in a new code block range,
> or defining  "variant selector" characters that would force one or
> other interpretation.
>
> Unicode combining characters following the base have always been a bit
> tricky in TeX so I wondered whether the engine (or font) developers
> (as opposed to macro level hacker like myself) have a view on what is
> a reasonable input form here.
>
> You could either reply here or as a comment on the above Blog.
>
> I attach a (latex) text file that produces more or less the same
> output in xetex and luatex showing that by default neither a simple
> combining character like e-acute nor VS1 work but the combining acute
> at least can perhaps be made to work but VS1 seems tricky as the base
> is a \mathop atom so it's not really amenable to being combined with
> the following variant selector character.
>
> Since current combining character use seems tricky I'm worried about
> the suggestion to use that method for selecting the entire script
> alphabet.
>
> (The combining acute could be normalised away by running filter to NFC
> form but don't do that as it's just standing in for a possible new
> character to switch script forms:-)
>
> Tests 0 and A show that both combining forms work fine in text, but
> math is the issue here....
>
> David
>
>
>
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