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

Will Robertson will at wspr.io
Wed Feb 10 22:52:18 CET 2016

On 11 Feb 2016, at 8:06 AM, David J. Perry <hospes.primus at verizon.net> wrote:
> 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?

Hi David P,

[My knowledge here is entirely as an end user of XeTeX maths features — I didn’t have any hand in implementing them.]

My understanding here is that Variant Selector acts like a character to produce a difference glyph (analogous to a ligature), so doesn’t need shaping/positioning information.

For maths typesetting, this is activated with “script=math”, and there are a bunch of (~50, too tired to look up the number now) font parameters stored directly within the font to handle most spacing needs.

However, I am not sure about the technical side of applying maths accents like \ddots. These are placed not as combining marks but with the XeTeX primitive \Umathaccent (which can also place the accent below the letter, etc.). I can only assume that a font designer can provide additional details to fine-tune the spacing for certain cases; to me it would seem necessary. But these accents can be stretched over multiple glyphs, so perhaps not.

Hope this helps,

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