[XeTeX] Handling of combining and variant selector characters in math
David J. Perry
hospes.primus at verizon.net
Thu Feb 11 00:05:16 CET 2016
Thanks for the reply, Will.
On 2/10/2016 4:52 PM, Will Robertson wrote:
> 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.
That's true, but the fonts that I am familiar with have a substitution
lookup (similar indeed to a ligature) that says "replace character X
followed by VS1 with character Y." I saw nothing like that in XITS.
> 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.
Yes, I saw a bunch of OT features in the font but nothing that looked
familiar in terms of accent positioning and the like. Which may be my
own ignorance, of course.
> 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.
I wonder how this is done (and I agree it seems needed). It doesn't
seem to be with the usual OT mechanisms.
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