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

[David J. Perry]

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.