[XeTeX] Fractions using fontspec

[Will Robertson]

On 2009-03-12 16:00:28 +1030, "R (Chandra) Chandrasekhar" 
<chandra at ee.uwa.edu.au> said:

> I am using TeXLive 2008 from the DVD and kept updated regularly. I am
> trying to get fractions using the [Fractions=On] feature in fontspec.

I think the fractions feature has been labelled "troublesome" and it's 
recommended not to use it. I'll have to address this in the fontspec 
documentation at some stage. Oh wait; I might be getting mixed up with 
the "Ordinal" feature. Erm, Jonathan or Adam?

I don't have these two fonts so I can't investigate in detail, but I 
might have some answers below.

> 1. Why is the digit 1 in Lining style or uppercase in each of the
> plainly written fractions?

Not sure...it might be a conflict between the font and XeTeX's handling 
of the fraction feature; like I said above, best to avoid it.

> 2. Why does the digit 4 in the \vfrac{} version look different from the
> digit in the plainly written fraction, but only in the roman family?

I suspect that \vfrac is actually doing the right thing here and 
selecting the correct glyph; because it's been drawn for the smaller 
size it looks different.

Try this to see the difference:

    \textsubscript{1234}\par\textsubscript*{1234}

(and/or with \textsuperscript as well.)

In the first case, you should get the "proper" subscript numerals; in 
the second case, simply a rescaling down of the originals. (Although 
the situation might be more complicated if there are optical sizes as 
well.)

> 3. Is the a quirk of the version of the Adobe Minion Pro and Myriad Pro
> I am using?

I think everything looks like it should, give or take.

> 4. What is the preferred method of getting fractions using solidus
> notation in XeLaTeX for OTF fonts?

Do you mean by using a solidus in the source? I'm afraid (as far as I 
know) that there is no robust solution across all fonts, since it 
depends on how the font is constructed. Your best choice is to use 
\vfrac.

Cheers,
Will