[lltx] $n\choose k$-issue with OpenType math fonts

[Ulrik Vieth]

On 03/04/2011 11:29 PM, Philipp Stephani wrote:
  > A1, A2, A3, A5, A6, A7, A8, A9, A11, C1, C2, C3, C5, C6, C7, C8, C9,
> C10, C11, X5, X9, X10, X11 look good.

> A4, C4, X4: upper and lower part are too far away from each other; as
> there is no fraction bar, they should be closer to each other than the
> numerator/denominator in ACX5.

> A10: numerator and denominator are too far away from the fraction bar.

> X1, X3, X6, X7, X8: lower part is too far to the bottom.
> X2: parentheses are too high.

> The issues that occur with Computer Modern are: 4, 10, i.e. those that
> also appear with A and C; these are more-or-less design problems;
> someone should check if they also occur in Word. X has more issues;
> LuaLaTeX + A/C can be declared okay-ish, but not perfect IMO.

I think the case of A4 using too large a size of delimiters with Asana
was a bug in LuaTeX 0.60.2 that was fixed in 0.61 upwards. (There was
an off-by-one error in LuaTeX jumping to the biggest size too early, and 
Asana happened to have fewer sizes of delimiters than the others.)

> It would be interesting to know whether there are established
> conventions regarding the vertical gaps in fractions vs. binomials.
> Personally I'd prefer a smaller gap for the latter since the bar is absent.

For a traditional TeX font, the minimum gap in gneralized fractions
without a bar will be 3x rule thickness in text style and 7x rule in 
display style, which may be quite large, depending on rule thickness.
Fortunately, OpenType MATH allows to specify the parameters separately, 
so it becomes possible to compensate for a larger rule thickness.

(See figure 3 and table 4 on p.25 of my paper
http://www.tug.org/TUGboat/tb30-1/tb94vieth.pdf)

Regards, Ulrik.

P.S: In any case, there is something very strange with xits-math.