Font dimensions related to \overline about firamath HOT 7 CLOSED

I just found out about the excellent papers by Bogusław Jackowski and Ulrik Vieth, where the authors illustrated Appendix G with drawings and discussed how to set various math font dimensions.

The resources include papers and slides, so I will provide a link that already has them gathered. They can be found at
http://www.gust.org.pl/projects/e-foundry/math/index_html
starting from “In the preparation for providing math typesetting support with the TeX Gyre fonts”

from firamath.

Perhaps the two “rule thickness” entries should be changed to 50 from the current 66.

from firamath.

According to Microsoft's OpenType specification, these constants are defined as:

• overbarVerticalGap
  Distance between the overbar and the (ink) top of he base. Suggested: 3 × default rule thickness.
• overbarRuleThickness
  Thickness of overbar. Suggested: default rule thickness.
• overbarExtraAscender
  Extra white space reserved above the overbar. Suggested: default rule thickness.

The default rule thickness of fira math regular is 66. But I'm not sure whether the other constants should be changed. Still, I need to investigate it thoroughly.

from firamath.

There are definitely something not quite right in the math table. I managed to find two three more:

Axis heights do not match?

TeX sees an unusual rule thickness

Many rule thicknesses

FractionRuleThickness and RadicalRuleThickness are set to 76, while OverbarRuleThickness and UnderbarRuleThickness are 66, although all of them are recommended to be set as the default rule thickness.

from firamath.

As FontForge's documentation says,

These values are stored in tfm files (should you generate a tfm file with your font).

So I think these TeX info has nothing to do with the OpenType MATH table, and hence simply leave them as the default values.

from firamath.

@RuixiZhang42 Even after going through that, I'm still at a loss as to how to have a thinner \overline that's a bit closer to the top of a number in Fira Math + XeTeX.

from firamath.

@ppw0 You can try something like this (no recommended unless you know what you are doing):

% only works with XeLaTeX + unicode-math (actually fontspec)
\setmathfont{Fira Math}[
  FontAdjustment = { % <- a secret option (undocumented)
    \fontdimen 53 \font = 0.75 \fontdimen 53 \font % <- reduce OverbarVerticalGap   to 75% of its original
    \fontdimen 54 \font = 0.5  \fontdimen 54 \font % <- reduce OverbarRuleThickness to 50% of its original
    \fontdimen 55 \font = ...
    % you will probably want to change underbar as well:
    \fontdimen 56 \font = ...
    \fontdimen 57 \font = ...
    \fontdimen 58 \font = ...
  }
]

from firamath.