`.X` very tight in xelatex/lualatex

[pkra]

The period seems to have virtually no space to the right.

E.g.

$$\begin{array}{c}
X_{\alpha }.X_{\beta }\\
X_{\alpha }\mathpunct{.}X_{\beta }
\end{array}$$

xelatex rendering with STIXTwoMath:

For comparison, xelatex rendering with Latin Modern Math:

For another comparison, lualatex rendering with STIXTwoMath:

so I'm wondering if it's a font issue.

[tiroj]

The /period glyph has spacing on the right, but is naturally spaced to the upright letters. Some math italic letters, and notably the X extend beyond the left sidebearing and end up colliding with the punctuation unless additional space is applied. I’m not sure whether this is something that can be addressed at the font level. The OpenType MATH table includes an italic correction offset, but only defines this at the top right, not at the lower left.

The X_{\alpha }\mathpunct{.}X_{\beta } example looks good, I think, and better than the Latin Modern Math example that ends up with too much space to the right of the period.

[pkra]

Thanks for the clarification @tiroj.

cc @davidmjones in case he wants to do something on the xelatex end (or not and just close this).

[pkra]

From mathjax/MathJax#3529 (comment)

@pkra, thanks for the link to your report for the STIX2 fonts. I don't think tiroj is correct about the space to the right of the period, however. This is FontForge output for the period:

You can see that the right bearing is right against the ink of the period. I downloaded the latest version (which seems to be from 2021) just to be sure it hadn't changed.

It looks like there might be something wrong with the definition for this character, as the hinting (the green and purple rectangles) seems not to be in the right place for the period outline, or more accurately, the outline is probably in the wrong place. The left bearing may be the problem. Perhaps it is meant to be half as big?

@tiroj was this perhaps fixed already or maybe a problem from elsewhere (e.g. the build process)?

[khaledhosny]

From mathjax/MathJax#3529 (comment)

@pkra, thanks for the link to your report for the STIX2 fonts. I don't think tiroj is correct about the space to the right of the period, however. This is FontForge output for the period:

The bad spacing in the screenshot is a bug in FontForge, that was recently fixed but not in a release yet. So unless the font is modified with FontForge, it should have any relation with the issue here.

[khaledhosny]

Some math italic letters, and notably the X extend beyond the left sidebearing and end up colliding with the punctuation unless additional space is applied. I’m not sure whether this is something that can be addressed at the font level. The OpenType MATH table includes an italic correction offset, but only defines this at the top right, not at the lower left.

On experience, -ve left sidebearings are nearly impossible to support in math. So I tend to avoid them. MATH cutins can help with presubscripts, but for regular symbols we would need regular kerning and many (if not most) math implementations don’t apply regular kerning between math symbols.

[pkra]

The bad spacing in the screenshot is a bug in FontForge, that was recently fixed but not in a release yet. So unless the font is modified with FontForge, it should have any relation with the issue here.

Thank you for the pointer, @khaledhosny !

[RuixiZhang42]

[M]any (if not most) math implementations don’t apply regular kerning between math symbols.

AFAIK, regular kerning is supported only with classical TFM-based fonts in any flavors of TeX, i.e., the classical Computer Modern Math and Latin Modern Math (not the OpenType versions). The kerning instructions in, say mathit.mf, are indeed followed. For OpenType math fonts though, almost no layout engine applies regular kerning. So simple expressions such as $V,$ (in $(U,V,W)$) and $Y.$ (in $X,Y.$) look bad.