Inconsistent script sizes with STIX Two and unicode-math

Question

With unicode-math, it is recommended to use \symup to get upright symbols in math-mode, and to use \textnormal to get upright text, e.g. for text label subscripts. When using the STIX Two fonts, these two commands give inconsistent results.

\documentclass{article}

\usepackage{amsmath}
\usepackage{unicode-math}
\setmainfont{STIX Two Text}
\setmathfont{STIX Two Math}

\begin{document}
\[
    \textnormal{AVfi}_{\textnormal{AVfi}_\textnormal{AVfi}} \qquad \symup{AVfi}_{\symup{AVfi}_\symup{AVfi}}
\]
\end{document}

In the left case, we get nice kerning, and ligatures, but the font size in the subscripts are bad. In the right case, the font size in the subscripts is good, but the kerning and ligatures are bad.

I think the problem is that STIX Two Text doesn't contain the correct glyphs for optical sizes in subscripts.

Is it possible to get good kerning, ligatures, and sizing without modifying the font files? If not, is there an easy way to achieve the desired result using fontforge? I can live without the ligatures, if this causes problems.

Answer 1

Optical sizes for letters are chosen at certain font sizes in math mode.

Here's an experiment: you can see that the +ssty=0 or +ssty=1 features are not honored in text (second line).

What you can do is disabling optical sizes in math (fourth line).

\documentclass{article}

\usepackage{amsmath}
\usepackage{unicode-math}
\setmainfont{STIX Two Text}[
  SizeFeatures={
    {Size={9-}},
    {Size={6-9},RawFeature={+ssty=0}},
    {Size={-6},RawFeature={+ssty=1}},
  },
]
\newfontfamily{\stixmath}{STIX Two Math}[
  SizeFeatures={
    {Size={9-}},
    {Size={6-9},RawFeature={+ssty=0}},
    {Size={-6},RawFeature={+ssty=1}},
  },
]
\setmathfont{STIX Two Math}
\setmathfont{STIX Two Math}[
  version=test,
  script-features={},
  sscript-features={}
]

\begin{document}

AVfi{\footnotesize AVfi}{\tiny AVfi}

{\stixmath AVfi{\footnotesize AVfi}{\tiny AVfi}}

$\textnormal{AVfi}_{\textnormal{AVfi}_\textnormal{AVfi}}$

{\mathversion{test}$\symup{AVfi}_{\symup{AVfi}_\symup{AVfi}}$}

$\symup{AVfi}_{\symup{AVfi}_\symup{AVfi}}$

\end{document}

Note that the font size used by \textnormal in subscripts is exactly the same as the one used by \symup; just add

$\textnormal{\detokenize\expandafter{\the\font}}$

$\scriptstyle\textnormal{\detokenize\expandafter{\the\font}}$

$\scriptscriptstyle\textnormal{\detokenize\expandafter{\the\font}}$

\detokenize\expandafter{\the\textfont2}

\detokenize\expandafter{\the\scriptfont2}

\detokenize\expandafter{\the\scriptscriptfont2}

at the end of the previous example and you'll see

The difference in size is only an optical effect due to the subscripts obtained with \symup are affected by ssty.

Answer 2

Math fonts and math mode are designed for math, which means that they are mostly meant for single math symbols and not for text words. If you really want to print a word in a subscript, it would more logical to explicitly switch to text mode -- and in text mode you naturally can adjust the font sizes:

\documentclass[fleqn]{article}
\usepackage{xcolor}
\usepackage{amsmath}
\usepackage{unicode-math}
\setmainfont{STIX Two Text}
\setmathfont{STIX Two Math}

\newcommand\mytext[1]
{%
  {\mathchoice
  {\mbox{\normalfont\normalsize\color{green}#1}}%
  {\mbox{\normalfont\normalsize\color{blue}#1}}%
  {\mbox{\normalfont\footnotesize\color{red}#1}}%
  {\mbox{\normalfont\fontsize{5.5pt}{6pt}\selectfont\color{red!50!green}#1}}%
  }%
}


\begin{document}
$   \mytext{AVfi}_{\mytext{AVfi}_{\mytext{AVfi}}} \qquad \symup{AVfi}_{\symup{AVfi}_{\symup{AVfi}}} $

\[
    \mytext{AVfi}_{\mytext{AVfi}_{\mytext{AVfi}}} \qquad \symup{AVfi}_{\symup{AVfi}_{\symup{AVfi}}}
\]

\end{document}