Math and text font size not the same

Question

This is a follow-up question to Switching from PDFLaTeX to LuaLaTeX. Using user187802's answer, I have the following setup:

\documentclass{article}

\usepackage{fontspec}
\setmainfont[
  Extension      = .otf,
  UprightFont    = *-regular,
  BoldFont       = *-bold,
  ItalicFont     = *-italic,
  BoldItalicFont = *-bolditalic,
  Ligatures      = {
    TeX,
    CommonOff
  }
]{NewCM10}

\usepackage[
  math-style   = TeX,
  warnings-off = {
    mathtools-colon
  }
]{unicode-math}
\setmathfont{NewCMMath-Regular.otf}
\setmathfont[
  version  = bold,
  FakeBold = 2
]{NewCMMath-Regular.otf}

\usepackage{polyglossia}
\setdefaultlanguage{danish}

\begin{document}

Test.\footnote{`12345' versus `$12345$'}

\end{document}

As can be seen in the output, neither the font size nor the font boldness in text mode and math mode are the same. (It doesn't just occur in footnotes but elsewhere the difference is more subtle.)

How can I get the exact same font size and font boldness for text mode and math mode when using the NewCM10 font?

Answer 1

Apparently, digits scale differently in NewCM-Math.

You can assign the text font to the digits.

\documentclass{article}

\usepackage{polyglossia}
\usepackage{fontspec}
\setmainfont[
  Extension      = .otf,
  UprightFont    = *-Regular,
  BoldFont       = *-Bold,
  ItalicFont     = *-Italic,
  BoldItalicFont = *-BoldItalic,
  Ligatures = {
    TeX,
    CommonOff
  },
  NFSSFamily=ncm,
]{NewCM10}

\usepackage[
  math-style = TeX,
  warnings-off = {
    mathtools-colon
  }
]{unicode-math}
\setmathfont{NewCMMath-Regular.otf}
\setmathfont[
  version = bold,
  FakeBold = 4,
]{NewCMMath-Regular.otf}

\setdefaultlanguage{danish}

% fix digits
\DeclareSymbolFont{textdigits}{TU}{ncm}{m}{n}
\SetSymbolFont{textdigits}{bold}{TU}{ncm}{b}{n}

\Umathcode`0="7 \symtextdigits `0
\Umathcode`1="7 \symtextdigits `1
\Umathcode`2="7 \symtextdigits `2
\Umathcode`3="7 \symtextdigits `3
\Umathcode`4="7 \symtextdigits `4
\Umathcode`5="7 \symtextdigits `5
\Umathcode`6="7 \symtextdigits `6
\Umathcode`7="7 \symtextdigits `7
\Umathcode`8="7 \symtextdigits `8
\Umathcode`9="7 \symtextdigits `9

\newcommand{\test}{%
  12345 versus $12345$\par
  \sbox0{12345}\the\wd0\ versus \sbox0{$12345$}\the\wd0\par
}


\begin{document}

\test {\bfseries\boldmath\test}

\bigskip

{\footnotesize\test {\bfseries\boldmath\test}}

$a+b=c$

{\boldmath$a+b=c$}

\end{document}

Without the \Umathcode lines, the output is

Answer 2

The problem is that LaTeX's Unicode math sets the math Unicode font with ssty font feature by very specific way. It uses +ssty=0 for textstyle, +ssty=1 for script style etc. I used in OpTeX only ssty=0, ssty=1 etc., without the + sign. I am unable to find the documentation of the plus sign in this font feature (the https://learn.microsoft.com/en-us/typography/opentype/spec/features_pt#ssty doesn't mention this) but I see that if it is used then the definitive variant of letters are selected not by the number specified for this feature, but by the size used for the font.

Compare:

LaTeX: [NewCMMath-Regular.otf]:mode=base;script=math;language=dflt;+ssty=0; at 8.0pt
OpTeX: [NewCMMath-Regular]:mode=base;script=math;ssty=0;mathsize=1; at 8.0pt

LaTeX selects script alternative shapes for the digits 1234 in loaded font, OpTeX selects basic shapes scaled to 8pt, which looks better in the footnote size. OpTeX selects script alternative shapes when ssty=1, i.e. only when scriptstyle in math mode is used.