fontspec no-math option changes the math bold alphabet

Question

I have a setup that works well with pdflatex and that I would like to carry it over to xelatex or lualatex, but fontspec seems to be changing the math-bold alphabet -- even if called with the [no-math] option. Consider the file:

% To be run with pdflatex
\documentclass{report}

\usepackage[lite]{mtpro2}

\begin{document}
$abcdefg....$

$\mathbf{abcdefg...}$

$\mbf{abcdefg...}$
\end{document}

which produces:

Now moving over to xelatex the equivalent should be:

% To be run with xe-lua-latex    
\documentclass{report}

\usepackage[no-math]{fontspec}
\setmainfont{Times New Roman}

\usepackage[lite]{mtpro2}

\begin{document}
$abcdefg...$

$\mathbf{abcdefg...}$

$\mbf{abcdefg...}$
\end{document}

The option [no-math] here is required to keep fontspec from using CM fonts inside math-mode...but then if one use it, the results are:

that is, the \mathbold alphabet is changed - the mid-line uses a different weight.

I am well aware of the question Fontspec changes the bold math and tried everything there and it does not seem to apply.

Answer 1

You have made some misunderstood claims here: 1) \mathbf does not give you lite-bold; 2) \mbf does not give you heavy-bold. I’d like to first correct some misconceptions and then present a solution.

In your examples you are using the \mathbf alphabet, not the \mathbold alphabet. In fact, mtpro2 declares the latter by

\DeclareMathAlphabet{\mathbold}{LMP1}{mtt}{b}{it}

which means that the \mathbold alphabet is bold and italic. You must purchase the complete version of mtpro2 to get this alphabet. So your lite examples have nothing to do with \mathbold. The \mbf alphabet, however, does exist with the lite version, and \mbf is indeed a separately designed bold upright math font. All heavy math fonts are available only with the complete version.

Under the NFSS (New Font Selection Scheme) of LaTeX2e, the commands \mathrm, \mathbf, \mathit, \mathsf, \mathtt, etc., switch to text fonts in math mode. So when you type

$\mathbf{a-b}$

The a and the b will be printed in bold upright face of the text font, while the hyphen - behaves like the minus sign because you are in math mode. The \mathXX commands only affect what’s called the \mathalpha class (usually includes all digits, lower and upper Latin letters).

\documentclass{article}
\usepackage{fontspec}% `no-math' is no longer needed since v2.7b (2019/02/12)
%\setmainfont{Times New Roman}
\setmainfont{Comic Sans MS}% For illustration purpose
\usepackage[lite]{mtpro2}
\begin{document}
$012abcdefg$, `no-math' is no longer needed, digits stay as MathT\i me\par
$\mathbf{012abcdefg}$, text font in math mode\par
$\mbf{012abcdefg}$, real bold upright math alphabet
\end{document}

See? The \mathbf command switches to text font (Comic Sans MS) here.

If you look closely in your pdfLaTeX example, then you should realize that \mathbf switches to Computer Modern Bold, which seems to be “semi-bold” because the default is Computer Modern Bold Extended. Maybe this is why you think \mbf is heavy: It isn’t.

---

Solution

You have Times New Roman for Windows, Times for MacOS, Nimbus Roman No. 9 L for Linux. These are all Times-like digitization of the same/similar metal types. In the TeX world, you have TeX Gyre Termes. For cross-platform considerations, I would suggests TeX Gyre Termes (IMHO, it has better support in, of course, TeX).

Unfortunately mtpro2 uses fixed font attributes. This is because the legacy Times family, called ptm, has bold face in the b series. Let’s help mtpro2 to re-declare related commands by using implicit font attributes:

\documentclass{article}
\usepackage{fontspec}% `no-math' is no longer needed since v2.7b (2019/02/12)
%\setmainfont{Times New Roman}
\setmainfont{TeX Gyre Termes}% For the TeX world, truly cross-platform
\setsansfont{TeX Gyre Heros}
\setmonofont{TeX Gyre Cursor}
\usepackage[lite]{mtpro2}
\DeclareSymbolFont{operators}{\encodingdefault}{\rmdefault}{\mddefault}{\updefault}
\SetSymbolFont{operators}{bold}{\encodingdefault}{\rmdefault}{\bfdefault}{\updefault}
\DeclareMathAlphabet{\mathbf}{\encodingdefault}{\rmdefault}{\bfdefault}{\updefault}
\DeclareMathAlphabet{\mathit}{\encodingdefault}{\rmdefault}{\mddefault}{\itdefault}
\DeclareMathAlphabet{\mathsf}{\encodingdefault}{\sfdefault}{\mddefault}{\updefault}
\DeclareMathAlphabet{\mathtt}{\encodingdefault}{\ttdefault}{\mddefault}{\updefault}
\SetMathAlphabet{\mathit}{bold}{\encodingdefault}{\rmdefault}{\bfdefault}{\itdefault}
\SetMathAlphabet{\mathsf}{bold}{\encodingdefault}{\sfdefault}{\bfdefault}{\updefault}
\SetMathAlphabet{\mathtt}{bold}{\encodingdefault}{\ttdefault}{\bfdefault}{\updefault}

\newcommand*\test[1]{\texttt{\string#1}: $#1{012abc}$}

\begin{document}
Normal math: $012abc$\par
\test\mbf\par
\test\mathrm, $\sin$\par
\test\mathbf\par
\test\mathit\par
\test\mathsf\par
\test\mathtt

Switch to \verb|\boldmath| now:\par
\boldmath
\test\mathrm, $\sin$\par
\test\mathbf\par
\test\mathit\par
\test\mathsf\par
\test\mathtt
\end{document}

---

P.S. The no-math option is no longer needed since fontspec v2.7b (2019/02/12). See this CTAN announcement.

Answer 2

The fontspec package allows you to set the math font with \setmathrm, and the \mathbf font with the BoldFont = option of \setmathrm. (ETA: Thanks to Ruixi Zhang. As of March 2019, this is bugged if you select [no-math].)

The example in the manual for loading a bold NFSS font as \mathbf is:

\AtBeginDocument{\DeclareMathAlphabet\mathbf{U}{eur}{b}{n}

Ruixi Zhang has a great answer that expands on this.

Other options include mathastext and, in XeLaTeX, mathspec. You could also drop the legacy 8-bit fonts entirely and use unicode-math with something like STIX Two Math.