7

The following MCE displays \unit{\micro\gram} as expected, that is with the (upright mu) Micro Sign character:

\documentclass{article}
\usepackage{fontspec}
\usepackage{siunitx}
\setmainfont{TeX Gyre Schola}
% \setmainfont{RobotoFlex-VariableFont.ttf}
\begin{document}
\begin{itemize}
\item Greek Small Letter Mu/Unicode char 03BC: μ/\char"03BC
\item Micro Sign/Unicode char 00B5: µ/\char"00B5
\item \verb|\unit{\micro\gram}|: \unit{\micro\gram}
\end{itemize}
\end{document}

enter image description here

However, if the TeX Gyre Schola font is replaced with the Roboto Flex font, \unit{\micro\gram} is displayed with a slanted mu, so not with a Micro Sign:

enter image description here

I assume this is a bug in the Roboto Flex font, but is there anything that can be done on the (Lua)LaTeX side to resolve the issue?

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2
  • 1
    Unsure whether you can call this a bug. It rather seems to be a choice of the font designers to let the micro sign have an italic glyph. See: fonts.google.com/specimen/Roboto+Flex?preview.text=3%20%C2%B5m … but the other Roboto fonts indeed seem to have an upright glyph Commented 19 hours ago
  • @JasperHabicht In any case, to my knowledge, this choice would go against the recommendations of the International Bureau of Weights and Measures (BIPM). Commented 17 hours ago

2 Answers 2

Reset to default
4

I can offer a workaround:

\documentclass{article}
\usepackage{fontspec}
\usepackage{siunitx}

\setmainfont{RobotoFlex-VariableFont.ttf}

%\DeclareSIPrefix{\micro}{\symbol{"03BC}}{-6}
\DeclareSIPrefix{\micro}{\textup{μ}}{-6}

\begin{document}

\verb|\unit{\micro\gram}|: \unit{\micro\gram}

\end{document}

Curiously, swapping the comments results in the same output, but removing \textup makes the symbol print in italics.

output

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3

The siunitx package uses 00B5 as default to get an upright mu (see https://github.com/josephwright/siunitx/issues/822#issuecomment-3228015058). You can overwrite this definition to use 03BC again. This should be done after \begin{document} because siunitx sets the default in \AtBeginDocument.

MWE:

\documentclass{article}
\usepackage{fontspec}
\usepackage{siunitx}
\setmainfont{RobotoFlex-VariableFont.ttf}
\begin{document}
\ExplSyntaxOn
\siunitx_declare_prefix:Nne \micro  { -6 } { \__siunitx_unit_non_latin:n { "03BC } }
\ExplSyntaxOff
\begin{itemize}
\item Greek Small Letter Mu/Unicode char 03BC: μ/\char"03BC
\item Micro Sign/Unicode char 00B5: µ/\char"00B5
\item \verb|\unit{\micro\gram}|: \unit{\micro\gram}
\item \verb|\unit{\ug}|: \unit{\ug}
\end{itemize}
\end{document}

Result:

screenshot showing upright mu in unit

There might be unintended side effects of this redefinition that I am not aware of, test carefully (or wait until Joseph Wright provides a proper explanation and solution...)

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