MetaPost & gmp package. How to use diacritical signs inside mpost?

Question

First of all, I've read this question: Proper use of the emp package.

But...

How to use diacritical signs inside mpost? Like ā, ä, ā, ä, ǟ, ḑ, ē, ī...

The project was created on overleaf.com.

\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[shellescape,latex]{gmp}
\gmpoptions{everymp={input expressg;}}
\begin{document}
\begin{figure}[htp]
\centering
\begin{mpost}
z0 = origin;
drawroundedbox(0, 2.5cm, 3cm, 5mm)(
  \btex {\begin{tabular}{@{}c@{}} ā, ä, ā, ä, ǟ, ḑ, ē, ī... \end{tabular}} etex
);
\end{mpost}
\end{figure}
\end{document}

It produces nothing.

I was using this site to test the MetaPost.

Answer 1

With your choice of packages, the simple

\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\begin{document}

ā, ä, ā, ä, ǟ, ḑ, ē, ī

\end{document}

would raise the same errors, since ā ǟ ḑ ē ī are not supported by the utf8 option, even if \usepackage[T1]{fontenc} is loaded. So it's no surprise they don't work in an mpost environment. On the other hand

\documentclass[12pt]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[shellescape,latex]{gmp}

\gmpoptions{everymp={input expressg;}}
\usempxpackage[T1]{fontenc}
\usempxpackage[utf8]{inputenc}

\begin{document}

\begin{figure}[htp]
\centering
\begin{mpost}
z0 = origin;
drawroundedbox(0, 2.5cm, 3cm, 5mm)(
  \btex {\begin{tabular}{@{}c@{}} ä, ă, ö \end{tabular}} etex
);
\end{mpost}
\end{figure}

\end{document}

works flawlessly, producing

This works, by teaching LaTeX how it should deal with the unknown characters:

\documentclass[12pt]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[shellescape,latex]{gmp}

\gmpoptions{everymp={input expressg;}}
\usempxpackage[T1]{fontenc}
\usempxpackage[utf8]{inputenc}
\usempxpackage{newunicodechar}
\mpxcommands{%
  \newunicodechar{ā}{\=a}%
  \newunicodechar{ē}{\=e}%
  \newunicodechar{ī}{\={\i}}%
  \newunicodechar{ḑ}{\d{d}}%
  \newunicodechar{ǟ}{\={ä}}%
}

\begin{document}

\begin{figure}[htp]
\centering
\begin{mpost}
z0 = origin;
drawroundedbox(0, 2.5cm, 3cm, 5mm)(
  \btex {\begin{tabular}{@{}c@{}} ā, ä, ā, ä, ǟ, ḑ, ē, ī \end{tabular}} etex
);
\end{mpost}
\end{figure}
\end{document}

Of course, the same declarations should be made for the main document, if you want those characters also in normal text.