Export Greek font (italic, upright, bold) from 'newtxmath' package with libertine option

Question

Is there a way to export just a single symbol from the font used by newtxmath with libertine option? Being be more specific, I would need both the italic and the upright Greek letters (in bold too) without loading the newtxmath package. For instance, this

\documentclass{standalone}
\usepackage[libertine]{newtxmath}
\begin{document}
$\tau\uptau\boldsymbol{\uptau}$
\end{document}

produces that How can I export these three characters (respectively italic, upright and bold)? I tried with something like

\DeclareSymbolFont{NEWTXletters}{OML}{ntxmi}{m}{it}
\SetSymbolFont{NEWTXletters}{bold}{OML}{ntxmi}{b}{it}
\DeclareMathSymbol{\newtxtau}{\mathord}{NEWTXletters}{28}

in the preamble, as suggested by @Ruixi Zhang and @Sebastiano and others; that is fantastic, but so I only get the Greek italic letters of the default option, which is not libertine! In newtxmath.sty there is something like DeclareMathSymbol{\tauup}{\mathord}{lettersA}{28}, but I don't know how to export it. Moreover, I tried to export them as .eps files, but that's not the same thing.

Thanks in advance!

Answer 1

Original (with newtxmath and libertine option)

\documentclass[border=2]{standalone}
\usepackage[libertine]{newtxmath}
\begin{document}

$\tau\uptau\boldsymbol{\uptau}$

\end{document}

With the imported symbols

\documentclass[border=2]{standalone}
\usepackage{amsmath,bm}

\DeclareFontFamily{OML}{nxlmi}{\skewchar\font=127}
\DeclareFontShape{OML}{nxlmi}{m}{it}{
   <-6.3> s*[1] nxlmi035
   <6.3-8.6> s*[1] nxlmi037
   <8.6-> s*[1] nxlmi03
}{}
\DeclareFontShape{OML}{nxlmi}{b}{it}{
  <-6.3> s*[1] nxlbmi035
  <6.3-8.6> s*[1] nxlbmi037
  <8.6-> s*[1] nxlbmi03
}{}

\DeclareFontFamily{U}{ntxmia}{\skewchar\font=127}
\DeclareFontShape{U}{ntxmia}{m}{it}{
   <-> s*[1] nxlmia
}{}
\DeclareFontShape{U}{ntxmia}{b}{it}{
   <-> s*[1] nxlbmia
}{}


\DeclareSymbolFont{ntxlibletters}{OML}{nxlmi}{m}{it}
\SetSymbolFont{ntxlibletters}{bold}{OML}{nxlmi}{b}{it}
\DeclareSymbolFont{ntxlibupletters}{U}{ntxmia}{m}{it}
\SetSymbolFont{ntxlibupletters}{bold}{U}{ntxmia}{b}{it}

\DeclareMathSymbol{\tau}{\mathord}{ntxlibletters}{"1C}
\DeclareMathSymbol{\uptau}{\mathord}{ntxlibupletters}{"1C}

\begin{document}

$\tau\uptau\boldsymbol{\uptau}$

\end{document}

The output is the same.

Adjust the factor [1] in the font shape declarations to match the font you use. It's a magnification factor; using s*[0.9] reduces the font size at 90%.