Define fallback font for specific Unicode characters in LuaLaTeX

Question

This closed question prompts me to propose a more general question and a solution I don't think has been demonstrated here.

When compiling with LuaLaTeX and using Unicode input, how do you set up a fallback font for specific Unicode characters that are absent from the main font?

I would welcome other approaches or critiques of the one I demonstrate.

Answer 1

We can define a command to switch to a fallback font using fontspec.

Then we can use newunicodechar to map the missing Unicode characters to a command that switched to the fallback font for those characters.

\documentclass{article}
\usepackage{fontspec, newunicodechar}
\setmainfont{PT Serif}
\newfontfamily{\fallbackfont}{Linux Libertine O}[Scale=MatchLowercase]
\DeclareTextFontCommand{\textfallback}{\fallbackfont}
\newunicodechar{ɔ}{\textfallback{ɔ}}
\newunicodechar{ϱ}{\textfallback{ϱ}}

\begin{document}
Hellɔ woϱld.
\end{document}

Output of pdffonts showing that both fonts are used:

name                                 type              emb sub uni object ID
------------------------------------ ----------------- --- --- --- ---------
NUOAQT+PTSerif-Regular               CID TrueType      yes yes yes      4  0
UAEGUX+LinLibertineO                 CID Type 0C       yes yes yes      5  0

EDIT:

We could also define a general command for setting up missing characters. It takes an optional argument which defaults to \textfallback, but a different font could be inserted in case an additional fallback font was needed.

\newcommand{\fallbackchar}[2][\textfallback]{%
    \newunicodechar{#2}{#1{#2}}%
}
\fallbackchar{ɔ}
\fallbackchar{ϱ}