How to substitute a glyph for a single Greek character?

Question

I am using Minion Pro (with fontspec under LuaLaTeX), and the character ϱ(= \varrho, but in text mode) doesn't show.

input: ϱυϑμὸν
output: �υϑμὸν

Is there a way to globally remap only this character to a different font? I can, of course, use a different font for every instance (or all Greek text, for that matter) but would rather not.

Answer 1

I am answering my own question, as it allows me more formatting than a simple comment, and to sum up the various answers and comments I have received. I have since discovered that my font is missing another character, ɔ, so my solution deals with both of them:

\documentclass{scrartcl}
\usepackage{newunicodechar}
\usepackage{fontspec}
\setmainfont{MinionPro.otf}   % Any defective font will do. Font files kept in the 
                              % current directory to eliminate all path issues etc.
% The fallback font:
\newfontfamily{\gentium}[Scale=MatchLowercase]{GentiumPlus-Regular.ttf}
% (Only) if character is not found in current font, fall back to Gentium:
\newunicodechar{ϱ}{\iffontchar\fontϱ ϱ\else{{\gentium ϱ}}\fi} \newunicodechar{ɔ}{\iffontchar\fontɔ ɔ\else{{\gentium ɔ}}\fi}
\begin{document}
{\huge Hellɔ woϱld!}
\end{document}

Answer 2

I don’t have Minion Pro to test this, but you can set the character active with newunicodechar, look up whether the current font contains ϱ with \iffontchar, and return the character if so, e.g.

\iffontchar\font`ϱ\symbol"03F1\else\symbol"03C1\fi

In practice, you might want to wrap this further in \ifmmode to use \varrho in math mode, e.g.

\ifmmode%
  \varrho%
\else%
  \iffontchar\font`ϱ%
    \symbol"03F1%
  \else\symbol"03C1%
  \fi%
\fi

You hit on using Gentium as a fallback font, instead of ρ as a fallback glyph, which is a great idea. You might want to load the supplemental font with Scale=MatchLowercase so the letters come out the same size. Also be sure to enclose the font change in a group, so it does not bleed through to the following text.

\newfontfamily\gentium{Gentium}[Scale=MatchLowercase]
\newunicodechar{^^^^03f1}{%
  \ifmmode%
    \varrho%
  \else%
    \iffontchar\font`ϱ%
      \symbol"03F1%
    \else{{\gentium\symbol"03F1}}%
    \fi%
  \fi}