# Automatically substitute missing glyph with another in LuaLaTeX

For some reason the russian letter ё is missing in some fonts, which leads to

warning  (file c:/Users/mishk/OneDrive/Stuff/fonts/PTSerifPro-Regular.otf) (cff
): 'seac' character deprecated in type 2 charstring
! error:  (file c:/Users/mishk/OneDrive/Stuff/fonts/PTSerifPro-Regular.otf) (cf
f): Type2 Charstring Parser: parsing charstring failed: (status=-1, stack=5)
!  ==> Fatal error occurred, no output PDF file produced!


However it is pretty always acceptable to substitute it with the letter е, but this code doesn't work:

\newunicodechar{ё}{\iffontchar\fontё ё\else{е}\fi}
\newunicodechar{Ё}{\iffontchar\fontЁ Ё\else{Е}\fi}


Can I do the automatic substitution without declaring ё to be an active character?

Full LuaLaTeX MWE:

\documentclass{book}

\usepackage{polyglossia}

\usepackage{newunicodechar}

\setmainlanguage{russian}

\newcommand{\MyPath}{C:/Users/mishk/OneDrive/Stuff/fonts/}

\newfontfamily{\cyrillicfont}{PTSerifPro}[
Path            =   \MyPath,
Extension       =   .otf,
UprightFont     =   *-Regular,
]

\newunicodechar{ё}{\iffontchar\fontё ё\else{е}\fi}
\newunicodechar{Ё}{\iffontchar\fontЁ Ё\else{Е}\fi}

\begin{document}

ааа еее ёёё

\end{document}


In LuaTeX you can define your own substitution rule as a font feature, see How to adjust font features in LuaTeX?

\documentclass{article}
\usepackage{fontspec}

\directlua{
name = "ediaeresis",
type = "substitution",
prepend = true,
data = {
["ё"] = "e",
["Ё"] = "Е",
}
}
}

\setmainfont{PT Serif}[RawFeature=+ediaeresis]

\begin{document}

ё Ё

\end{document}


• Wow this is an amazing feature! – ShreevatsaR Feb 22 '18 at 1:37
• I tried using this technique to hunt down the source of a mysterious occurrence of ^D (U+0004) in a document, but it appears not to work for characters in that range. I used luacode* from the luacode package and added ["\x04"] = "X" to the data array, to no avail. So this is all very mysterious. Is this feature documented anywhere? I feel uncomfortable doing cargo cult programming. – Harald Hanche-Olsen Mar 1 '18 at 11:51
• @HaraldHanche-Olsen It's documented in “Fonts out of ConTeXt”. – Henri Menke Mar 1 '18 at 20:16
• Ah, thanks. I might not have found that in a million years. – Harald Hanche-Olsen Mar 1 '18 at 21:09