Change em-dash character in an OpenType font with xetex

Question

In a (OTF) font I'm using with xetex, loaded with the mapping=tex-text option, I found that the em-dash character is too thin, so I would like to replace it with a heavier one. For example, suppose I want to replace it with an \hrule (or, equivalently, a \vrule). What should I do? How are characters mapped to ligature combinations?

Answer 1

XeTeX can use the powerful method of mapping files.

Locate on your TeX system the file tex-text.map and copy it into the working directory as baskerville-dash.map.

Open the file with a text editor and modify it to look like

; TECkit mapping for TeX input conventions <-> Unicode characters

LHSName "baskerville-dash"
RHSName "UNICODE"

pass(Unicode)

; ligatures from Knuth's original CMR fonts
U+002D U+002D           <>      U+2013  ; -- -> en dash
U+002D U+002D U+002D    <>      U+2015  ; --- -> horizontal bar (was U+2014)
U+2014                  <>      U+2015  ; em dash -> horizontal bar

U+0027                  <>      U+2019  ; ' -> right single quote
U+0027 U+0027           <>      U+201D  ; '' -> right double quote
U+0022                   >      U+201D  ; " -> right double quote

U+0060                  <>      U+2018  ; ` -> left single quote
U+0060 U+0060           <>      U+201C  ; `` -> left double quote

U+0021 U+0060           <>      U+00A1  ; !` -> inverted exclam
U+003F U+0060           <>      U+00BF  ; ?` -> inverted question

; additions supported in T1 encoding
U+002C U+002C           <>      U+201E  ; ,, -> DOUBLE LOW-9 QUOTATION MARK
U+003C U+003C           <>      U+00AB  ; << -> LEFT POINTING GUILLEMET
U+003E U+003E           <>      U+00BB  ; >> -> RIGHT POINTING GUILLEMET

Save the file and run

teckit_compile baskerville-dash

that should produce a file baskerfille-dash.tec.

Now test the new mapping file:

\documentclass{article}

\usepackage{fontspec}

\setmainfont[Mapping=baskerville-dash]{Baskerville}

\begin{document}

a---b—c

\end{document}

Between b and c in the input file there's U+2014 (em dash).

Here's the output with the default tex-text mapping

Now keep the baskerville-dash file along with the TeX source, or place it in some place that XeTeX will look in for files, presumably

mkdir -p $(kpsewhich -var-value TEXMFHOME)/fonts/misc/xetex/fontmapping/baskerville-dash
mv baskerville-dash.* $(kpsewhich -var-value TEXMFHOME)/fonts/misc/xetex/fontmapping/baskerville-dash

or a similar trick for other operating systems.

Run XeLaTeX on the test file again for being sure all went well.

Answer 2

Since you're using the "Monotype Baskerville" font, you actually have a choice of two weights for the em-dash symbol: \char"0214 (the default) and \char"0215. The former glyph is indeed extremely thin; the latter is a lot thicker, and it may even be too thick for your taste. Your call.

If you like the latter form, you could set up a macro via an instruction such as

    \newcommand\thickemdash{\char"0215}

so that you don't have to remember the Unicode character slot of the glyph.

The following MWE runs on my system (MacOSX 10.11.1., MacTeX2015) under both XeLaTeX and LuaLaTeX.

% !TEX TS-program = xelatex
\documentclass{article}

\usepackage{fontspec}
\setmainfont{Baskerville} % Monotype Baskerville

\begin{document}

a---a a\char"2014a a\char"2015a

\end{document}