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<p>Modern typography provides technologies for tweaking various aspects
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of the font’s presentation. The general idea is to make the typeface
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flexible enough for adapting to a variety of typographic requirement.
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Common features include tabular numerals, stylistic alternatives for
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select glyphs (e.g. slashed zero), ligatures, small caps, ordinal
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figures. Each font family will have support for its own classes.</p>
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<p>GNU/Linux users can access <a href="https://en.wikipedia.org/wiki/OpenType_feature_tag_list">these
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features</a> by
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leveraging the <code>fontconfig</code> library’s rule declaration. There may be
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differences between distros on where the system-wide defaults are
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placed. But user-specific settings should normally be defined at
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<code>~/.config/fontconfig/conf.d</code>. Here is an overview of my current
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settings (see <a href="https://gitlab.com/protesilaos/dotfiles">my dotfiles</a>):</p>
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<pre><code>.config/fontconfig/conf.d/
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├── 10-hinting-full.conf
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├── 10-sub-pixel-rgb.conf
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├── 11-lcdfilter-default.conf
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├── 20-unhint-small-hack.conf
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├── 45-generic.conf
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├── 45-latin.conf
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├── 50-enable-terminus.conf
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├── 60-generic.conf
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├── 60-latin.conf
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├── 80-alegreya-fontfeatures.conf
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├── 80-alegreya-sans-fontfeatures.conf
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├── 80-condensed-large-dejavu.conf
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├── 80-firago-fontfeatures.conf
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└── README
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</code></pre>
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<p>The numbering matters as it determines the sequence for loading these
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rules. This is what Debian provides on the matter:</p>
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<blockquote>
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<p>conf.d/README</p>
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<p>Each file in this directory is a fontconfig configuration file. Fontconfig
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scans this directory, loading all files of the form [0-9][0-9]*.conf.
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These files are normally installed in /usr/share/fontconfig/conf.avail
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and then symlinked here, allowing them to be easily installed and then
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enabled/disabled by adjusting the symlinks.</p>
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<p>The files are loaded in numeric order, the structure of the configuration
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has led to the following conventions in usage:</p>
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<pre>
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Files begining with: Contain:
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00 through 09 Font directories
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10 through 19 system rendering defaults (AA, etc)
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20 through 29 font rendering options
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30 through 39 family substitution
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40 through 49 generic identification, map family->generic
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50 through 59 alternate config file loading
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60 through 69 generic aliases, map generic->family
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70 through 79 select font (adjust which fonts are available)
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80 through 89 match target="scan" (modify scanned patterns)
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90 through 99 font synthesis
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</pre>
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</blockquote>
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<p>Rules are written in XML. See <a href="https://www.freedesktop.org/software/fontconfig/fontconfig-user.html">this
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spec</a>
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for all available constructs.</p>
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<h2>Trying things out</h2>
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<p>Now on to implementing our newfound knowledge! Say you have
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downloaded <a href="https://bboxtype.com/typefaces/FiraGO">FiraGO</a> and placed
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a directory containing its font files somewhere it can be read by your
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operating system. Font files can end in either <code>.otf</code> or <code>.ttf</code>. On
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Debian, user-specific typefaces are read from <code>~/.local/share/fonts</code>.
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So FiraGO would be like this:</p>
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<pre><code>.local/share/fonts/FiraGO
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├── FiraGO-BoldItalic.otf
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├── FiraGO-Bold.otf
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├── FiraGO-Italic.
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└── FiraGO-Regular.otf
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</code></pre>
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<p>Then inside the local <code>conf.d</code> we add <code>80-firago-fontfeatures.conf</code>,
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which contains the following:</p>
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<pre><code><?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
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<fontconfig>
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<description>Enable select opentype features for FiraGO.</description>
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<!--
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Typographic features are provided on a per-typeface basis.
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Tables with all available features are available here:
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https://en.wikipedia.org/wiki/OpenType_feature_tag_list
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Also read the fontconfig user spec:
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https://www.freedesktop.org/software/fontconfig/fontconfig-user.html
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-->
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<match target="font">
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<test name="family" compare="eq" ignore-blanks="true">
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<string>FiraGO</string>
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</test>
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<edit name="fontfeatures" mode="append">
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<string>tnum on</string> <!-- tabular numbers -->
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<string>zero on</string> <!-- slashed zero -->
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</edit>
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</match>
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</fontconfig>
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</code></pre>
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<p>Focus on the part inside of the <code><edit></code> tags. This is where we
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toggle the features that FiraGO supports (as documented in its
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download page). As you can see, I only need tabular numbers and a
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slashed zero. This makes FiraGO far better as a UI font, while not
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detracting from its already superb ability to present body copy.</p>
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<p>To target another font family, simply replace “FiraGO” from this part:
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<code><string>FiraGO</string></code>. I have tried this method with a number of
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typefaces, all delivering the expected results.</p>
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<p>Pro tip: combine FiraGO with Fira Code (<code>apt install fonts-firacode</code>)
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for a consistent typographic experience across your desktop
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environment.</p>
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