Please add <group> → <axis> → <family>

This is a part of: #200

Preamble

The optical size axis behaves slightly differently than the other standard OpenType axes.

While width / weight / slant can be selected by the user in traditional font selection menus, optical size is supposed to be applied by default, at specific sizes.

Therefore, it behaves more like a complete family replacement, at some sizes, than a style attribute.

Purpose

To accommodate optical size, step can accept lists of family or match elements, with the same syntax and rules as group.

Proposed syntax

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<!-- SPDX-License-Identifier: MIT -->
<fontconfig>
  <group>
    <family>@canonical_family_name@</family>
    <axis>
      <name>@axis_name@</name>
      <step>
        <name>@step_name@</name>
        <value>@step_value@</name>
        <family>@technical_family_name@</family>
      </step>
    </axis>
  </group>
</fontconfig>

Behaviour

An optical sizing step with a name different from Regular behaves like a group with a target of @canonical_family_name@ SPACE @step_name@. (Regular corresponds to @canonical_family_name@, and therefore can not be re-declared, except for its axis value).

Like agroup, it can contain family elements, match elements, etc.

It reuses the weight, width, and slant step values of the containing group. The reuse and aliasing implied by optical sizing unification are the reason it is not desirable to push grouping of an optical axis step in a separate group element.

That would probably not scale well to multiple special axes, but optical size is the only one specified so far and it’s hard to plan for things that have no formal existence in the OpenType spec.

The corresponding font files are used to satisfy a query for the @step_value@ of the axis, using the same distance calculation as in #198. Fontconfig should apply optical sizing automatically, unless explicitly requested otherwise by apps.

The Regular step of optical size is mapped by default to the fonts without any optical size axis in group. Its default value is 11.

Exemple

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<!-- SPDX-License-Identifier: MIT -->
<fontconfig>
  <group>
    <target>Bar</target>
    <axis>
      <name>optical size</name>
      <step>
        <name>Display</name>
        <value>24</name>
        <family>BigFoo</family>
      </step>
    </axis>
  </group>
</fontconfig>

Real world Exemple

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<!-- SPDX-License-Identifier: MIT -->
<fontconfig>
  <group>
    <target>PT Sans</target>
    <match>
      <family>PT Sans Narrow</family>
      <style-prefix>Narrow</style-prefix>
    </match>
    <axis>
      <name>optical size</name>
      <step>
        <name>Caption</name>
        <value>8</name>
      </step>
    </axis>
  </group>
</fontconfig>

In that real-world case the technical family mapped to the Caption optical size step already has the target naming, so it does not need an explicit family or match declarations.

Any query for PT Sans, Bold at size lower or equal to 8, should return PT Sans Caption, Bold data.

Past alternatives

Something similar to:

<?xml version='1.0'?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<!-- SPDX-License-Identifier: MIT -->
<fontconfig>
  <match target="scan">
    <test name="family">
      <string>PT Sans</string>
    </test>
    <edit name="fontversion">
      <int>2</int>
    </edit>
  </match>
  <match target="scan">
    <test name="fullname">
      <string>PT Sans Narrow</string>
    </test>
    <test name="family">
      <string>PT Sans Narrow</string>
    </test>
    <test name="style">
      <string>Regular</string>
    </test>
    <edit name="family" mode="prepend">
      <string>PT Sans</string>
    </edit>
    <edit name="style" mode="prepend">
      <string>Narrow</string>
    </edit>
    <edit name="fontversion">
      <int>1</int>
    </edit>
  </match>
  <match target="scan">
    <test name="fullname">
      <string>PT Sans Narrow Bold</string>
    </test>
    <test name="family">
      <string>PT Sans Narrow</string>
    </test>
    <test name="style">
      <string>Bold</string>
    </test>
    <edit name="family" mode="prepend">
      <string>PT Sans</string>
    </edit>
    <edit name="style" mode="prepend">
      <string>Narrow Bold</string>
    </edit>
    <edit name="fontversion">
      <int>1</int>
    </edit>
  </match>
  <alias binding="same">
    <family>PT Sans</family>
    <accept>
      <family>sans-serif</family>
    </accept>
  </alias>
  <alias>
    <family>sans-serif</family>
    <prefer>
      <family>PT Sans</family>
    </prefer>
  </alias>
  <match>
    <test name="family">
      <string>PT Sans</string>
    </test>
    <test name="size" compare="less_eq">
      <double>8</double>
    </test>
    <edit name="family" mode="prepend" binding="same">
      <string>PT Sans Caption</string>
    </edit>
  </match>
  <alias binding="same">
    <family>PT Sans Caption</family>
    <accept>
      <family>PT Sans</family>
    </accept>
  </alias>
  <alias binding="same">
    <family>PT Sans</family>
    <accept>
      <family>PT Sans Caption</family>
    </accept>
  </alias>
</fontconfig>