feature

Maple Mono Feature Module

A utility module for managing OpenType font features like stylistic sets, character variants, and ligatures.

Why

Most open-source font projects manage OpenType feature files manually. While fonttools' ast module enables feature file automation, the lack of documentation made implementation challenging. This module reimplements the ast functionality to generate .fea files and integrate them during builds.

Overview

The feature/ module uses an AST approach to programmatically define OpenType features.

Key Components

• ast.py: Core utilities for defining OpenType features.
• regular.py: Entry file for regular features.
• italic.py: Entry file for italic features.
• base/: Foundational classes and features (e.g., numbers, cases, localized forms).
• calt/: Default ligatures.
• cv/: Character variants.
• ss/: Stylistic sets.

Usage

Custom Tags

Create custom tags using utilities in calt/tag.py.

The font includes built-in tags with full-round borders. Use subst_liga to customize trigger text:

subst_liga(
    source="TODO:",
    target="tag_todo.liga",
    lookup_name="todo_colon"
)

For additional tags, use tag_custom:

tag_custom(
    [
      (":attention:", "[attention]"),
      ("_noqa_", "(noqa)"),
      # ("_alter_", "<alter>"),
    ],
    bg_cls_dict,
)

This converts:

:attention: _noqa_

into a styled tag:

Limitations

1. Custom tags lack spacing optimization.
2. Tags may break with letter spacing > 0. See #381.
3. Tags inherit text color. See #381.

Remove Infinite Ligatures

To disable infinite ligatures for =, -, ~, and #, set __USE_INFINITE = False in calt/_infinite_utils.py.

Variable Font Features

Two strategies exist for feature freezing:

1. Move ligature rules to calt (e.g., ss08)
2. Direct glyph substitution (e.g., cv01)

Currently, method 2 isn't supported in variable format. In V7.0, all variable format variants are identical except for family name. Use --apply-fea-file flag as needed.

Features now load dynamically via Python. The logic in common.py implements method 1. Enable font ligatures to use all features in variable fonts (not recommended).

AST Utilities

Core utilities for defining OpenType features:

Clazz

Represents a class of glyphs.

from source.py.feature.ast import Clazz, subst

cls_digit = Clazz("Digit", ["zero", "one", "two", "three"])
cls_digit.state()
subst(cls_digit.use(), "a", "b", "c")

Generated fea string:

@Digit = [zero, one, two, three];
sub @Digit a' b by c;

Lookup

Defines a lookup block for substitutions.

from source.py.feature.ast import Lookup, subst

lookup_example = Lookup(
    name="example_lookup",
    desc="Example substitution",
    content=[
        subst("a", "b", None, "c"),
    ],
)

Generated fea string:

# Example substitution
lookup example_lookup {
  sub a b' by c;
} example_lookup;

Feature

Represents an OpenType feature.

from source.py.feature.ast import Feature

feature_example = Feature(
    tag="calt",
    content=[
        lookup_example,
    ],
)

Generated fea string:

feature calt {

  # Example substitution
  lookup example_lookup {
    sub a b' by c;
  } example_lookup;

}

Create

Generates the final OpenType feature file content.

from source.py.feature.ast import create

fea_content = create([feature_example])
print(fea_content)

Generating Features

Features apply automatically during build. To update fea files manually:

uv run task.py fea

Example feature generation:

from source.py.feature import generate_fea_string

fea_string = generate_fea_string(italic=False, cn=True)
print(fea_string)