Font Merge Engine

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The core font merge engine behind OFL Font Baker. The code in this directory is MIT-licensed and can be freely used in other projects.

Merges a base font (typically CJK) with a sub font (typically Latin or kana) into a single font file, replacing glyphs and OpenType features via cmap-based mapping.

For details on features and background, see the OFL Font Baker repository.

OFL Fonts Only

This library only accepts fonts licensed under the SIL Open Font License (OFL). If an input font's name table (nameID 13) does not contain an OFL license string, loading fails with an error.

Merged output is automatically tagged with OFL-compliant copyright and license metadata.

Installation

Existing users should upgrade with pip install --upgrade ofl-font-baker. Version 0.4.10 requires Brotli 1.2.1 or later, including when an older Brotli is already installed.

pip install ofl-font-baker

Requires Python 3.9+. fonttools and brotli are installed automatically.

Usage

Provide a JSON config on stdin. The engine supports two modes:

cat config.json | python3 merge_fonts.py

Path Mode

Specify output file paths explicitly via export.path. Only the files whose paths are provided are written.

{
  "baseFont": {
    "path": "/path/to/base.otf",
    "scale": 1.0,
    "baselineOffset": 0,
    "axes": []
  },
  "subFont": {
    "path": "/path/to/sub.ttf",
    "scale": 1.0,
    "baselineOffset": 0,
    "axes": [],
    "excludeCodepoints": ["U+2460-U+24FF", "U+203B"]
  },
  "output": {
    "familyName": "My Font",
    "weight": 400,
    "italic": false,
    "width": 5
  },
  "export": {
    "path": {
      "font": "/out/MyFont-Regular.otf",
      "woff2": "/web/MyFont-Regular.woff2"
    }
  }
}

Package Mode

Specify export.package to create a complete output directory with font files and metadata.

{
  "baseFont": { "path": "/path/to/base.otf", "scale": 1.0, "baselineOffset": 0, "axes": [] },
  "subFont": { "path": "/path/to/sub.ttf", "scale": 1.0, "baselineOffset": 0, "axes": [], "excludeCodepoints": [] },
  "output": { "familyName": "My Font" },
  "export": {
    "package": {
      "dir": "/exports/MyFont-Regular"
    }
  }
}

Output (JSON manifest on stdout):

{
  "outputDir": "/exports/MyFont-Regular",
  "fontPath": "/exports/MyFont-Regular/MyFont-Regular.otf",
  "woff2Path": "/exports/MyFont-Regular/MyFont-Regular.woff2",
  "oflPath": "/exports/MyFont-Regular/OFL.txt",
  "settingsPath": "/exports/MyFont-Regular/Settings.txt",
  "configPath": null,
  "files": [...]
}

Progress is emitted as JSON lines on stderr.

baseFont / subFont

When the sub font carries a glyph name that the base font also uses but at a different codepoint (e.g. Inter encodes U+0298 ʘ as uni25CE while Noto Sans JP uses uni25CE for U+25CE ◎), the merge engine renames the sub-font glyph to {name}.sub so the base outline at the stranded codepoint stays intact. A warning is emitted to stderr for each rename. This is unconditional — same name with disjoint codepoint sets always means unrelated glyphs.

output

export.path

All keys are optional. Only files whose paths are specified are written. woff2 requires font.

export.package

Creates a complete output directory. Always generates font, WOFF2, OFL.txt, and Settings.txt.

Tests

python3 -m pytest python/tests/ -v

Test fonts live in python/tests/fonts/.

License

The source code in this directory is licensed under the MIT License. See LICENSE for details.

Test fonts under tests/fonts/ follow each font's own license (OFL, etc.) and are not covered by the MIT notice.

Other parts of the parent repository (OFL Font Baker) are licensed under AGPL-3.0-or-later and are not covered by this MIT notice.