ofl-font-baker 0.4.0

Font merge engine — combine two font files into one via cmap-based glyph replacement

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

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

Key	Default	Description
path		Font file path (required).
scale	1.0	Outline scale factor.
baselineOffset	0	Vertical offset in font units.
axes	[]	Variable-font axis values: [{"tag": "wght", "currentValue": 400}, ...]. Empty for static fonts.
excludeCodepoints	[]	Sub-font only. Codepoints to keep sourced from baseFont. Each entry is "U+XXXX", "U+XXXX-U+YYYY" (inclusive range), or a raw integer. Useful for Latin + CJK merges that want CJK-conventional shapes for symbols (e.g. ①, ◯, ※, Ⅰ, ℃). Ignored on baseFont.

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

Key	Default	Description
familyName	"Font"	Output font family name.
postScriptName	""	PostScript name (nameID 6). When empty, derived from familyName by stripping chars outside printable ASCII 33–126 and []{}<>()/%, clamped to 63 bytes.
version	"1.000"	Version string (nameID 5). A Version prefix is prepended automatically if missing.
weight	400	Font weight (100–900).
italic	false	Whether the output is italic.
width	5	Font width class (1–9).
upm	(from base)	Target units-per-em. When different from the base font, all metrics and outlines are scaled.
manufacturer	""	Manufacturer name (nameID 8).
manufacturerURL	""	Manufacturer URL (nameID 11).
copyright	""	Additional copyright string appended to source copyrights.
trademark	""	Additional trademark string appended to source trademarks.
metricsSource	"base"	Which font's vertical metrics (OS/2, hhea) to use. "base" keeps the base font metrics and expands only when the sub font is larger. "sub" overwrites with the sub font's metrics.
metadataMode	"merge"	Identity policy. "merge" (or null / unset) rewrites name / OS/2 / head identity records as a fresh derivative work — current default. "inheritBase" passes the base font's identity through untouched; "inheritSub" does the same with the sub font (errors if subFont is absent). Only the output.* fields you explicitly set act as overrides on top of the inherited identity, and OFL nameID 13/14, designer, vendor ID, and head.created/modified are left alone. Useful for intermediate artifacts or merges that should not self-identify as a new derivative (e.g. adding Noto Hentaigana glyphs into Noto Sans JP).
hinting	"strip"	TrueType output hinting policy. "strip" (also "unhinted" / "none") removes TT bytecode, fpgm, prep, and cvt , while keeping maxp valid for unhinted outlines. "ttfautohint" (also "autohint") first writes the final stripped TTF, then runs an external ttfautohint binary over the finished font. CFF output keeps its separate CFF hint-preserving path.

export.path

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

Key	Description
font	Font file (OTF/TTF) path.
woff2	WOFF2 file path.
ofl	OFL.txt path.
settings	Settings.txt path.
config	ExportConfig.json path.

export.package

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

Key	Default	Description
dir		Output directory path (required).
overwrite	false	Allow overwriting an existing directory.
bundleInputFonts	false	Copy input fonts into a source/ subdirectory and write ExportConfig.json with relative paths. Makes the package self-contained and reproducible.

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.