OpenType Font Subsetting

Documentation:

This is a simple, no-std-compatible library that provides OpenType font subsetting, i.e., retaining only glyphs and other related data that correspond to specific chars. The subset can then be saved in the OpenType (.otf / .ttf) or WOFF2 format.

As an example, it is possible to subset visible ASCII chars (' '..='~') from a font that originally supported multiple languages. Subsetting may lead to significant space savings; e.g., a subset of Roboto (the standard sans-serif font for Android) with visible ASCII chars occupies just 19 kB in the OpenType format (and 11 kB in the WOFF2 format) vs the original 457 kB.

The motivating use case for this library is embedding the produced font as a data URL in HTML or SVG, so that it's guaranteed to be rendered in the same way across platforms.

Features

Can read and write fonts to/from OpenType and WOFF2 (the latter via an opt-in crate feature).
Supports variable fonts. This allows to embed a continuous set of fonts across one or more dimensions, such as font weight or width, into a single file.
Provides general info about fonts, e.g., font naming / license info and variation axis parameters.
Single dependency for WOFF2 (de)compression; no-std compatible.

Design philosophy

Keep parsing simple. The library generally assumes that the input is well-formed, and defers parsing when possible. For example, simple glyph data is not parsed at all because it's not necessary for subsetting; it can be copied as opaque bytes.
Keep focus. The library is focused on subsetting. For example, it doesn't strive to parse all tables from the OpenType spec.
Keep dependencies lean. The library has the only opt-in dependency (brotli) to support WOFF2 serialization. The library is unconditionally no-std-compatible.

Usage

Add this to your Crate.toml:

[dependencies]
font-subset = "0.1.0"

Subsetting

use std::collections::BTreeSet;
use font_subset::{Font, ParseError};

// Load the Fira Mono monospace font (~129 kB in the OpenType format).
let font_bytes = include_bytes!("../examples/FiraMono-Regular.ttf");
// Parse the font.
let font = Font::opentype(font_bytes)?;
// Ensure that the font license permits embedding and subsetting.
let permissions = font.permissions();
assert!(permissions.embedding.is_lenient());
assert!(permissions.allow_subsetting);

// Create a subset.
let retained_chars: BTreeSet<char> = (' '..='~').collect();
let subset = font.subset(&retained_chars)?;
// Serialize the subset in OpenType and WOFF2 formats.
let ttf: Vec<u8> = subset.to_opentype();
println!("OpenType size: {}", ttf.len());
assert!(ttf.len() < 20 * 1_024);

let woff2: Vec<u8> = subset.to_woff2();
println!("WOFF2 size: {}", woff2.len());
assert!(woff2.len() < 15 * 1_024);
Ok::<_, ParseError>(())

Known limitations

glyf + loca transforms are not supported when writing / reading WOFF2 files (yet?).
Subsetting drops advanced layout tables like GPOS, kern etc.
Some table data (e.g., maxp fields like "maximum points in a non-composite glyph") are not updated in the subset font. Looks like some other subsetters (e.g., allsorts) do not update them either.

Alternatives and similar tools

allsorts is a library working with OpenType / WOFF / WOFF2 fonts, including their subsetting. It's more versatile, but at the cost of having more deps and requiring the standard library (although there is a no-std fork). Its subsetting logic also produces fonts not parseable by browsers because of a missing OS/2 table.
subsetter is a specialized subsetting library. but it's geared towards PDF subsetting specifically (i.e., again not covering the motivating SVG use case).

License

All code is licensed under either of Apache License, Version 2.0 or MIT license at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in font-tools by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

font-subset 0.1.0